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[Articles Home]  [Add Article]  

Dipole or Tripole?

Tom Thompson (W0IVJ) on May 20, 2007
View comments about this article!

Introduction
Recently, I wrote an article about toroids. The comments that ensued quickly centered around using ferrites as choke baluns on coaxial feed lines connected to balanced antennas such as dipoles. These baluns are used to attenuate the common mode current that flows on the outside of the coaxial feed line. I was certainly aware of this application of ferrites; in fact, I employ them on my antennas. However, in thinking further, I realized that I had no quantitative feel for the currents and the power that might be dissipated in the ferrites. I found a nice article by L. B. Cebik, W4RNL on common mode current in coaxial feed lines. Tom Rauch, Jr., W8JI, also has a nice discussion on the subject. These two articles convinced me that I could model the common mode situation using EZNEC. Row Lewallen, W7EL, the author of EZNEC also has a discussion of modeling radiating feed lines in his EZNEC help file. Walt Maxwel, W2DU, also, has an excellent discussion of this subject in chapter 21 of his book "Reflections II". With this assurance, I set out to put some numbers on these common mode currents. In this article I will attempt to show how these common mode currents occur, and what pitfalls one may encounter in attenuating these currents. Finally, I will discuss how one can make use of the "outside of the shield phenomenon" to construct a center fed dipole that can be easily suspended from a balcony with the feed point away from the building.


Skin Effect

At DC the current in a wire has a uniform current density. That means that the current in the wire is equally distributed in the wire. If you could plot the current, as it would appear from looking at the wire, end on, it would appear as a circle of equal density. AC current density is different. As the frequency is increased, the current density tends to be greater at the surface of the wire. Looking at the wire, end on, the current now appears as a donut with a hole in the center. But this is a fuzzy donut. That is to say the edge of the donut in the center does not have a sharp edge but disappears gradually into a hole. When people talk of skin depth, they mean the depth of penetration where the current density is e^-d times the current density at the surface of the conductor where e = 2.718... and d = the number of skin depths.. If d = 1, then that is one skin depth, and the current density is e^-1 or 37 percent of the current density at the surface. The skin depth of copper at 160 meters is about 0.002 inches. So, if you look at the current density at a depth of 0.01 that is 5 skin depths or e^-5 = 0.007 which is less than 1% of the current density at the surface.


Dipole or Tripole
When one feeds a dipole with a coaxial cable, the feed line consists of the center conductor, the dielectric around the center conductor, and the inside of the shield. The current that flows on the center conductor actually just flows on the outside of the center conductor. The return current just flows on the inside of the shield. That makes up the transmission line. I measured the thickness of the shield on a piece of coax to be about 0.02 inches. From the calculation above, the current density is down to less than 1% before it gets half way through the shield. But the outside of the shield is also connected to one leg of the dipole. Due to the skin effect, the outside of the shield does not know about what current is flowing on the inside, so it looks as if it was a separate wire. Since the transmission line part of the coax has opposing currents that are shielded from the antenna environment, one can place the source between the dipole legs and represent the coax shield as a third wire connected to the side of the source that is connected to the shield of the coax. This may be viewed in Figure 1.


Figure 1: Diagram of Common Mode Current in a Dipole Transformed into a Model Representation.

One could say that at this point we have a tripole instead of a dipole due to this third wire. Please note that the velocity factor of the coax affects the length of the transmission line part of the coax but not the length of the third wire part of the coax. For example, the velocity factor of a typical RG-58 coaxial cable is 0.67. It has a vinyl jacket that has a velocity factor of 0.95. Therefore when you figure the electrical length of the coax, you must use 0.67 for the transmission line part and 0.95 for the outside shield.


Dipole Model

EZNEC allows us to model this radiating feed line using the transformation shown in Figure 1. I set the power to 1500 watts and placed the dipole over perfect ground in order to best illustrate the common mode effects. Figure 2 shows the dipole model in an X-Y-Z Cartesian plane. The generator is at the midpoint between the two legs of the dipole labeled 1 and 2. The outside of the shield is represented by the vertical wire labeled 3. The red, curved line represents the current distribution along the wires. A load is placed at the top of the vertical wire. This load will represent our balun later on. Initially the load is set to 0 + J0 so it has no effect.


Figure 2: EZNEC Model of a 1/2 Wave Dipole ~ 1/2 WL Over Perfect Ground with Feed Line Not Grounded and Driven with 1500 Watts.

Figure 2 shows that the current imbalance in the two dipole legs and the feed line current are minimum. The current distribution along the two legs of the dipole, as indicated by the red curve, is exactly what you would expect. It reaches a maximum at the center of the dipole and is minimum at the two ends. The voltage distribution is just the opposite, reaching a maximum at the two ends and going through a minimum at the center. The feed point impedance, also, looks correct for a dipole in this configuration. Now we will ground the feed line and see what happens. Figure 3 shows the result of grounding the feed line at the bottom. In a real situation, this grounding might be a ground strap tied to the shield of the coax as it enters the shack, a ground strap tied to the tuner, amplifier, or transceiver or the ground wire on the AC receptacle in the shack. In fact, since the length is important, you must know what it is.


Figure 3: EZNEC Model of a 1/2 Wave Dipole ~ 1/2 wl Over Perfect Ground with ~1/2 wl Feed Line Grounded and Driven with 1500 Watts.

You can see from Figure 3 that the current distribution and the feed point impedance are very different from Figure 2. In fact, if we subtract the 5.9 Amp current in the outside shield of the feed line from the 6.4 Amp current in dipole leg 1, you get the 0.5 Amp current left in dipole leg 2. The current that should be flowing in dipole leg 2 is now flowing in the outside shield of the feed line. Not only is the current distribution altered, but the feed point impedance looks more like a vertical than a dipole. In fact, you could view this as an inverted vertical with one radial. This is not a good situation. Not only does this alter the radiation pattern that we expect, it causes radiation in the shack. Remember that the ground connection at the bottom end of wire 3 could be very close to equipment in the house. It is as we have moved the ground up to the leg of the dipole and provided a low impedance path for the current to flow.

I have chosen a feed line length that maximizes the feed line radiation for demonstration purposes. If you chose the feed line to be close to 1/4 wl and leave the end open instead of grounded, feed line currents will also flow on the outside of the coax. The current distribution on the feed line will look like just the upper half of the distribution on wire 3 in Figure 3. The feed line lengths that maximize common mode currents are odd multiples of 1/4 wl and multiples of 1/2 wl. In order to determine how far you have to stay away from these critical lengths with a grounded feed line end, I used EZNEC to determine the currents for various lengths of grounded feed lines. Figure 4 shows the results of those calculations.

Figure 4: Dipole Leg Current Imbalance and Feed Line Common Mode Current vs Length of a Grounded Feed Line. 1500 Watts Drive Power

You can see from Figure 4 that the dipole leg current imbalance and the feed line common mode current are correlated, which is exactly what you would expect. Also, you can see that if you stay at least 1/8wl away from multiples of 1/2 wl feed line lengths, the imbalance and common mode problem is greatly mitigated. It is also interesting to note that the problem diminishes as the feed line gets longer. I thought this was due to loss, but I ran the model again with zero loss and this phenomenon still occurs.

Altering the feed line length will change the impedance that the transmitter sees in the shack if the antenna feed point impedance does not exactly match the transmission line characteristic impedance. But if you are operating close to a length that causes high common mode currents, the feed point of the antenna is actually changed. Now you see the impedance being transformed as a function of the feed line length, when measured in the shack, and the feed point impedance at the antenna is being actually being changed as a function of feed line length. This becomes a measurement nightmare if you are trying to make antenna impedance measurements from within the shack.

In order to illustrate this further, the same dipole that is shown in Figure 3 was modeled exactly the same but with 1/8 wl of feed line added. The results are shown in Figure 5.

