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Very Basic Primer
Alan Applegate (K0BG)
on
May 4, 2008
View comments about this article!
Todays amateur radio operators are a lucky bunch for a variety of reasons. For example, today's transceivers are far superior to those of yesteryear. Selectivity and sensitivity have improved, while the cost of ownership has decreased in comparison. And, although basic antenna design hasn't changed all that much, new innovations like the SteppIR, have brought them into the 21st century. Even our shack tools have improved.
When I first became an amateur, there were two devices one almost couldn't do without. One was a VTVM (vacuum tube voltmeter), and the other one was an SWR bridge. I don't remember what I paid for the Heathkit VTVM I purchased in 1970, but at the time I thought the price was outrageous. Fact is, I still have it, and it still works! My first SWR bridge, along with about 5 others, has come and gone. Nowadays, there is a new, easily affordable device that's becoming as ubiquitous as an SWR bridge, and that device is an antenna analyzer.
Allow me to digress. An SWR bridge gives you the ratio of forward to reflected voltage, and provides a mechanical way of displaying it. What they can't display is the complex impedance of the device under test; an antenna for example. An antenna analyzer can do just that (and a lot more), if you know how to use it. Let's illustrate it this way.
Say your SWR bridge readout tells you your antenna SWR is 1.25:1; a very acceptable figure. Assuming the antenna is at its true resonant point, the actual resistive portion of the impedance could be either 40 ohms, or 62.5 ohms. While the actual value isn't all that important, it can become important when you're trying to set up the antenna in the first place.
As an example, one of the easiest methodologies to match a remotely-tuned mobile antenna is with a simple, shunt, base coil like the one shown in the photo. Properly adjusted, the coil, and a small amount of capacitance borrowed from the antenna (the antenna is adjust slightly high in frequency), form an LC matching network. This network transforms the nominal 25 ohms impedance of the antenna to the requisite 50 ohms needed by our transceivers. There are many other examples to be sure, but this one best illustrates the issue at hand; properly reading the resonant point with an antenna analyzer.
An antenna analyzer can do a lot more than just measure the complex impedance of a mobile antenna. You can use one to test ferrite beads, adjust antenna traps, measure coax loss, and a whole lot more. But for this basic exercise, we're going to limit ourselves to just one, very important point. And that is, how can you tell when your antenna (or network) is resonant?
For some unknown reason, all too often the consensus of opinion is, that resonance is always at the same point as the lowest SWR. Nothing could be further from the truth! An antenna's resonant point is when the reactive component is neither inductive or capacitive (X=Ø). The resistive component is really immaterial at this point; it is what it is.
Just for the record, I don't have any pecuniary interests in any amateur radio related company. While there are several, inexpensive antenna analyzers on the market, the one I own is an MFJ-259B. Therefore, it's easy for me to take the necessary photos to illustrate the point I'm trying to make here. That is not to say, that other manufacturers units can't do these measurements. They can, albeit the methodology might be different.
Referring to the photo, note that the reactive component (X=Ø), and the resistive component (R=23). This is an actual readout of my mobile antenna, without any matching device(s). Note the SWR is 2.1:1.
Referring to the bottom photo, note that the reactive component (X=4), and the resistive component (R=46). (The reason X has a small value is due to stray capacitance from my body as I was taking the photo). In both cases the antenna is at it's resonant point, yet the SWR reading is different. This is the actual readout of my mobile antenna after the shunt, base coil was installed.
It is a little hard to illustrate by using photos, but in the first example, if you were to increase the frequency on the 259B slightly upwards, you would notice that the apparent SWR will decrease, but the reactive component will increase (X=+j). This is basically what happens when you're using an SWR bridge. In other words, you don't know the actual resonant point unless you use an analyzer.
As I alluded to previously, the actual resonant point isn't all that important, unless you're trying to adjust a shunt, base coil, or perhaps an antenna trap, or any of a myriad of other antenna measurements. The point that needs to be driven home here is, the only time an SWR readout is reasonably accurate, is when the antenna in question is resonant, and that's when X=Ø!
If you want to know how to properly perform all of the things an antenna analyzer can do, and how to determine whether the reactive readout (X) is inductive (+j), or capacitive (-j), download the User's Manual for the MFJ-259B here.
Alan, KØBG
http://www.k0bg.com
This article has expired. No more comments may be added.
