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Author Topic: 43 Ft Vertical Blatant Lie  (Read 42347 times)
RFRY
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« Reply #15 on: September 21, 2012, 03:14:08 PM »

I can draw a 1-2 inch arc at 500 watts on 160 meters with a 43 ft vertical over 20-30 60-80 ft long radials at the antenna base.

However, the voltage at the top of the loading coil needed for system resonance of an electrically short monopole depends much more on the electrical length and width of that monopole than on the r-f ground loss in whatever set of buried radials is in place.

The electrical length and width of a such a monopole determine the reactive term of the impedance at its base, which then determines the coil inductance needed to achieve system resonance at a given frequency.

Shorter/thinner monopoles need more XL at the feedpoint to achieve system resonance, and the r-f voltage at the top of the loading coil needed to resonate such short/thin monopoles for a given applied power and frequency rises accordingly.
« Last Edit: September 21, 2012, 03:20:19 PM by RFRY » Logged
W8JI
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« Reply #16 on: September 21, 2012, 05:45:34 PM »

The current through the reactance determines voltage drop across the reactance.

A lossy ground and lossy coil will reduce current for a given available power. That will reduce voltage.

The simple way to look at this is a certain radiator will requires a certain current, and a certain base voltage, to radiate a certain total power. With a normal diameter 43 foot vertical, the base voltage on 160 meters to actually radiate 500 watts is about 14,000 volts.

If we changed it to a thin wire, or relaced the top with a thin whip, the voltage would increase.

If someone is running 500 watts without long arcs at the base insulator and a normal thickness 43 ft vertical, they have lost significant efficiency someplace.
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W5DXP
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« Reply #17 on: September 22, 2012, 10:14:30 AM »

The current through the reactance determines voltage drop across the reactance.

When we are dealing with RF waves flowing through transmission lines or large air-core loading coils, there's no evidence that the current is the independent variable while the voltage is the dependent variable. The attenuation of an RF wave through the reactance is responsible for a drop in both the voltage and current. In a constant Z0 environment, as exists end-to-end in a large air-core loading coil, the ratio of voltage to current is a constant equal to the characteristic impedance of the loading coil. The attenuation factor of the coil for RF waves reduces the voltage and current by an equal percentage. One does not "determine" the other. These effects are easily demonstrated using the EZNEC helix option.

The lumped circuit model does not work well for RF networks. When an RF signal is attenuated, the E-field (voltage) and H-field (current) are attenuated by the same percentage. The ratio of V/I, i.e. the impedance, is constant for a constant characteristic impedance. Transmission lines and large air-core loading coils are not lumped circuits and do not obey Kirchhoff's lumped circuit laws. That's why the distributed network model had to be developed.

One cannot understand standing-wave antennas without understanding the basics of wave theory, the concepts of which are not very difficult. When one sees oneself in a mirror, one is observing EM wave reflections.

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KB4QAA
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« Reply #18 on: September 22, 2012, 11:27:31 AM »

Not the coils again!  Argghh.
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W5DXP
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« Reply #19 on: September 22, 2012, 12:49:39 PM »

Not the coils again!  Argghh.

What Tom said is absolutely true for lumped-circuit coils. Unfortunately, the loading-coil needed to turn a 43' monopole into a 160m antenna is nowhere near a lumped-circuit coil since the 43' monopole is only 30 degrees long on 160m. A quick calculation with the following inductor calculator at:

http://hamwaves.com/antennas/inductance.html

indicates that loading coil would have to be ~20 degrees long on 160m which will cause an error when using the lumped-circuit model that assumes all lumped-circuit coils are zero degrees long.

And it's not just coils, it is also true for transmission lines. The lumped-circuit model assumes that all wires are zero degrees long. Is the voltage across a transmission line determined by the current through the transmission line? Of course not! Both the voltage and current are determined by the applied power, the Z0 of the line, and the magnitude of the forward and reflected waves. There are no resistors, inductors, or capacitors anywhere.

Question: Which is better? An easy to understand invalid statement? Or a more difficult to understand valid statement? I cannot bring myself to assert or agree with a statement that I know to be invalid. If that's a character flaw, I apologize.
« Last Edit: September 22, 2012, 12:56:34 PM by W5DXP » Logged
N2EY
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« Reply #20 on: September 22, 2012, 01:28:03 PM »

I'm not positive how or why 43 foot verticals were created, but it appears they were an accident caused by using the wrong type of balun at the bottom. The very first 43ft antennas to appear were sold with a 4:1 voltage balun at the base, and that reverse excited the coax shield with RF.