Figure 5: EZNEC Model of a 1/2 Wave Dipole ~ 1/2 wl Over Perfect Ground with ~5/8 wl Feed Line Grounded and Driven with 1500 Watts.

You can see that by just adding 1/8 wl of feed line, the dipole leg current imbalance is corrected, the common mode current in the feed line is reduced, and the feed point impedance of the antenna is restored to a reasonable value.



Enter the Common Mode Choke or Choke Balun

Selecting the right feed line length may not be the easiest way to get rid of common mode feed line currents. We may be operating from an upstairs apartment where a good ground is not easily obtained. Our feed line may feed multiple antennas where the feed line length would have to vary with frequency. The common mode choke sometimes called a choke balun is the answer. Common mode chokes come in a variety of flavors. Essentially a choke balun is a current balun, and may be as simple as several turns of feed line wound in a nice solenoid coil about 6 inches in diameter. I have used these since the 1960's on antennas for 14 MHz and above. These chokes have the advantage that the impedance is entirely reactive and thus heating is not an issue, since there is no lossy resistive part of the impedance. The disadvantage of this kind of choke is getting a high enough impedance at the lower frequencies. Ferrite cores and rods have also been used for many years. Unfortunately, they have been used in voltage balun applications where the whole transmitter power passes through the windings. Overheating and core saturation in this application have given the ferrite a bad reputation. When you use a ferrite in a common mode application, the stresses on the core are much less, but you still have to be careful! We will discuss some of these precautions later, but right now let's just see what a common mode choke will do. In a previous toroid article, I discussed a common mode choke consisting of 15 turns of RG-58 coax wound on an FT-240-43 core. This has an impedance of 5000 - J5000 at 7 Mhz. If we create a load of this value at the upper end of wire 3 in our model shown in Figure 3, we get the results shown in Figure 6.

Figure 6: EZNEC Model of a 1/2 Wave Dipole ~ 1/2 wl Over Perfect Ground with ~1/2 wl Feed Line Grounded and Driven with 1500 Watts. A Common Mode choke Consisting of 15 turns of RG-58 on FT-240-43 Core is Employed. Z = 5000 - J5000. 1.33 Watts (0.004 db) is Lost in the Core with 1500 Watts Drive.

You will notice that the common mode choke balances the dipole leg currents very well, the feed line common mode current is greatly reduced, and the feed point impedance is correct. Furthermore, the power lost in the choke is negligible even at maximum drive power. You might notice that the reactive part of the impedance is negative which is capacitive. This occurs because the self resonant point of the choke is below 7 MHz. But this doesn't matter as long as the magnitude of the impedance is sufficiently high and the resistive part of the impedance doesn't dissipate too much power.

I have always heard that a voltage balun does not work well when the SWR is high. But what about the common mode choke which is a current balun? How does it do when the SWR is high? With EZNEC this is easy to evaluate. I just changed the frequency from 7 MHz to 6 MHz and then to 8 MHz. The choke impedance magnitude does not vary much over this range, so I left the choke impedance at 5000 - J5000 for these calculations. If you use the actual values, the numbers only change a bit. The object of this test is to show that high SWR does increase the stress on the choke. Table 1 below shows the results.


Table 1: Effects of High SWR on Choke Stress


Frequency (MHz) Impedance (Ohms) SWR Dipole Leg 1 Imax (A) Dipole Leg 2 Imax (A) Feed Line Imax (A) Loss in Choke (Watts) Loss in Choke (db)
7 68 - J0.6 1.35 : 1 4.7 4.7 0.017 1.3 0.004
6 54 - J236 22 : 1 5.3 5.2 0.066 20 0.058
8 96 + J257 16 : 1 4.0 4.0 0.072 16.5 0.048

Although high SWR does stress the choke, an FT-240 core is probably sufficiently large to take it, especially if you take into account a less than 100% duty cycle encountered in most amateur operations. In fact, the SWR's shown here are extreme. Under normal SWR conditions, the choke probably could handle the stress without any trouble whatsoever.



Ferrite Beads

What about ferrite beads? Again referring to the toroid article, Figure 11 shows 7 ferrite beads of the FB-1020-43 variety yielding a 7 MHz impedance of 300 + J500. Table 2 shows the results of various numbers of these beads slipped over the coax at the feed point.

Table 2: FT-1020-43 Ferrite Bead Choke Balun Chart

Number of Beads Zbead @ 7 MHz Zfeedpoint Dipole Leg 1 Imax (A) Dipole Leg 2 Imax (A) Feed Line Imax (A) Loss in Choke (Watts) Loss in Choke (db)
1 45 + J112 64 + J4.8 4.8 4.4 0.893 35.9 0.11
4 205 + J308 66 + J1.2 4.8 4.6 0.3 18.8 0.06
7 500 + J300 69 + J0.6 4.7 4.6 0.195 11.4 0.03

It is evident from this chart that you can easily overstress a single bead by not placing enough beads on the feed line. Not only is the power that needs to be disspated more, but the suface area of a single bead is smaller than the surface area of several beads. It is like placing a bunch of resistors in series to increase the power disspation capability.



Pattern Alterations

Now that we have established that these common mode currents on the feed line are flowing, let's see what it does to our pattern. We normally are not too concerned about our dipole patterns unless we have the dipole mounted high and in the clear. But the directional antenna is a different matter. We have probably put a lot of effort and money to establish a radiation pattern that has good gain and front-to-back ratio. We don't want that destroyed by feed line radiation, that can upset the pattern of a directional array by radiating itself, or by upsetting the current distribution in the elements that will alter both gain and front-to-back ratio. But how bad is it? I modeled a 3 element Yagi to test this. No particular effort was made to optimize either gain or front-to-back ratio.Figure 7 shows the pattern when the feed line is exactly vertical beneath the Yagi.

Figure 7: Feed Line Common Mode Current Effects on a 3-Element Yagi with a Grounded, ~1/2 wl, Vertical Feed Line Beneath the Yagi.

When you extend the feed line so that it is not vertical, but partially horizontal beneath the Yagi, the effects are even more extreme. Figure 8 shows these effects.

Figure 8: Feed Line Common Mode Current Effects on a 3-Element Yagi with a Grounded, ~1/2 wl, Slanted Feed Line Beneath the Yagi.

The Yagi plot with the balun in Figure 8 looks different than the one in Figure 7 because I lowered the antenna so I could slant the feed line and still reach ground. Nevertheless, the important factor to note is the distortion in the pattern from not having the common mode feed line choke.


Radiation Onto the Feed Line

What about radiation from the antenna onto the feed line? Common sense tells us that common mode current on the feed line should be lower if the feed line leaves the antenna at a right angle. But, does the choke balun mitigate this type of common mode current? In order to test this, I placed the FT-240-43 choke balun on the feed line model shown in Figure 5. I then rotated the feed line so that it was under Leg 1 of the dipole and then under leg 2 of the dipole instead of being at a right angle to the dipole. Table 3 shows the results of this test.

Table 3: Effects of Feed Line Position on Feed Line Common Mode Current and Choke Stress

Feed Line Position Impedance (ohms) Dipole Leg 1 Imax (A) Dipole Leg 2 Imax (A) Feed Line Imax (A) Loss in Choke (Watts) Loss in Choke (db)
At Right Angle 68 - J0.64 4.7 4.7 0.03 1.45 0.004
Under Leg 1 76 + J1.6 4.4 4.4 0.81 13.7 0.04
Under Leg 2 67 + J2.5 4.7 4.7 0.85 5.7 0.016

It is apparent from Table 3 that the position of the feed line does make a difference on the amount of feed line common mode current, feed point impedance, and balun stress. Also, it is apparent that feed line common mode current has three sources: 1) Feed line length, 2) Feed line position, and 3) Feed line termination. Also, the balun cannot eliminate that part of the feed line common mode current that is the result of the antenna radiation.