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Very Basic Primer
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by N5EAT on May 4, 2008
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I'm at work so I can't test this. Will your transceiver output go up when x=0 and swr=2.1 if you adjust the antenna so the swr=1:1: or does the transceiver like the reactance at zero best?
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RE: Very Basic Primer
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by M0JHA on May 4, 2008
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For some unknown reason, all too often the consensus of opinion is, that resonance is always at the same point as the lowest SWR. Nothing could be further from the truth! An antenna's resonant point is when the reactive component is neither inductive or capacitive (X=Ø). The resistive component is really immaterial at this point; it is what it is.
//////////////////////////////////////////////////
i agree , lots of people especially those who only ever buy cheap commercial antennas seem to think 1.1 swr is resonance.
billy. uk
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RE: Very Basic Primer
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by NV2A on May 4, 2008
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Alan, thanks for a great article. You've made the point very well. I'll be printing off this article and keeping it with my 259B. Now to re-check everything!
73's
Ray......
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RE: Very Basic Primer
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by W8JI on May 4, 2008
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I've got a little bad news for people.
Lowest reactance, no matter what the fixed resistance value, is always lowest SWR.
For a given resistance lowest SWR is always lowest reactance so if the SWR crosses minimum without a resistance change it is resonance.
There is NO combination of reactance in addition to resistance that has a lower SWR than zero reactance.
For example if I have a 30 ohm resistor and I measure SWR while adding rectance in series, the lowest SWR is always with zero ohms reactance. Always. There is never a combination of additional reactance added to a pure resistance that will give a 1:1 SWR.
There is one thing that can seem to violate this rule. If the resistance of the antenna is changing at a much faster rate than the reactance is changing with frequency, lowest SWR might not be at resonance.
This is because the resistance value is changing faster than the reactance value with frequency. I haven't thought about how common it is in practical antennas, but it would be very rare in large common antennas that come to mind at the moment. All the antennas I can think of at the moment change resistance slower than they change reactance, so lowest SWR would be resonance.
73 Tom
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RE: Very Basic Primer
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by N2UGB on May 4, 2008
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Billy, this is a serious comment. With one pound worth two dollars, join the other Brits on a BA shopping spree to the States. Fill an empty suitcase wth ham gear purchased over here.
Or are the VAT enforcers getting tougher.
Still FT-817 QRP?
73
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RE: Very Basic Primer
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by W6TH on May 4, 2008
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.
At the antenna point when XL=XC=0 then what is left, the radiation resistance?
Of course this is taken at a certain height, but raising or lowering the antenna, will cause a change because of the coupling coefficient,antenna above ground.
.:
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Very Basic Primer
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by N2DIG on May 4, 2008
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This article made me recheck my antenna.
Tom said:
"I've got a little bad news for people.
Lowest reactance, no matter what the fixed resistance value, is always lowest SWR."
I got some different results with my 40 Meter inverted V.
MHz---SWR---X---R
2.73----5.8-----0-----8
7.07----1.0-----4-----50
8.77----5.3-----0-----9
14.05---5.8-----0-----8
16.74---5.8-----325---0
19.41---4.3-----11----0
21.62---1.3-----13----61
21.46---1.4-----8-----70
23.50---3.5-----52----73
25.39---3.4-----0-----15
27.93 thru
28.20---3.0-----0-----161
31.12---3.6-----0-----13
33.60---3.2-----0-----195
50.73---1.3-----0-----66
100.8 1.1-----0-----58
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RE: Very Basic Primer
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by M0JHA on May 4, 2008
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quote "Lowest reactance, no matter what the fixed resistance value, is always lowest SWR"
/////////////////////////////////////////////
?????? but the lowest swr at resonance may be no good for a 50 ohm input of a solid state radio...
billy uk.
still qrp..
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RE: Very Basic Primer
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by K3AN on May 4, 2008
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N2DIG, can I presume you were making those impedance measurements right at the antenna terminals, and not at the shack end of your feedline?
I'm guessing that's not the case.
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Very Basic Primer
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by N2DIG on May 4, 2008
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K3AN,
I undestand about the feedline issue, I was only commenting about Tom's statement - That Lowest X is not always lowest SWR.