I think that's pretty close.

I suspect that the "magic" 43 foot length came from three things:

1) it's close to 5/8 wave at 20 meters

2) it's mechanically manageable for many hams

3) with a so-so ground system and typical ham coax & tuners, the swr on most bands isn't too bad.

And like many antennas, it all depends on what you're comparing it to.

73 de Jim, N2EY
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W5WSS
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« Reply #21 on: September 22, 2012, 03:01:28 PM »

43ft vertical works well as a longer range HF dx antenna for 5 bands where the percentage to 1 wavelength ranges from 5/8 wave on 20m to around a quarter wavelength on 60m.

The pattern manifests in this design because of the physical length of the radiating vertical element.

  A loading coil can be constructed and then added and utilized as a trap for the 80m band where the 43ft vertical can then be made to work as a quarter wave or thereabout.

For higher band use the 43ft version is too long to expect equivalency vs itself compared to it's performance on lower bands when low angle dx work is ones objective.

In all cases where an auto tuner and balun is properly placed at the antenna base omitting any coaxial cable in the lossy untuned zone between the tuner and balun the antenna will rival a trap vertical that is designed to be a quarter wave radiator provided the tuner and balun are meeting the requirements of high voltage exposure when the antenna is a half wave in length.

The cost of such an antenna system exceeds that of a readily available commercial trap vertical but is considerably cheaper than an automatically physically adjustable version which is also an option and probably the best multi band vertical design to date.

One could quick change three fixed length vertical radiators for high medium or low band use with wire but the length to diameter ratio increases the antenna base voltage exponentially as frequency applied is lowered.

Keeping the tuner and balun and counterpoise the same.

I used a 22ft for 30m to 1om pursuant to low angle dx work

I used a 43 ft for the same objective expecting low angle production from 20m to 60m

a 66ft vertical wire or obviously an Aluminium tube is a better choice for low band low angle work there is no magic.
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RFRY
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« Reply #22 on: September 23, 2012, 03:53:00 AM »

A quick calculation ...  indicates that a loading coil would have to be ~20 degrees long on 160m which will cause an error when using the lumped-circuit model that assumes all lumped-circuit coils are zero degrees long.

A 43' monopole is about 28 degrees in electrical length (height) at 1.8 MHz, so by the logic in the above quote, the base loading coil needed to produce first system resonance needs to be "62 degrees long."

The XL of that coil offsets the XC of the electrically short monopole to produce system resonance.  But that doesn't mean the coil adds the equivalent radiating length of 62 degrees to the physical length of that monopole.  A  43', base-coil-resonated/driven monopole on 160m is still only ~28 degrees in height, and has the radiation pattern/directivity of a ~28-degree monopole.

If the loading coil added such physical degrees of length to the antenna system, then the radiation resistance of the system with the loading coil would be about the same as if a 1/4-wave linear radiator was used -- which does not occur.  It remains about the same as before the coil was added.

The loading coil adds about the same value to radiation resistance as a linear conductor of the same overall length as the coil, end-end -- not to the total length of wire used in the coil.

The link below taken from Kraus' ANTENNAS..., 3rd edition develops this:

http://i62.photobucket.com/albums/h85/rfry-100/Helically-woundVertical.gif


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W8JI
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« Reply #23 on: September 23, 2012, 04:48:08 AM »

I think that's pretty close.

I suspect that the "magic" 43 foot length came from three things:

1) it's close to 5/8 wave at 20 meters

2) it's mechanically manageable for many hams

3) with a so-so ground system and typical ham coax & tuners, the swr on most bands isn't too bad.

And like many antennas, it all depends on what you're comparing it to.

73 de Jim, N2EY

I actually wonder if the antenna came from cutting Cebik's 88 ft dipole in half and making it vertical, or if it came came from an experiment where someone built a 5/8th vertical for 20. Whatever the origin, they obviously used the wrong matching system, excited the coax shield, and thought they had an invention.