Balcony Antenna

Since this coaxial skin effect causes a third wire to be connected to a dipole, I kept trying to put it to some good use. I travel a lot and sometimes run my ham station in rooms that have balconies. I have been known to stick one end of a fiberglass pole out from the balcony with a counterpoise inside the room. This has the disadvantage of half the antenna being in the room and the high current part of the antenna being near the building. After thinking about this for a while, the antenna shown in Figure 9 occurred to me.

Figure 9: 20 Meter End Connected, Center Fed Balcony Antenna Made From RG-58 Coax.

This antenna makes use of the 3 wire effect caused by the skin effect on the shield of a coaxial cable. I have tried antennas like this in the vertical configuration with the shield folded back on itself, but they never seemed to work quite right. Now I know why! First of all the, there is no need to fold the shield back. The skin effect will isolate the inside from the outside. Also, a trap is needed to "stop" the outer shield at the 1/4 wl point. Since this is a high impedance point anyway, the impedance of the trap needs to be very high at the operating frequency. Figure 10 shows the EZNEC model of the balcony antenna.

Figure 10: EZNEC Model of Balcony Antenna

Theoretically, this antenna should work. If you suspend it over the balcony on a fiberglass pole, all the radiation is outside of the building and the radiation pattern is away from the building. I run a 600 watt, home-brew, solid state amplifier when I travel. If I operate SSB, then the trap will dissipate 8 watts when I transmit. It will dissipate about 19 watts on CW. The mass of the trap should be able to handle this. The trap was calculated on the basis of an unloaded Q of 300 for the coiled feed line. This may be optimistic. If Qu is lower than 300, the trap losses will go up.

I decided to build this antenna to see if it would really work. Figures 11 and 12 show the finished product.

Figure 11: Two Ends of the Balcony Antenna Showing the Trap and the Center Conductor of the Antenna.

Figure 12: Balcony Antenna in Position for Testing. SteppIR Antenna is Shown Above.

The antenna was built by taking about 40 feet of RG-58 coax and wrapping 14 turns around a thin wall, 1 inch pvc pipe. A 50 pF, 15 kV, NPO capacitor fits inside the pipe and is connected from shield to shield on the coiled RG-58. The coax extends for 17 feet and then the center conductor extends another 17 feet beyond that. The antenna was tested on 20 meters between Boulder, CO and Madison, WI and Boulder and Arlington, VA against the 3 element SteppIR shown in figure 12. The Yagi is at 40 feet. One end of the balcony antenna is at 35 feet and the other end is at 15 feet. Two people in Madison showed 5 db difference while one showed no difference. The person in Arlington showed 20 db difference. In order to insure that the dipole was not coupling into the Yagi and giving it an unfair advantage, I modeled them both together as they appear in Figure 12. There was minimum interaction between the two. In fact the Yagi reduced the gain of the dipole by 1 db, and the effect of the dipole on the yagi was less than 0.2 db. This is not an exhaustive test by any means, but the balcony antenna at least acts as an antenna and proves the concept.


Conclusions

The skin effect that forces RF current to the outside of a conductor causes a dipole fed with coaxial cable to become a tripole. This third wire of the antenna system can cause RFI in the shack and upset the radiation pattern of the antenna. Careful selection of the feed line length can mitigate this effect. Using a choke balun can also mitigate this effect making the feed line length less critical. However, care must be exercised when constructing the choke balun in order to prevent excessive dissipation in the core if a ferrite is used. Finally, this tripole effect can be put to good use in the construction of a balcony antenna that is center fed from the end.

I hope this analysis has been as instructive for you as it was for me!

Member Comments:
This article has expired. No more comments may be added.
 
Fan Dipole  
by KA4KOE on May 20, 2007 Mail this to a friend!
I heartily applaud the author's treatise. Any commentary on the now ubiquitous fan dipole???

Thanks

Philip Neidlinger
KA4KOE
 
Baluns  
by KA4KOE on May 20, 2007 Mail this to a friend!
Seriously,

I recently had to install a choke balun at the antenna feed of a 20 element LPDA, KMA50500, due to feedline radiation. The VSWR and directivity have indeed improved. Used about a foot of clip-on ferrite chokes from the Wireman.

Philip
KA4KOE
 
Dipole or Tripole?  
by N9MW on May 20, 2007 Mail this to a friend!
Very nice article, well written and lots of practical information to think about.
 
Dipole or Tripole?  
by WA1RNE on May 20, 2007 Mail this to a friend!
A very well written and thought out article.

A couple of comments:


"Essentially a choke balun is a current balun, and may be as simple as several turns of feed line wound in a nice solenoid coil about 6 inches in diameter. I have used these since the 1960's on antennas for 14 MHz and above. These chokes have the advantage that the impedance is entirely reactive and thus heating is not an issue, since there is no lossy resistive part of the impedance. "


>> Close-wound solenoid type coaxial chokes work fine *up to about 14 Mhz, then after that the interwinding capacitance starts to degrade the choke impedance at higher frequencyies. One way to reduce interwinding capacitance of a single layer air wound choke used for 20 meters and up is to add spacing between turns to reduce the capacitance. This makes the choke longer but can make a big improvement in the available choke impedance. Here's good explanation of this and includes an easy to use program for calculating the best combination of choke dimensions vs turns and winding pitch: http://www.smeter.net/feeding/selfres3.php

The "end connected, center fed" balcony antenna is really just an end fed wire, not a center fed dipole. It's a clever idea as it can be put together in a pinch. But given the main content of this article, I'm surprised the author didn't try adding a common mode choke at the feed point of this antenna.


....WA1RNE
 
RE: Dipole or Tripole?  
by AA2LD on May 20, 2007 Mail this to a friend!
Great article. This is the main reason my Windham 80 'dipole' works so well with its vertical radiator, essentially the coax feed line. The choke at the base of the vertical radiator is essential. This makes the Windham 80 rock and roll for a dipole. Highly recommend it.
 
Dipole or Tripole?  
by WA8MEA on May 20, 2007 Mail this to a friend!
GREAT article! I love all of the graphs, charts and formulas.

But when all is said and done, it is still called:

ANTENNA THEORY.

Reason? You can take ANY antenna, move it to the other side of the neighborhood and it will act/react totally different. There are SO MANY variables that go into the performance of an antenna, we often forget. Even the make-up of the ground beneath you, drain fields, grass vs. snow cover and buried treasures like metal salvage.

That's why I'm not a big fan of baluns. You still cannot undo what Mother Nature throws at you. So I ditched the balun back in 1971 and never looked back.

And I think my stubborn behavior has slopped over into my business practice. HOWEVER, while some folks swear AT baluns...a lot of hams swear BY them.

In a few weeks we will be offering portable dipoles with 1:1 and 4:1 baluns. We will give the people what they want. And that's what a good business should do.

73, Bill - WA8MEA
http://HamRadioFun.com
 
Dipole or Tripole?  
by N0AH on May 20, 2007 Mail this to a friend!
Another reason why a Hustler 4BTV backyard vertical will outperform any neighborhood dipole for DX on any given Sunday.For dipoles to work, you need height. In theory, this article is amazing- lots of hard work, well document, quantitative, (some questionable sources), but over all text book stuff- I just hate to see third party quotes from those who claim that they are their own- not the author.....just certain calls he has mentioned- Excellent models......thank you for your hard work and efforts. You know your stuff-

73 Paul
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.

...A very well written and thought out article...

I go along with the post of WA1RNE, which is fool proof and tried to perfection.

I am not fond of baluns and not any longer used, however I do use the quarter wave sleeving on my coaxial dipoles that seem to work well with absolutely no heating or heat dissipated...This may also be called a Tripole if one delve; To search deeply and laboriously in theory that proves itself at low cost.

Thanks for a well written article.