George
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RE: Very Basic Primer
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by W4VR on May 4, 2008
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Obtaining the feedpoint R and X values is easy to do on a mobile antenna or ground-mounted vertical. On a dipole or elevated antenna it's a little more complicated. You can do that either with a 1/2 wave piece of coax and use an MFJ or Autek impedance meter at the other end, or get one of the newer Auteks and plug in your transmission line length if you know what it is. As for me, I have all those fancy instruments but always do it the quick and dirty way on horizontal antennas, cut the antenna for the lowest SWR which is close enough for government work. For vertical antennas the feedpoint R is so low in most cases that a feedpoint measurement is necessary to design an appropriate matching network.
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RE: Very Basic Primer
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by W9PMZ on May 4, 2008
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"Say your SWR bridge readout tells you your antenna SWR is 1.25:1; a very acceptable figure. Assuming the antenna is at its true resonant point, the actual resistive portion of the impedance could be either 40 ohms, or 62.5 ohms. While the actual value isn't all that important, it can become important when you're trying to set up the antenna in the first place."
Alan,
This is a very good example of measurement uncertainty in RF power measurement. Actually, with a SWR of 1.25:1 this creates an area that is a circle at the end of the real power. The 40 ohms or 62.5 ohms is complex. So if you can imagine the real power as a vector, the SWR sets up a circle about the end point where a device will measured the power.
What this means, is that without unity SWR, to measure the real power, you have to know the return loss (SWR) and its associated phase angle.
Assuming that the phase angle is 0 degrees then return loss will create an error in the measurement by adding the error to the real power; and conversly if the phase angle is 180 degrees then the return loss will create an error in the measurement by subtracting the error to the real power.
Uncertainity is not to be confused with accuracy. They are seperate factors that have to be considered when you make a RF power measruement.
73,
Carl - W9PMZ
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RE: Very Basic Primer
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by AB7E on May 4, 2008
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N2DIG, you should know something is not accurate with your readings when a totally reactive load (zero R) gives you SWRs as low as approximately 5:1.
Also, I know of no 40m inverted-V on this earth that would give zero reactance at so many non-harmonically related different frequencies, including one at 2.73 MHz.
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RE: Very Basic Primer
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by N6AJR on May 4, 2008
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As a point of interest here is a little something to play with on the subject of resonance. You will need a mfj 259 and a field strenf=ght meter, and a couple of antennas.
You will need to use a mfj 259 antenna analyzer ( or equivlent), and a couple of antennas in the same range but different frequencies, like a 2 meter antenna and an air band antenna.
First put on the 2 meter antenna and tune the analyzer from the low end to the hi end of the 114 to 170 position on the 259b.
If you watch the field strength meter it will be at or near the zero mark through most of the sweep. When you get int the 144 to 148 range you will see the FSM show an increase then a decrease in power as you go through the 2 m antenna's resonance range.
From 114 to about 144 mhz the Field AStrength Meter will be at the low end. It will jump up through the 144-148 mzh zone and then drop low from 148 or so till the 174 mark.
Now put on the air band antenna. Do the same sweep with the 259b. You will see the field strength meter now run up from about 118 to about 122 mhz and be on the bottom on all other frequencies. This is when the air band antenna is resonant.
When the antenna is in its resonant portion it "couples more power to the aether" . The whole time the mfj 259b is putting out the same couple of miliwatts across the band.
As you enter the resonant " sweet spot" of the antenna it transmits you best signal. You have the best eficiency in transfering power to the air waves.
This is one reason why a Steppir antenna is so good, because it can be tuned for that " sweet spot" on every part of every band in its range.
So resonance and swr are not every thing, but they do make a difference.
Just my opinion, you're milage may vary... :)
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RE: Very Basic Primer
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by K0BG on May 4, 2008
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The point of all of this is simply that too many folks don't know how to properly use a 259B, and in this case using it to adjust a base loading coil on a mobile antenna. In this case, the SWR readingdoesn't mean much, but knowing the R value at resonance (X=Ø) does.
There is obviously more to the story, and this is why I said it was very basic.
Alan, KØBG
www.k0bg.com
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RE: Very Basic Primer
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by W8JI on May 4, 2008
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George,
When you measure at the end of a transmission line you add another set of complex variables to the system.
It is however an absolute that lowest SWR always occurs at the crossing point of zero reactance IF the resistance is not changing faster than the reactance with frequency.
There are several interesting things that happen when the feedline is added into the system. One very interesting thing is for a certain set of feedline characteristics reactance can be either positive *or* negative signs both above and below the lowest SWR frequency!! This screws up some analyzers that look at reactance change with frequency variation to determine the sign of reactance.