The source of the original 43 ft vertical has a tendency to take antenna designs in one application and make convert them to others based largely on feeder radiation. For example, he sells a 40-10 meter groundplane antenna. We know there cannot be any such thing, and that on bands where the "groundplane" radials are not 1/4 wave long the mast and feedline become major radiators. If built a 40-10 groundplane, it would have three or four resonant radials on every band from 40 - 10 meters and a 1/4 wave electrical element on 40-10. Otherwise, it isn't really one. It might be a single band groundplane fed as a random wire on other bands, but it sure as heck is not a 40-10 groundplane.
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W8JI
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« Reply #24 on: September 23, 2012, 05:09:52 AM »

A 43' monopole is about 28 degrees in electrical length (height) at 1.8 MHz, so by the logic in the above quote, the base loading coil needed to produce first system resonance needs to be "62 degrees long."

The XL of that coil offsets the XC of the electrically short monopole to produce system resonance.  But that doesn't mean the coil adds the equivalent radiating length of 62 degrees to the physical length of that monopole.  A  43', base-coil-resonated/driven monopole on 160m is still only ~28 degrees in height, and has the radiation pattern/directivity of a ~28-degree monopole.

If the loading coil added such physical degrees of length to the antenna system, then the radiation resistance of the system with the loading coil would be about the same as if a 1/4-wave linear radiator was used -- which does not occur.  It remains about the same as before the coil was added.

The loading coil adds about the same value to radiation resistance as a linear conductor of the same overall length as the coil, end-end -- not to the total length of wire used in the coil.

The link below taken from Kraus' ANTENNAS..., 3rd edition develops this:

http://i62.photobucket.com/albums/h85/rfry-100/Helically-woundVertical.gif




This all started out years ago when someone thought a loading coil always represented some "missing antenna electrical degrees" and, as such, claimed the current drop across those "missing degrees" occurred in every loading coil.

The rule is, of course, if the coil does not have significant displacement current compared to the current entering the coil, it has to have the same current at the far end. It really comes down to how much the electric field and size allows displacement currents.

A small compact loading inductor would have virtually no current delay or taper from end-to-end, while the very same reactance in another physically large inductor (with lots of stray C and displacement current) might have noticeable current taper and phase difference in current at each end.

I really don't understand what this is so difficult for a few people to understand, unless they intentionally are being difficult as possible as entertainment.

73 Tom
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W5DXP
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« Reply #25 on: September 23, 2012, 08:42:56 AM »

A quick calculation ...  indicates that a loading coil would have to be ~20 degrees long on 160m which will cause an error when using the lumped-circuit model that assumes all lumped-circuit coils are zero degrees long.

A 43' monopole is about 28 degrees in electrical length (height) at 1.8 MHz, so by the logic in the above quote, the base loading coil needed to produce first system resonance needs to be "62 degrees long."


Sorry, that is not true.  My posting was based on a paper Smith Chart and was somewhat inaccurate. I've use MicroSmith for the following. Consider the following transmission line:

Source---13 deg Z0=450 ohm line---+---30 deg Z0=60 ohm line---open

Would you say that since the 60 ohm section is 30 degrees long, the 450 ohm section must be 60 degrees long to create a 1/4WL (90 deg) stub? If so, that's just not true.

Physically, the line is 43 degrees long. Electrically, the line is 90 degrees long. Question: Where are the missing 47 degrees? Answer: The impedance discontinuity between the 450 ohm line and the 60 ohm line provides a 47 degree phase shift at the impedance discontinuity point.

Now, replace the 13 degree 450 ohm section above with a 13 degree loading coil having a characteristic impedance of 4500 ohms and replace the 30 degree 60 ohm section above with a 30 degree long whip having a characteristic impedance of 600 ohms. That's an estimate of the conditions when feeding a 43' monopole on 160m through a loading coil to achieve resonance.

There are three phase shifts in the system, not two.

1. The large air-core loading coil provides a delay, i.e. a phase shift of 13 degrees above.

2. The impedance discontinuity between the top of the coil and the 43' monopole provides a point phase shift of 47 degrees.

3. The 43' monopole provides a phase shift of 30 degrees.

13+47+30 = 90 electrical degrees

We know the antenna is 90 degrees long because the phase shift from the feedpoint to the tip end of the antenna is 90 degrees thus effecting a purely resistive feedpoint impedance.

More info at: http://www.w5dxp.com/shrtstub.htm

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W5DXP
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« Reply #26 on: September 23, 2012, 09:20:42 AM »

This all started out years ago when someone thought a loading coil always represented some "missing antenna electrical degrees" and, as such, claimed the current drop across those "missing degrees" occurred in every loading coil.