W6TH
.:
 
Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
Chris, WA4RNE said:

"Close-wound solenoid type coaxial chokes work fine *up to about 14 Mhz, then after that the interwinding capacitance starts to degrade the choke impedance at higher frequencyies. One way to reduce interwinding capacitance of a single layer air wound choke used for 20 meters and up is to add spacing between turns to reduce the capacitance. This makes the choke longer but can make a big improvement in the available choke impedance. Here's good explanation of this and includes an easy to use program for calculating the best combination of choke dimensions vs turns and winding pitch: http://www.smeter.net/feeding/selfres3.php"

I wanted to put some numbers on this type of balun, so I wound an 8.5 turn solenoid type coil of RG-58 on a 6.5 inch diameter cansister and slipped it off of the form. I then measured the impedance of this balun at the HF amateur bands. The table below shows the results.

Frequency___Impedance______Magnitude of Impedance
1.8____________1 + J200_______200
3.7____________6 + J430_______430
5.35__________11 + J698_______698
7.2___________45 + J1173_____1174
10.1_________570 + J3627_____3671
14.2_________621 - J3751_____3802
18.1_________124 - J1462_____1467
21.3__________71 - J990_______993
24.9__________66 - J720_______723
28.5__________58 - J572_______575

The self resonance of this balun is at 12.1 MHz, so the reactance is capacitive for frequencies above 12.1 MHz. However, it is the magnitude of the impedance that is important, and it should be approximately greater than 1000 ohms to be completely effective.

I then wound another balun that consisted of 11 turns on a 4.75 inch diameter plastic cansister. This balun was wound with about 1/2 inch space between the windings, and was left on the plastic canister. The impedance measurements of this coil are shown below:

Frequency___Impedance______Magnitude of Impedance
1.8____________1 + J110_______110
3.7____________1 + J229_______229
5.35___________2 + J343_______343
7.2____________6 + J492_______492
10.1__________27 + J817_______817
14.2__________73 + J1683_____1685
18.1________2082 + J6812_____7123
21.3________2420 - J5120_____5663
24.9_________552 - J1826_____1908
28.5_________350 - J1291_____1338

The self resonant frequency of this balun has moved up to 19.4 MHz, so this balun is more effective on the higher bands.

Chris, WA4RNE, also,said:

"The "end connected, center fed" balcony antenna is really just an end fed wire, not a center fed dipole. It's a clever idea as it can be put together in a pinch. But given the main content of this article, I'm surprised the author didn't try adding a common mode choke at the feed point of this antenna."

The antenna described is NOT an end-fed antenna. Since it is 1/2 wl long, if it was end fed, the feed point impedance would be high not low. The feed line on this antenna consists of the outer part of the center conductor and the inner part of the shield. The dipole legs consist of the unshielded center conductor at the point the shield terminates and the outer part of the shield of the coax at the point of the shield termination to the tuned trap. The feed line is actually inside of one leg of the dipole. This doesn't matter because the currents on the feed line are balanced and shielded.




 
Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
Pardon me Chris, I have moved you south. That should be WA1RNE instead of WA4RNE in the previous post.

Sorry,

Tom W0IVJ
 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
WA8MEA comments on "antenna theory" :

"Reason? You can take ANY antenna, move it to the other side of the neighborhood and it will act/react totally different"

I recently built and erected this antenna:

http://www.n3ox.net/projects/2017moxon

And up to a slight (and mostly inconsequential) misadjustment of the very touchy L-network for 17m, it works exactly like the model predicts.

I built it to a few millimeter accuracy and put it up the height above ground that I had in my model and **measured** the F/B and SWR to be practically spot on on 20m, and F/B on 17m was exactly what the model predicted as well. Regrettably, I can't actually measure gain to strengthen my case.

- - - - - -

I've tweaked the matching network a bit for better SWR on 17m, but other than that I built the thing carefully, put it up, and it works just like the EZNEC model says it works.

Being able to adjust and empirically measure antennas is very important... maybe the most important thing for most folks, but let's not use the word "theory" as a dirty word.

For a model to reflect reality, it has to capture all the essential features of the antenna... this article covers that well... throw in the third wire with a load corresponding to your balun and you'll have a much more accurate model.

For horizontal antennas a good distance above the ground clutter with a decent balun and a symmetrically placed feedline about 90 degrees to the plane of the antenna, even a model that doesn't include the feedline is going to be valid, and that is my experience with modeling and building of VHF antennas and the new Moxon. It just does what the model says.

I did discover that on 20m with the Moxon pointed north, there was a bit of an increase in SWR. That's where the high-voltage very critical region of the antenna where the element tips come together gets quite close to my 60 foot low band vertical.

I found that the SWR change was least bad when I switched the 60 foot vertical to the 40m position, which puts an 8 microhenry inductor between the vertical and ground. So I built a model that included the vertical, and it showed that several hundred ohms inductive was pretty close to the optimum load to ground to minimize currents in the vertical.

8uH on 20m is about 700 ohms.

- - - - - -

You've got to build a decent model, and you especially must not make any sort of claims that conflict with reality based on what the model says, but I've found "antenna theory" to have great predictive power, and that's the test of a good model.

I can use EZNEC and some other programs to build an antenna with an L-match that works the first time.

I totally agree with you, Bill, if you're talking about a 40m dipole down among trees, sheds, and lawn gnomes... and more importantly, near large metallic objects... there are a lot of factors that can influence an antenna. However, that doesn't make theory and modeling useless... you just need to recognize when something is significant enough to include in your model..

73,
Dan
 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
It occurs to me that my claims that my antenna "worked just like the model" are total BS if I don't actually give you some numbers...

F/B was within a couple dB of the model on both bands.

SWR minimum on 20m was within 50kHz of predicted and good enough to stay under 2:1 for most of the band.

The best I can say about the gain is that it's more than a delta loop with the lowest point at 16 feet on 20m from tests I did while I had the Moxon and a delta loop up simultaneously.

Also, I don't want to sound like I'm knocking experimeters... it's more fun and better to put wire in the air and make some contacts than to hack around on a computer all day.

I've just had a good experience with properly applied antenna models and careful measuring in making a relatively complicated antenna that works well with almost no tweaking.

I've got great respect for someone who can, for example, experimentally optimize a four-band quad by spending a whole weekend pruning wires and taking notes. I don't have the patience, so I do the model first.

73,
Dan
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
W0IVJ.

Did you install these on you balcony antenna and then do some more investigating? If so, could you please pass along more information if there is any change with it connected to your antenna?

I wanted to put some numbers on this type of balun, so I wound an 8.5 turn solenoid type coil of RG-58 on a 6.5 inch diameter cansister and slipped it off of the form. I then measured the impedance of this balun at the HF amateur bands. The table below shows the results.

Frequency___Impedance______Magnitude of Impedance
1.8____________1 + J200_______200
3.7____________6 + J430_______430
5.35__________11 + J698_______698
7.2___________45 + J1173_____1174
10.1_________570 + J3627_____3671
14.2_________621 - J3751_____3802
18.1_________124 - J1462_____1467
21.3__________71 - J990_______993
24.9__________66 - J720_______723
28.5__________58 - J572_______575

The self resonance of this balun is at 12.1 MHz, so the reactance is capacitive for frequencies above 12.1 MHz. However, it is the magnitude of the impedance that is important, and it should be approximately greater than 1000 ohms to be completely effective.

I then wound another balun that consisted of 11 turns on a 4.75 inch diameter plastic cansister. This balun was wound with about 1/2 inch space between the windings, and was left on the plastic canister. The impedance measurements of this coil are shown below:

Frequency___Impedance______Magnitude of Impedance
1.8____________1 + J110_______110
3.7____________1 + J229_______229
5.35___________2 + J343_______343
7.2____________6 + J492_______492
10.1__________27 + J817_______817
14.2__________73 + J1683_____1685
18.1________2082 + J6812_____7123
21.3________2420 - J5120_____5663
24.9_________552 - J1826_____1908
28.5_________350 - J1291_____1338

The self resonant frequency of this balun has moved up to 19.4 MHz, so this balun is more effective on the higher bands.