Also meters are not perfect. As little as one bit of error out of 255 bytes in a MFJ-259 can cause a reactance to show when there isn't one.
If I have a transmission line on a dipole, even a half wave line, it can skew the results and make you think the antenna is resonant where it is not resonant. This is because the half wave line is ONLY a half wave at one frequency!
It's best we don't worry too much about things like this unless we really want to understand what is happening.
As long as our radio is happy and we are happy that's most of what matters to most of us. It is a fact however that there is no amount of additional reactance that will reduce SWR into any value of resistance. The lowest SWR of a ten ohm load resistance is when the series reactance is zero, and the lowest SWR of a 1000 ohm resistance is again when series reactance is zero.
73 Tom
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RE: Very Basic Primer
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by N4KC on May 4, 2008
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N6AJR, I'd be interested in seeing your experiment repeated at HF using one antenna, first fed with very low loss transmission line and again fed with coaxial cable.
Don N4KC
www.n4kc.com
www.n4kc.blogspot.com
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RE: Very Basic Primer
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by AA4PB on May 4, 2008
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AJR: I submit that you are seeing the increase because at resonance the reactance is zero, the SWR is the lowest, and more power is coupled from the feed line into the antenna. It is not because a resonant antenna inherently "couples more power to the aether".
If you design the proper impedance match between the coax and the antenna, a non-resonant antenna will radiate as much power as a resonant antenna. There is nothing magic about a physically resonant element length.
If I could figure a lossless way to match a 10-foot wire on 75M it would radiate the same amount of power as my 120 foot dipole.
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RE: Very Basic Primer
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by W4LGH on May 4, 2008
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AA4PB said..."If I could figure a lossless way to match a 10-foot wire on 75M it would radiate the same amount of power as my 120 foot dipole."
What kind of statement is that? IF, LOSSLESS? I mean come on..If I could figure out how to make perpetiual
motion, I could solve the worlds energy problems, and build my own City of Duhbi, here in the Gulf of Mexico. You can make any length of wire radiate at any frequency, but at what loss? Hell an antenna cut for resonance of a given frequency is already at a loss!
Just like the argument in a previous post, an antenna can NOT have gain! You can design it to look like gain in a favored direction, but what the additional loss in the non-favored direction? I bet enough to average the overall of the antenna to be back at 0.
I like my antenna to be resoante at the center of the bands I use, and a couple that I have cut for certain operating frequencies. Enjoy...
73 de W4LGH - Alan
http://www.w4lgh.com
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RE: Very Basic Primer
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by N6AJR on May 4, 2008
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Like I said, YMMV. I know that when I do that with a Field Strength Meter, it reads high where the antenna is in resonance, the design point , or what ever is written on the tag. The FSM only reads radiated power from same said antenna, . I do not perport to know or understand why.
And go ahead and do your experiment on HF and let us know the results. I use a variety of antennas that work, some better than others, and promote the "fan Dipole" because it is a cheap, and effective way to get on HF for not much $$$.
Many other antennas work better, but a lot work worse too. I use mostly coax fed antenbnas because I physically can't climb any more, so it is easier for the folks who do my antenna work to run coax rather than route twin lead. I find the twin lead more efficient, but the coax works and is useable in my situation.
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RE: Very Basic Primer
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by N4JTE on May 4, 2008
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As usual very good info from K0BG but I must take issue with the swr/ resonance not being a valid measurement. Perhaps on a mobile setup the MFJ is golden but for backyard wire dipoles and wire beams I would humbly suggest sticking to the swr meter on your radio to find the best length/ efficency for an antenna, taking into account the whole circuit, feedline length ,etc, is extremely important to getting the most from the antenna. I use the MFJ at the antenna to verify resonant physical length of my wires, hopefully taking into account the nearby factors that will effect resonance, after that I go back to the radio and check swr, if it is a dipole I will expect about 1.2 to 1, if I am experimenting with a wire beam I know my swr is gonna be higher but I take comfort in knowing that my orginal driven element was optimixed.
If you are trying to get the most from your one element antenna, adjust length for lowest swr and you will be heard as best as possible.
Bob N4JTE
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RE: Very Basic Primer
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by N2XE on May 5, 2008
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"I've got a little bad news for people.
Lowest reactance, no matter what the fixed resistance value, is always lowest SWR."
If the radiation resistance is greater than the transmission line impedance, that is true. If the radiation resistance is less than the transmission line impedance it is not.