It is true that is how it started. But many years ago, I said that both sides were wrong in that argument. There are not two, but three phase shifts involved. You are a lot like Obama still blaming Bush for all of his problems. The argument is almost ten years old and what was said ten years ago just doesn't matter anymore. Please see my reply to RFRY above.

Quote
The rule is, of course, if the coil does not have significant displacement current compared to the current entering the coil, it has to have the same current at the far end. It really comes down to how much the electric field and size allows displacement currents.

I'm sure that you are aware that the concept of "displacement current" has been relegated to the myth pile as a close look at displacement current requires that the speed of light limit be violated. Displacement current around a device has been proven by quantum physics to be photon flow through the device which has the distinct advantage of not violating the speed of light limit.

Quote
A small compact loading inductor would have virtually no current delay or taper from end-to-end, while the very same reactance in another physically large inductor (with lots of stray C and displacement current) might have noticeable current taper and phase difference in current at each end.

As Reagan would say: "There you go again." You and a few others are the only ones to hold to the ridiculous notion that a humongous Texas Bugcatcher 80m loading coil is "small and compact". (Did your significant other ever hit you over the head with one of those things?)Smiley Those large air-core loading coils used on 160m and 80m are NOT "small and compact" so they do not meet the requirements of a lumped-circuit model. That is easy to see by looking at a resonant helically wound antenna. Ask yourself, if a helical-wound vertical is electrically 90 degrees long and resonant, how long is half a helical-wound vertical? Why wouldn't it be 45 degrees long? Actually, it can be proven to be close to 45 degrees long by looking at the feedpoint impedance the phase of which tells us how long the antenna is electrically.

Quote
I really don't understand what this is so difficult for a few people to understand, unless they intentionally are being difficult as possible as entertainment.

I personally don't think you are that naive. Please prove that my dial-Z0 shortened stub analysis is invalid for loaded mobile antennas.

Here is an EZNEC model of an 80m Texas Bugcatcher coil (72uH) loaded with a 2571 ohm resistor at 4 MHz. The source current is the same magnitude as the load current. The source voltage is the same magnitude as the load voltage. The delay for both voltage and current is 42 degrees, i.e. voltage and current are in phase at both ends of the coil. How does your lumped-circuit analysis explain those results?

http://www.w5dxp.com/bugctchI.zip
« Last Edit: September 23, 2012, 09:25:38 AM by W5DXP » Logged
G3TXQ
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« Reply #27 on: September 23, 2012, 09:49:44 AM »

Does any of this alter Tom's statement that a 43ft vertical used on 160m will have a very high feedpoint voltage to ground, even at modest powers. It seems to me that was the point he was making?

Steve G3TXQ
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RFRY
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« Reply #28 on: September 23, 2012, 10:09:53 AM »

Sorry, that is not true.  My posting was based on a paper Smith Chart and was somewhat inaccurate. I've use MicroSmith for the following. etc

Cecil, please refer to the analysis linked below.  Note that this system is resonant, as would be linear conductor long enough to be first self-resonant at this frequency (about 132 feet).

Now note that although the coil-loaded system is resonant, the Rr of the loaded antenna system is 2.57 ohms, while the Rr of the unloaded 1/4-wave monopole would be around 36 ohms.  That reduced Rr, and the r-f loss in the loading coil (assumed here to be 8 ohms) means that the loaded system radiates about 20.5% of the applied power, whereas a linear, 1/4-wave monopole with no loading coil would radiate about 95% of the applied power, for the same 2-ohm r-f ground connection loss.

The groundwave field at 1 km for the 1/4-wave with 1 kW applied power would be about 300 mV/m, where the 43' loaded monopole produces about 134 mV/m (other things equal).

Obviously the loaded 43' monopole does not have the characteristics and performance equivalent to that of a linear radiator having 90 degrees of physical length (height).

http://i62.photobucket.com/albums/h85/rfry-100/43-ft_Monopole_160m_zps65267a54.gif
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N3OX
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« Reply #29 on: September 23, 2012, 12:09:19 PM »

Does any of this alter Tom's statement that a 43ft vertical used on 160m will have a very high feedpoint voltage to ground, even at modest powers.

Very relevant...
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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