When connected to the antenna and tests run, please pass along the information of any changes occurred, thanks.
.:
 
RE: Dipole or Tripole?  
by N6AJR on May 20, 2007 Mail this to a friend!
Actually hes already has the best antenna around, he is hanging it off a steppir.....

and BTW you can probably adjust the dipole by running the steppir in or out too. nice to have an adjustable capacity hat..
 
RE: Dipole or Tripole?  
by WA1RNE on May 20, 2007 Mail this to a friend!

"The antenna described is NOT an end-fed antenna. Since it is 1/2 wl long, if it was end fed, the feed point impedance would be high not low."

Tom;

Thanks, your right, in terms of operation I misinterpreted the diagram and description of the antenna.

I would think that using the coil with less interwinding capacitance, the Q should go up as well and losses should come down.


...WA1RNE



 
RE: Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
Vito, W6TH, wrote:

"Did you install these on you balcony antenna and then do some more investigating? If so, could you please pass along more information if there is any change with it connected to your antenna?"

The balcony antenna does not need a balun since the outside of the coax shield, which is the "third wire" on a dipole, is actually one leg of the balcony antenna. In fact, since this is the case, if you put a choke balun at the feed point of the balcony antenna, the antenna would not work. There is a trap on the balcony antenna, but it is 1/4 wl from the center of the antenna to "stop" the current flow on the outside of the shield, which is one leg of the dipole. The coax that continues from there to the transmitter is just feed line. There is no current on the outside of the shield from the trap to the transmitter because of the trap.

Tom W0IVJ
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
Tom, Same here,
Thanks, your right, in terms of operation I misinterpreted the diagram and description of the antenna.


N30X, Dan Some information please.

3 times to the 8th power divided by 7 times to the 6th power equals 42.857 times 3.28 equals 140.57 feet.

Or does the EZNEC use the 468 divided by 7 equals 66.857 feet times 2 equals 133.714?

W6TH
.:
 
RE: Dipole or Tripole?  
by NXET on May 20, 2007 Mail this to a friend!
I don't see the tripole thing. When you say the 3rd member, which is the foil it is not seperate from the wire braid. Therefor it is part of the shielding that makes up the wire braid part. If it had a dilectric between it then I could but it doesn't. Therefore, its as the manufacture says, improved-- shielding and becomes part of the wire braid that makes up the ouside of the coax. Yes the current goes on the outside too due to sphearical skin effect What goes up comes back in RF unless, you have one side of the coax grounded and no balun at the antenna feed point. If you do, then you only have an improved shielding for the coax... sorry

Look, as Cebik is quick to point out. Any antenna that is not a quarter, half,5/8's or full wave is not much of a antenna and your fooling yourself it you think any variation of anything other its going to work as good as the quarter,half, 5/8's or full wave. I find it amusing that some new break through in ham radio antenna turns ot to be a little whip about 6 inches tall that the maker claims works as well as a half wave dipole. they keep trying but, in the end, here on eham you see reports of how they really do work by some who actually have tried them. The rest I think are ringers that couldn't tell one end of a diode from the other yet, try and convince you that in less than 2 weeks they have found it to be a outstanding antenna that you should go out and waist your money on.

Again anything short of the real McCoy--- in the proven field of antennas which are a quarter, half, 5/8's or full wave are just someones idea of a good time and a waist of yours.
 
RE: Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
Chris, WA1RNE wrote:

"I would think that using the coil with less interwinding capacitance, the Q should go up as well and losses should come down."

Chris, you are exactly right. I threw this antenna together just to see if my concept was correct. I had the 15 pF doorknob capacitor, which is inside the pvc pipe, on hand, so I just wound it up and tried it. You actually want the unloaded Q to be as high as possible to cut down the losses. At some point, I'll put a carrier on and see how warm the trap gets. Another day, another experiment :)

Tom W0IVJ
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.

Tom W0IVJ,

Thanks, I am familiar with the info and have used that before, yes it could be called a Tripole. Much too much to read Tom, so had to go back and recheck.

There are other means to prevent RF on the outside of coax and the good systems do use the Coax sleeve method. Also methods to parallel a quarter wave from the feedpoint down.

The more I read, the better your post. Being an old man, I skimp read and was told to slow down.

73, W6TH
.:
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
NXET
Look, as Cebik is quick to point out.

Cebik "paraphrases"

He Cebik helps restatement of a text or passage in another form or other words, often to clarify meaning.

This makes for easy reading for folks not familiar with an electron knowledge. Most of his work is a copy of text from another source and Cebik rephrases it for simplicity.

Others through the internet do the same, as history repeats itself so does knowledge.

By the way, this post has me reading and rereading about the inner coax and the outer center conductor so feel at home.

W6TH
.:
 
RE: Dipole or Tripole?  
by K8MHZ on May 20, 2007 Mail this to a friend!
"I don't see the tripole thing."

I am with you. If there are three poles at any given time in the waveform they should be able to be named. In the electric field that would be positive, negative and ???? In the magnetic field we would have N, S and ???

I think this entire tripole thing comes from a lack of understanding of the term 'pole'.

A conductor is not a pole. At RF frequencies coax has three conductors, not three poles. Those conductors are illustrated in the drawing.

 
RE: Dipole or Tripole?  
by W9OY on May 20, 2007 Mail this to a friend!
do you take into account the velocity factor of the coax shield + vinyl ie 95% in the calculation of the antenna length?

73 W9OY
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
Well, look at it this way and think in the terms of phase relationship.
.......There are three thingies going east and west and the third is going south. You have voltage, you have current and then you have phase which can be considered three.

To make it simple; a dipole is not a center fed zepp and a center fed zepp is not a dipole, yet many call a zepp a dipole and so does Cebik, because Cebik wants to be on the safe side and not cross reference Hertz.

So the final conclusion is a pole is a pole which cannot be denied. A pole is misinterpreted as a pole is an extremity of an axis through a sphere.

There you have it Mark.

.:
 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
"In the electric field that would be positive, negative and ????"

How about 0 degrees, 46 degrees and 72 degrees with the right magnitudes to make them all add up at the common point?

The current and voltage have an amplitude and phase at every point on the antenna system, but you only get equal amplitude and opposite phase currents on your antenna and the inside of your feedline if you have a perfectly balanced antenna with no current on the outside of the feedline.

Dan
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
N3OX Dan

What if one should run a coax fed antenna and tune the same as if it was a open feed line. What then would one see for a balanced line? Considering that the antenna itself was resonant.

.:
 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
Vito,

Do you mean what would be the difference in overall feedline commmon-mode current if you tried coax vs. ladder line?

It depends on a lot of factors, but if you used a decent link-coupled tuner or other system with very high common-mode impedance (really, really good balun), you could probably force the coax to be nearly as well balanced as the ladder line, and then no appreciable current will exist on the outside of the shield.

Current would only flow on the inside.

The analogous situation with ladder-line is to have more current in one leg than in the other.
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
I can see where it would be as good as open wire line and would be perfectly balanced. Might be a good try by using a one to one balun at the transmiiter end for a try out, but I myself would rather go for a link coupling to an inductor and a variable capacitor in parallel with the inductor. I don't know how much power could be put through it, but certainly worth the try for an experiment. There should be almost 100 percent cancellation and not much radiation off of the shield. Could be useful for those that have RF down the line problems.

I imagine a lot of high power would melt the coax and care should be taken.

.:


 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
A lot of people use a feedline choke (ferrites on the outside of the coax) for unbalanced feedline to help with these issues.
 
RE: Dipole or Tripole?  
by K0RFD on May 20, 2007 Mail this to a friend!
Shoot, I'm an "Extra Lite" but I still get it. Maybe that's because I have read Maxwell's paper.
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.

Who is Maxwell's, a Democrat in Grand Junction CO.

Never heard of her.

.:
 
RE: Dipole or Tripole?  
by W6TH on May 20, 2007 Mail this to a friend!
.
N30X

Actually Dan I have never had the use of unwanted feed line RF problems. A well designed antenna using coax should not have to use any more than the half wave antenna at resonance and the coax feed line.