Short vertical antennas are a good example of best SWR not being at resonance.
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RE: Very Basic Primer
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by N2XE on May 5, 2008
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Actually, that should state feedpoint resistance--i.e. radiation resistance plus losses. In the case of a short monopole vertical, ground loss is usually huge.
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RE: Very Basic Primer
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by WA5UHK on May 5, 2008
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My first instrument was an swr bridge too but my second one was a grid dip meter. It was all about resonance. I even found the resonance of the clothsline at my apartments but never had the nerve to try and feed & match it with my DX-20.
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RE: Very Basic Primer
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by W8JI on May 5, 2008
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"I've got a little bad news for people.
Lowest reactance, no matter what the fixed resistance value, is always lowest SWR."
If the radiation resistance is greater than the transmission line impedance, that is true. If the radiation resistance is less than the transmission line impedance it is not. >>>
There is NO series reactance that reduces SWR over the resistance alone.
If we have a short vertical with a series loading coil lowest SWR is at resonance. That's true if the resonant resistance is 10 ohms or 100 ohms.
73 Tom
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RE: Very Basic Primer
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by G3LBS on May 5, 2008
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Isn't the obsession with antenna resonance and SWR at antenna only important if you are using coax?
W2/G3LBS
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RE: Very Basic Primer
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by W5FYI on May 5, 2008
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John, N2XE, wrote, "If the radiation resistance is greater than the transmission line impedance, that is true [that lowest SWR occurs at resonance]. If the radiation resistance is less than the transmission line impedance it is not."
It's true in either case. A 10 ohm ± j0 in a 50 ohm cable has an SWR of 5.000:1. Adding ± j1 brings it up to 5.002:1; adding ± j5 brings it up even more, to 5.052:1; etc. Any additional reactance always increases the SWR no matter what the resistive component or line characteristic impedance is. Work the math.If you can come up with a specific example, I'll crunch the numbers to verify it for you.
He also wrote that ground losses become "really huge" for short vertical antennas. I can't agree. All references I have seen point to consistent ground resistances and losses. If, for instance, we started with a perfect ground with a normal vertical, what would change its "perfectness" if placed under a short vertical?
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Very Basic Primer
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by KC8ZEV on May 5, 2008
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Anyone ever compare the Palstar ZM30 to the MFJ 259B??? Always worry about MFJ quality control. Nice article explaining the what and the why behind tweaking an antenna.
73
KC8ZEV
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RE: Very Basic Primer
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by W9OY on May 5, 2008
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I'm rather enjoying the irony of the title of this article vs the subsequent responses.
73 W9OY
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RE: Very Basic Primer
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by N4CQR on May 5, 2008
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QUOTE: Anyone ever compare the Palstar ZM30 to the MFJ 259B??? Always worry about MFJ quality control.
Palstar ZM30 Reviews:
http://www.eham.net/reviews/detail/4782
Average rating: 4.8/5 form 37 reviews
MFJ 259B Reviews:
http://www.eham.net/reviews/detail/13
Average rating: 4.1/5 from 138 reviews
Craig N4CQR
Berea.KY.USA.Earth
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RE: Very Basic Primer
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by W8JI on May 5, 2008
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Some of what is attibuted to antenna behavior is actually bridge behavior or slight calibration issues.
This is a problem with ANY bridge, even $20,000 bridges.
As little as a single error bit out of 255 bits of data (a fraction of a percent) can result in a few ohms reactance showing when there isn't any.
We just have to live with that. It doesn't make any difference in performance.
73 Tom
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RE: Very Basic Primer
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by W9PMZ on May 5, 2008
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"QUOTE: Anyone ever compare the Palstar ZM30 to the MFJ 259B??? Always worry about MFJ quality control"
I wonder how many more MFJ-259Bs have been sold over the Palstar and out of that population the MFJ-259Bs were sold to users who didn't understand how to use it and then claimed "quality problems". I really can't understand this.
I have not had a MFJ product yet to work per the instruction manual and specification.
I've had my MFJ-259B now for 2 years with no issues.
73,
Carl - W9PMZ
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RE: Very Basic Primer
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by G3RZP on May 5, 2008
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AA4PB said:
>If you design the proper impedance match between the coax and the antenna, a non-resonant antenna will radiate as much power as a resonant antenna. There is nothing magic about a physically resonant element length<
A lossless Hertzian dipole (i.e. less than 0.05 wavelength long)is fractions of a dB down in comparison with a half wave dipole - all other things being equal. The problem is matching circuit losses. So a 100 foot long dipole fed with open wire feeder and a good tuner is going to be as effective on 3.5 MHz as a 132 foot resonant dipole fed with coax.
If antennas have to be resonant to be effective, how do terminated rhombics work?
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RE: Very Basic Primer
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by N3OX on May 5, 2008
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"I'm rather enjoying the irony of the title of this article vs the subsequent responses. "
That's because it's not a simple subject ;-)
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RE: Very Basic Primer
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by N4KC on May 5, 2008
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Lord help us! The dreaded "resonate" vs. non-"resonate" antenna brouhaha has reared its ugly head again.
Please, though, do everyone a favor and remember that it is a "resonant" antenna -- or even better, a "resonant" antenna SYSTEM, since we have yet to figure a way to get RF from the transmitter to the antenna in the sky without some kind of transmission line. Or how to maintain resonance (without a matching device) across a score of very wide ham bands. Or how to totally remove other minor factors that have an effect on resonance -- like dirt and trees and the wife's clothesline.
Don N4KC
www.n4kc.com
www.n4kc.blogspot.com
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RE: Very Basic Primer
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by VA3PEN on May 5, 2008
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I prefer using a grid dip meter to using an antenna analyzer when resonating a new antenna, and worry about the SWR later (e.g. I could use an antenna tuner to lower the SWR if it is higher than 2:1).
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Very Basic Primer
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by KC8ZEV on May 5, 2008
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Soldering jobs worse than a two year old would do, loose screws rattling around inside, missing hardware........these are some of the MFJ quality control issues I am refering to, not lack of product knowledge. My experience with MFJ products has been both ends of the spectrum. Either it works fine or it has to go to the bench before it can be used. I have already read the reviews on both the Palstar and MFJ, was hoping someone here has used both and has a opinion. The Palstar is more $$$ for sure. Question is .......is there any performance difference?? An antenna analyzer is a useful piece of equipment, without a doubt.
73
KC8ZEV
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Very Basic Primer
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by KE4ZHN on May 5, 2008
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With all this hoopla one must never forget one important fact. There will ALWAYS be loss in ALL antenna systems regardless of what feed line you use or what antenna it is. There is no such thing as the perfect antenna and there never will be unless some scientific breakthrough happens. The name of the game is maximum efficiency. Couple the most of your available transmitter power to the antenna so it can be radiated. Just because the antenna may not be perfectly resonant that doesnt mean it wont work. If its fed properly or in an efficient manner, it will radiate a signal. Of course there are limits to this, but swr is one of the most overblown worries in the amateur world. Just one more reason I love my balanced line so much. SWR? Who cares!
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RE: Very Basic Primer
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by WI7B on May 5, 2008
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"Antenna Tuner" => a device that physically changes the length of your antenna element(s) to a specific resonant frequency.
"Transmatch" => matches impedance into your tranmission line from your transmitter.
"Antenna Coupler" => matches impendance into your antenna from your transmission line.
All three have a different function. Not so basic or simple.
73,
---* Ken
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RE: Very Basic Primer
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by K7PEH on May 5, 2008
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One of the rather surprising aspects of ham radio is how much things actually do work even if you have hardly any knowledge of what makes them work. Or, even when you are naive about some of the basic requirements of a good antenna system.
When I was a Novice back in the mid-1960s I had a 40-meter dipole strung from a tree to a 15 foot mast sitting on the top of work shed in the back of our lot. That was enough room for my dipole which was center fed with coax. It worked. But, at that time, I knew nothing about using baluns, nothing about SWR, and absolutely nothing about using an antenna tuner (or, coupler, trans-match, or whatever). I am not even sure I was aware that these concepts or devices even existed.
When I got back into the hobby in 2004, I was much more mature and adept about researching these matters so before I strung up my first antenna, I knew quite a bit about the difficulties of match, balance, and troublesome SWR. But, I was also impatient so I strung up a simple wire where one end was nailed (I mean literally nailed) to the top of the back door frame on our house (about 7 feet off the ground) and the other end of this wire was attached to the top of the tool shed (about 10 feet up) on the back corner of our lot. There was enough sag that I had to duck under the wire when I was walking around the yard (and, my wife didn't like that part of this very much). This wire went straight to my Icom 756 non-pro that had an internal tuner. One of those "I can handle up to 3:1 jobbies".