Something is lacking in their design and could be as simple as a good ground system.

I have witnessed RF on the bug and microphone, but only with the use of the halfwave antenna and the single wire feed, never ever with a coax run.
.:
 
RE: Dipole or Tripole?  
by N3OX on May 20, 2007 Mail this to a friend!
Vito,

I had the same experience as that until I moved into some noisy locations.

If the impedance at the feedpoint is 50 ohms and the impedance of the path along the coax shield is, say, 1000's of ohms, then very little current will be present on the shield of the coax. You might only get a few tens of milliamps on the coax shield at 100W with a resonant dipole.

However, this path is bi-directional.

I reduced the noise on my 15m dipole by two whole S-units by adding a ferrite-bead choke balun at the feedpoint.

When I first became a ham in high school, I put up antennas at my parents' very quiet nearly rural location and never put baluns or feedline chokes on anything.

Now that I'm fighting electrical line noise and suburban electronics noise I wouldn't go without them.

73,
Dan
 
RE: Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
I'll try to address several comments in one post.

Sam, NXET, said:

"I don't see the tripole thing. When you say the 3rd member, which is the foil it is not seperate from the wire braid. Therefor it is part of the shielding that makes up the wire braid part. If it had a dilectric between it then I could but it doesn't. Therefore, its as the manufacture says, improved-- shielding and becomes part of the wire braid that makes up the ouside of the coax. Yes the current goes on the outside too due to sphearical skin effect What goes up comes back in RF unless, you have one side of the coax grounded and no balun at the antenna feed point. If you do, then you only have an improved shielding for the coax... sorry"

Sam, this is a hard concept to get. The reason there are essentially two conductors on the shield is because of the skin effect. The skin depth on the shield is such that 99% of the current flows in less than 1/2 the thickness of the shield. As far as RF is concerned, this allows two independent currents to flow on the shield, one on the outside and one on the inside. They don't interact for two reasons. 1) The current on the inside of the shield is balanced by the current on the center conductor and 2) The shielding effect of the shield isolates the inside of the shield and the center conductor from the fields on the outside of the shield. That leaves the outside of the shield to act like an independent wire. This is not anything new. Maxwell and others figured this out in the late 1800s, but it has just been obscured by all the other information available on antennas.
---------------------------------------------------------

Vito, W6TH, said:

"3 times to the 8th power divided by 7 times to the 6th power equals 42.857 times 3.28 equals 140.57 feet.

Or does the EZNEC use the 468 divided by 7 equals 66.857 feet times 2 equals 133.714?"

Vito, EZNEC uses a finite element analysis approach. It breaks various lines into very small segments and calculates the accunulated effects of the currents and thus the fields caused by those currents. So it doesn't care about such equations as 468/f or 492/f. It takes into account the effects of the conductor characteristics such as size, material, and dielectric. It, also, takes into account various ground types in terms of ground conductivity and dielectric. It assumes the ground is perfectly flat and infinite. It will allow you to specify two different ground types, however. It, also, assumes the ground is homogenious and on the surface. It is not perfect, but it proves to be a pretty good model.

This type of analysis is also used for thermal and mechanical analysis.

-------------------------------------------------------

Mark, K8MHZ, said:

"I am with you. If there are three poles at any given time in the waveform they should be able to be named. In the electric field that would be positive, negative and ???? In the magnetic field we would have N, S and ???

I think this entire tripole thing comes from a lack of understanding of the term 'pole'.

A conductor is not a pole. At RF frequencies coax has three conductors, not three poles. Those conductors are illustrated in the drawing."

Mark, maybe I used the wrong terminology when I used Tripole. I was simply trying to convey three legs of an antenna with "tri" as opposed to two legs of an antenna with "di". I am sorry if that was misleading.
---------------------------------------------------------

73 Tom W0IVJ
 
RE: Dipole or Tripole?  
by W0IVJ on May 20, 2007 Mail this to a friend!
Lee, W9OY, said :

"do you take into account the velocity factor of the coax shield + vinyl ie 95% in the calculation of the antenna length?"

Lee:

Yes, one must calculate one leg of the dipole using the center conductor size, insulation, and insulation thickness; and the other leg of the dipole using the shield size, insulation, and insulation thickness. The leg of the dipole that uses the shield starts at the shield termination and ends at the trap. The leg of the dipole that uses the center conductor starts at the shield termination and ends at the end of the center conductor.

By the way, I always enjoy your contributions to the Flex reflector.

73 Tom W0IVJ
 
RE: Dipole or Tripole?  
by NXET on May 21, 2007 Mail this to a friend!
do you take into account the velocity factor of the coax shield + vinyl ie 95% in the calculation of the antenna length?

73 W9OY

No the velocity factor is a function of the cap and inductance within the cable and how fast it can move down it. The shield and vinyl have nothing to do with it most of the time.

Again the shield is considered part of the outer braiding and not a current carrying item. it is just what belden says it is a shield only.
 
RE: Dipole or Tripole?  
by NXET on May 21, 2007 Mail this to a friend!
Ok. lets look at it this way. As maximum would do

If you have one wire that is hooked to the hot side of the alternating current and to a light. From the light you have two wires in next to each other, then we would have the I of the single wire being equally shared by the two other wires at half current each.

That is the tripole concept.

But, now lets take one of the two return wires and make one of them a little foil that can't carry much current but is contenious. while the other wire is twice as big in current capablilty as the single wire. How much current is going to flow in the foil and how much in the larger wire running with it??

Now while true that the shield has some capacity with the braiding its in constant contact with the braid thus it acts like part of the shielding matrix and not a seperate conductor.

Why did they put it in the cable in the first place?

Well lets go up in frequency to the UHF region. Can we see a problem with the braids weave spacing and inter capactance/inductance being setup between the weave?

Yes indeed then how to cure it was easy. Just add a foil that is smooth, contenious but still flexable and can handle the skin effect much better-- leaving the braid to handle the current flow.

Problem solved for UHF 1296 and up coax cableing.
 
RE: Dipole or Tripole?  
by N3OX on May 21, 2007 Mail this to a friend!
Take a giant aluminum box. Inside the box, which is completely closed; sealed to the outside world, is you, a vertical antenna and a battery powered transceiver with one side connected to the antenna and the other connected to the inside wall of the box. So, inside the box are your ham station and your ground mounted vertical antenna.

First, call CQ. You transmit and RF current courses up and down your vertical and in and out of the "ground plane". This sets up an electromagnetic field inside the box, and also sets up standing waves of RF current on the inside walls of the box.

Does anyone outside the box hear your CQ? RF current is flowing on the inside walls of the box... does it flow on the outside walls too and allow people to hear you, or does the box behave like the good shield it is and block any electromagnetic energy inside the box from getting out?

Likewise, you listen for a response. RF energy is all around outside the box, inducing RF currents on the outside surface of the box. Do you ever hear this RF?

Remember... you're contained entirely within a perfect shield... is there any transmission that can get in or out?

Now, let's say someone comes by and installs a coaxial cable in the wall of your box... with the shield connected to the wall right where it enters, and run the center conductor out into your box some distance. They do the same with the other end of the cable in someone else's box.

Now you call CQ. You set up electromagnetic fields inside your box and they excite the exposed center conductor of the coax... which is, after all, just another ground plane antenna in your box.

The electromagnetic energy can travel through the coaxial cable and excite electromagnetic standing waves in the box at the other end of the cable, right?

Can anyone outside your two shielded boxes connected by a cable hear your CQ? Will you ever hear anyone besides your neighbor in the other box?

Will stations outside the box be able to communicate with you by applying a voltage between your boxes, driving a current on the outside of the shield of your coaxial cable? Would it really be called a shield if they could?

73,
Dan
 
RE: Dipole or Tripole?  
by W6TH on May 21, 2007 Mail this to a friend!
.
Wow Dan, lets get back to basics.

It is well known that the center of a halfwave antenna is approximately 73 ohms impedance. We feed the antenna with a 50 ohm impedance of coax line, which we now have a VSWR of 1.45::1...Now we prune the halfwave antenna for a VSWR reading of 1::1. There should be no rf or standing waves on the coax; should be a perfect match.

.:
 
FROM THE SHORES OF TRIPOLEE  
by K4JSR on May 21, 2007 Mail this to a friend!
Ever since I read the article, some years back, in "Scientific American" about the search for the magnetic unipole I have suffered from "Dipolar Bisorder". And now Tom tells me that we have a "Tripole". This means that my antenna has three faces like Eve! I guess this means "Lobes at first sight"! AAAAAAARGH! I will not attach any Freudian significance to any of this shall always remain Jung at heart. As such, I shall always be a fan of dipoles.
As you can tell from this posting I am a lot like my
antenna system---I just don't get out much anymore!!

73 from the world famous Forget-Me-Not Rest QTH and
Old Fart Farm.

PS. Thank goodness we live in a great country like ours--with means for checks and baluns! This leaves me all choked up!
 
RE: FROM THE SHORES OF TRIPOLEE  
by W0IVJ on May 21, 2007 Mail this to a friend!
Dan, N3OX:

You seem to have a pretty good grasp on EM theory. What do you think is the cause for the diminishing peaks in feed line current as the feed line is lengthened by integer 1/2 wave lengths? This is shown in Figure 4. As I said in the text, it is not due to loss in the wire, because I modeled it with a lossless wire and the peak current still diminshed.

73 Tom W0IVJ
 
RE: Diminshing Current with Length  
by W0IVJ on May 21, 2007 Mail this to a friend!
Dan, N3OX:

You seem to have a pretty good grasp on EM theory. What do you think is the cause for the diminishing peaks in feed line current as the feed line is lengthened by integer 1/2 wave lengths? This is shown in Figure 4. As I said in the text, it is not due to loss in the wire, because I modeled it with a lossless wire and the peak current still diminshed.

73 Tom W0IVJ
 
RE: Diminshing Current with Length  
by WA2TNO on May 21, 2007 Mail this to a friend!
Dan N3OX,

I read your comment on one shielded box and two shielded boxes connected by coax. In a perfect world, I know exactly what the answer should be. What cracks me up, working with shielded rooms, shielded boxes and the cables that come between them, is how ingenius people can be in messing up the most simple shielding topology. I can only assume that you've been witness to the same kinds of foul ups that I've witnessed!!!

Be of good cheer...

Bruce
WA2TNO
 
RE: Diminshing Current with Length  
by W6TH on May 21, 2007 Mail this to a friend!
.
W0IVJ
You seem to have a pretty good grasp on EM theory. What do you think is the cause for the diminishing peaks in feed line current as the feed line is lengthened by integer 1/2 wave lengths? This is shown in Figure 4. As I said in the text, it is not due to loss in the wire, because I modeled it with a lossless wire and the peak current still diminshed.


Tom,

Read the halfwave lengths according to the theory of a long wire antenna....The first halfwave is resonant and the second added halfwave is no longer resonant so the wire will have to be pruned to bring back to resonance.

.:
 
RE: Diminshing Current with Length  
by W6TH on May 21, 2007 Mail this to a friend!
.
Tom,

Here is the formula to calculate for the two halfwaves, this should explain the length of the added halfwave wires.


............L in feet=492 (n-0.015/F Mhz..............

The "n" is the number of halfwaves.

.....(2-0.015)=1.985 X 492 =976.92/14 =69.758 Feet.....

.:
 
RE: Diminshing Current with Length  
by W0IVJ on May 21, 2007 Mail this to a friend!
Vito, W6TH, said:

"Tom,

Read the halfwave lengths according to the theory of a long wire antenna....The first halfwave is resonant and the second added halfwave is no longer resonant so the wire will have to be pruned to bring back to resonance.

Tom,

Here is the formula to calculate for the two halfwaves, this should explain the length of the added halfwave wires.


............L in feet=492 (n-0.015/F Mhz..............

The "n" is the number of halfwaves.

.....(2-0.015)=1.985 X 492 =976.92/14 =69.758 Feet....."


Vito,

I don't think this is a length issue since Figure 4 has data points 1/8 wl either side of 1 wl. It is something else, but I don't know what. Anyway, thank you for thinking about it and posting.

Tom
 
RE: Diminshing Current with Length  
by N3OX on May 21, 2007 Mail this to a friend!
W0IVJ asks: "What do you think is the cause for the diminishing peaks in feed line current as the feed line is lengthened by integer 1/2 wave lengths?"

Off the cuff, I'm going to say that the impedance at the antenna end of the shield at will depend on the exact details of the radiation from the line.

Compare the feed resistance of a 7/2 wave doublet to a 1/2 wave dipole... I think the long doublet will show something in the 150-ish ohms range, even though it's fed at a current maximum.

The radiation resistance of the 7/2 wave antenna is different from that of the 1/2 wave antenna... and so is the radiation resistance of a 1/2 wave shield different than that of a 7/2 wave shield.

WA2TNO says: "What cracks me up, working with shielded rooms, shielded boxes and the cables that come between them, is how ingenius people can be in messing up the most simple shielding topology"

I've spent a lot of time thinking about shield topology recently.

Yeah, it's especially bad when it's the manufacturer of a $3000 data acquisition card you're trying to use in a room with two variable frequency drives that chop at 10kHz and hit about 8kW output apiece.

And I'm building an experiment that will use 250kW motors instead... I've got some shielding ahead of me.

73,
Dan
 
RE: Diminshing Current with Length  
by W0IVJ on May 21, 2007 Mail this to a friend!
Thanks, Dan. I believe that is it. As the feed line gets longer in terms of integer 1/2 wave lengths, the impedance at the feed point goes up and consequently the current is smaller. That makes sense.

73 Tom W0IVJ
 
RE: Dipole or Tripole?  
by K8MHZ on May 21, 2007 Mail this to a friend!
"Mark, maybe I used the wrong terminology when I used Tripole. I was simply trying to convey three legs of an antenna with "tri" as opposed to two legs of an antenna with "di". I am sorry if that was misleading."

Thank you Tom.

I did not mean to distract from the work you put into the article which to me seems to be a fantastic amount. Thanks much for your efforts and for adding a great article to eHam.

Now since so many people here insist on the existance of a three pole antenna it must be true. Now with that I will set about to design a three pole single phase electric motor and it won't be like the one described in patent number 4,185,217 which really is a 2 pole phase shifted motor, usually called a capacitor start / capacitor run motor. (A condensor is the same thing as a capacitor).

In order for there to be a pole, there must be an opposite pole. That means there they must exist in pairs. There can be 2 pole things and 4 pole things but not 3 pole things. To me that is quite logical.

Thanks again Tom for your work.

73

Mark K8MHZ

 
RE: Dipole or Tripole?  
by N3OX on May 22, 2007 Mail this to a friend!
Mark,

What about a three phase motor run off of three phase voltage? None of those phases actually sum to zero with another *one* of the phases.

And, by analogy, what about a three phase transmitter (yes, you could build such a thing) feeding three 1/4 wave wires spaced 120 degrees apart using a three-conductor transmission line. That would certainly be a tripole. Three phases 120 degrees apart, three legs 120 degrees apart... the current in each leg will be 120 degrees out of phase with the current in the other two; never will it exactly oppose in two legs... so it's a true tripole.

If you had a two-phase transmitter (0 and 90 degrees) you could feed circularly polarized crossed yagi antennas with equal length feedlines directly.

So take two crossed dipoles. Feed them 90 degrees out of phase using a two phase 0/90 transmitter. You have two "two pole" things... one is maximum at 0 and 180 ... the other is maximum at 90 and 270... maybe you'd consider this a four-pole antenna.