I worked with that wire antenna for about a month before I finally put up an 80-meter dipole. It seemed to work fine. I even operated on 160 meters and 80 meters. No, I did not have the best signal and almost never flat 1:1 but it did work. But, no way am I going back to that either.
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RE: Very Basic Primer
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by KL7AJ on May 5, 2008
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PEH:
Same story here! Though I did have ONE extremely valuable tool....an R.F. current meter. In fact, I have an upcoming QST article dedicated to precisely that.
The R.F. current meter used to be the one common instrument in hamdom...and we've neglected it to our own detriment.
Eric
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RE: Very Basic Primer
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by WI7B on May 5, 2008
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The RF ammeter was THE instrument for measuring output by Marconi at the turn-of-the century. The buck stops there.
73,
---* Ken
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RE: Very Basic Primer
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by KL7HF on May 5, 2008
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After reading all the posts, it still comes back to
the engineering book's definition, and that is
"An antenna is considered resonant when there are
standing waves present".
Feeding it is a whole nuther matter.
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RE: Very Basic Primer
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by KL7AJ on May 5, 2008
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Ken:
Not only that, but it is STILL REQUIRED BY LAW for all A.M. Broadcast stations. It is indeed the bottom line.
eric
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RE: Very Basic Primer
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by N3OX on May 5, 2008
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"The RF ammeter was THE instrument for measuring output by Marconi at the turn-of-the century. The buck stops there.
"
http://www.w8ji.com/building_a_current_meter.htm
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm
73,
Dan
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RE: Very Basic Primer
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by KL7HF on May 6, 2008
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Hi, Eric:
There are still some of the thermocoupler based RF
Ammeters available from time to time in the surplus
market, such as Arrow. I've seen others based on
diode rectifiers and not sure I'd trust the accuracy,
but they may be better than I think. (I didn't think
the FCC required anything anymore. I havent seen a RI
at a broadcast station in years)
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RE: Very Basic Primer
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by G3LBS on May 6, 2008
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I've had an MFJ-835 Dual RF Ammeter on order at MFJ for 5 weeks - has anybody got one for sale please?
W2/G3LBS Allegany NY
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RE: Very Basic Primer
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by KL7AJ on May 6, 2008
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HF:
The FCC has recently allowed the use of transformer type ammeters...such as Pearson and Ion Physics current probes....so thermocouple ammeters are no longer required. But the license still requires direct power measurment using I^2*R. Proabably always will.
eric
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RE: Very Basic Primer
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by VE3IVM on May 6, 2008
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>C8ZEV
Question is .......is there any performance difference?? An antenna analyzer is a useful piece of equipment, without a doubt.
------
Rudy Severns, N6LF, tested a few of them. Try this page <http://www.makarov.ca/vna.htm> and follow the "Comparisons of different network analyzers.." link. You won't find a verdict wich one is the best but rather decide yourself based on the data provided.
Ivan
VE3iVM
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RE: Very Basic Primer
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by G3LBS on May 7, 2008
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If you don't have a dummy load which will cope with your megawatts you can use the analyzer first with your ATU to tune up your antenna system before you connect it to the rig.
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Real vs. Imag. Values
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by KC8PPD on May 8, 2008
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If I have a capacitor that measures -j50 ohm (at some frequency), can't I just connect it to my 50-ohm rig and get 1:1 SWR?
I always thought that impedance was just impedance. 50 ohm (real or imaginary) will give 1:1 SWR. I.e. If Mag(R+jX) is 50, then it will give me 1:1 SWR.
Wrong?
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RE: Real vs. Imag. Values
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by K7PEH on May 8, 2008
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The answer is wrong.
It is wrong because a capacitor, let's assume a perfect one as you describe by its impedance, cannot absorb power. Therefore, the power must be reflected totally which means the SWR is infinite, not 1:1.
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RE: Real vs. Imag. Values
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by KL7AJ on May 8, 2008
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YIP
The Smith Chart is your friend. Shows this fact quite clearly.
eric
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Very Basic Primer
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by KG5VK on May 11, 2008
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Great article
it could have been named why your swers won't matter if your x ain't zero :)
73
steve
KG5VK
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RE: Very Basic Primer
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by K8MHZ on May 18, 2008
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"If Mag(R+jX) is 50, then it will give me 1:1 SWR."
If all you have is a capacitor, then R = infinity, not 0.
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