Two dipoles, two pairs of "opposing poles".

Now, instead of a two phase transmitter, use a coax feed to force 0/90 currents.

Two dipoles, two pairs of "opposing poles", yet it's being fed from single-phase RF!!

You can't really tell the difference between the two just by looking at the antenna currents.

The same would go if you took the true tripole I mentioned first... the three legged one being fed by three-phase RF from a three-phase transmitter, and arranged for the currents to do the same thing with phasing networks. You really can't tell the difference if you get 120 degree apart currents flowing in those legs.

There's a difference between electrostatic and magnetostatic poles and motor and antenna "poles" based on phase relationships between currents driving them.

I'm not sure which one the dipole is ACTUALLY named after... maybe you wouldn't want to call the three-legged three-phase fed antenna a tripole, but I sure want to.

Dan
 
RE: Diminshing Current with Length  
by W9OY on May 22, 2007 Mail this to a friend!
As to diminishing current on increasing 1/2 wavelengths, how is common mode induced in the first place? Why does it distort pattern?

73 W9OY
 
RE: Diminshing Current with Length  
by K8KAS on May 22, 2007 Mail this to a friend!
You boys have to relax, you have gone over the edge. You all need to spend more time on the antenna range looking at far field results. A tenth of a db or less for the most part is not worth talking about.

This is Ham Radio and not RCA selling something to a broadcaster.
 
RE: Diminshing Current with Length  
by N3OX on May 22, 2007 Mail this to a friend!
"how is common mode induced in the first place? Why does it distort pattern? "

If you look (in EZNEC, probably) at the common mode impedance of a 1/2 wave grounded shield on a dipole by connecting a source in the shield wire instead of one of the dipole wires, you'll see that the impedance is something like 12 ohms resistive and that's connected directly to your antenna with an impedance of 50-60-70 ohms.

There's actually more current on the feedline than on the shield leg of the dipole in this case.

So you've got a big funny asymmetric T antenna instead of a dipole. Some people might not notice... if you never rotate your dipole, you won't notice the pattern change from all that radiation from the feedline... it might even be useful...

However, if you stick a little "noise" generator in the feedline down near ground level... well, that's why I never go without a choke balun anymore. I'm immersed in noise... the baluns actually help... which brings us to K8KAS's comment:

"You all need to spend more time on the antenna range looking at far field results. A tenth of a db or less for the most part is not worth talking about."

Hard to set up a valid HF antenna range in a suburban backyard. What about the fact that adding a 9 inch string of #43 beads to my 15m dipole dropped the noise level 2 S-units. Is that a tenth of a dB effect? I've measured my S-meter sensitivity with a step attenuator. 2 S-units in that particular range is about 9, maybe 10dB.

Having more common mode current on your feedline than you have current in the shield leg of your dipole is not a tenth of a dB effect.

"This is Ham Radio and not RCA selling something to a broadcaster."

Hmm... guess I should never have worried about my 15m dipole in the first place. I guess ham radio is all about only hearing signals that are stronger than S7 instead of stronger than S5...

73,
Dan
 
RE: Diminshing Current with Length  
by K8MHZ on May 22, 2007 Mail this to a friend!
"What about a three phase motor run off of three phase voltage? None of those phases actually sum to zero with another *one* of the phases."

Dan,

Phases are not poles. A pair of poles accompanies each phase, however. Those pole pairs are 120 degrees out of phase with each other in a three phase motor or generator.

Simplified, a phase is an occurrence whereas a pole is either an attraction or a repulsion. Phases can exist in any number. Poles only exist in pairs. Consider this, a pole can be out of phase but a phase can't be out of poles! :)

73

Mark K8MHZ
 
RE: Diminshing Current with Length  
by N3OX on May 22, 2007 Mail this to a friend!
"Poles only exist in pairs"

and, earlier,

"In the electric field that would be positive, negative and ????"


What's the electric field of a single electron alone in space?

Dan
 
RE: Diminshing Current with Length  
by K8MHZ on May 22, 2007 Mail this to a friend!
Hi Dan (again)

I think you are not realizing that a three phase motor, transmitter or generator has six poles, not three.

'Three phase' is just three individual single phase forces applied at equal intervals so the intervals overlap in a synchronous manner. Since each single phase presents a pair of poles, three phases present six poles, in the magnetic field that would be three Norths and three Souths alternating and occurring 60 degrees apart. (Like poles would be 120 degrees apart, or out of phase).

I think this explains it better than my previous attempt.
 
RE: Diminshing Current with Length  
by K8MHZ on May 22, 2007 Mail this to a friend!
"What's the electric field of a single electron alone in space?"

Lonely. :)

The only way an electron could be alone in space would be for it to be the only piece of matter left in existence. Since there would be no proton to act as a counterpoise there would be no field and no charge and no poles because all matter in the universe (that single electron) would be at the same potential.

When a proton came into existence the electron would no longer be alone in space and would resume it's normal charge, the proton would become positive and we would go from 0 poles to 2 poles in the most minute manner possible.

Does that work as an explanation?

 
RE: Diminshing Current with Length  
by N3OX on May 22, 2007 Mail this to a friend!
"Does that work as an explanation? "

Nope. The electric field of a single electron is directed radially outward from itself and falls off like 1/r^2. This way, the total energy in the field is finite and the field falls to zero at infinity.

The electron can exist alone in an infinite universe without breaking any laws of physics. It doesn't need a counterpart.

If you jiggle the lonely electron back and forth it will even emit electromagnetic energy. One electron... no counterpart at all.

In metals, it's the electrons that are the free charge carriers. There aren't any moving positive charges in your dipole antenna... the positive pole is merely the one depleted of electrons at that instant. It's not a terrible picture to imagine a gas of electrons sloshing over to one half of your dipole and back over to the other half every RF cycle.

You don't actually need the metallic crystal structure to make an antenna if you had some other way to grab onto and wiggle a bunch of electrons. You couldn't just do it with a gas of electrons, of course, because they'd all want to scatter.

There are no protons or other positively charged particles moving in a dipole antenna... just a bunch of electrons in pretty good lock step with each other.

So, we can go back and forth over whether or not "dipole" has something to do in particular with one leg being positively charged at some instant because of the overall charge distribution on the antenna ... but really, a dipole antenna relies on a particular phase relationship of current between the legs.

An interesting page I came across showing current distribution in various monopoles (uhoh... maybe a dirty word)

http://www.lago.demon.co.uk/PAGE7.HTM

73,
Dan
 
Dipole or Tripole?  
by KB9YGD on May 24, 2007 Mail this to a friend!
One of the best articles i have ever read and then reread and saved.Thank You Tom.73,Norm
 
RE: Update on End-Connected, Center-Fed Dipole  
by W0IVJ on May 26, 2007 Mail this to a friend!


Since I wrote this article, Ed, N9MW, has affectionately named the end-connected, center-fed dipole the SED for skin effect dipole. I was able to do an A/B comparison between the SED and a conventional dipole from Vermont to Wisconsin on 20 meters. There was no difference between the two antennas, so it looks like the SED might indeed be a viable portable antenna for a balcony or a situation where one of the antenna supports is at the transmitter.

I would like to thank all of you who took the time to read the article and comment. I think an open forum like EHAM is an excellent place to learn.

73, Tom W0IVJ
 
RE: Update on End-Connected, Center-Fed Dipole  
by VR2UIX on May 29, 2007 Mail this to a friend!
Very good article, great discussion. I've built the 2 meters band version and it works fine.

http://hk.myblog.yahoo.com/oliver-li/article?mid=566&prev=-1&next=560

VY 73, Oliver VR2UIX
 
RE: Update on End-Connected, Center-Fed Dipole  
by W0IVJ on May 31, 2007 Mail this to a friend!
Oliver,

I saw your 2 meter antenna. Nice work! One question...did you place a capacitor across the coil from shield to shield or just wind the coax to form an inductor?

73, Tom W0IVJ
 
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