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Author Topic: How to measure RF voltage across RF antenna choke?  (Read 1065 times)
K1ZJH
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« on: November 07, 2017, 07:06:51 PM »

How do you guys measure the RF voltage across a common mode choke? 

For example, one being used to isolate an elevated radial system from
stray earth return paths?

Pete
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W5DXP
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« Reply #1 on: November 07, 2017, 07:37:16 PM »

How do you guys measure the RF voltage across a common mode choke?

Most of us measure the impedance across the choke.
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K1ZJH
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« Reply #2 on: November 07, 2017, 07:39:52 PM »

I would tune elevated radials in parallel for minimum RF voltage across the common mode choke.

What inductance value should I shoot for?  Grin  besides the one that produces minimum voltage
drop.

Pete
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VK6HP
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« Reply #3 on: November 07, 2017, 09:23:14 PM »

I might be mis-understanding what you're trying to do but wouldn't you be better off measuring the common mode current in the feeder?  That's relatively easily done with a simple clip-on current transformer arrangement (build or buy).
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K1ZJH
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« Reply #4 on: November 08, 2017, 08:10:39 AM »

There were a few sites that suggested tuned elevated radials would result in minimal voltage
drop across the common mode choke used to decouple the radial system from external stray
ground return paths, such as the coax feedline or lightning ground rods.

I would assume that could be done using a RF current measuring device on the feedline below
the choke since it is a series circuit path and ground current losses would drop as the
the voltage across the balun increases , but the authors'  comments concerning "tuning for minimum
RF voltage across the balun" made wonder how they are measuring the voltage.  

I'd guess I could place a turn or two of wire around the balun toroids and measure the detected
voltage?  That is what I am tempted to try if there isn't a better suggestion.

The method they used was not mentioned.

This is for a short elevated radial system that will use a common resonating coil; so some method
of verifying at what point the entire system has minimal RF current losses is what is being asked.

Pete
« Last Edit: November 08, 2017, 08:28:20 AM by K1ZJH » Logged
W1VT
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Posts: 2480




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« Reply #5 on: November 08, 2017, 08:57:04 AM »

Full size quarter wave tuned radials are preferred by many hams because they not only act as a counterpoise, or the ground return part of the antenna, but also decouple the shield, minimizing common mode currents. 

The major deficiency of short radials is that they are no longer useful for minimizing shield currents.  Adding an inductor in series isn't going to turn the short radials into an effective shield decoupling mechanism.

Zack W1VT
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WB6BYU
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« Reply #6 on: November 08, 2017, 09:03:11 AM »

Quote from: K1ZJH

There were a few sites that suggested tuned elevated radials would result in minimal voltage
drop across the common mode choke used to decouple the radial system...




That could reduce the power dissipation in the balun, particularly if it were poorly designed.
But overall I think you're better off just measuring the common mode current - that gives
you a better measure of what you really care about, and is simpler to do.

For maximum decoupling, use a pair of feedline chokes a quarter wave apart on the coax.
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K1ZJH
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« Reply #7 on: November 08, 2017, 10:36:22 AM »

Full size quarter wave tuned radials are preferred by many hams because they not only act as a counterpoise, or the ground return part of the antenna, but also decouple the shield, minimizing common mode currents. 

The major deficiency of short radials is that they are no longer useful for minimizing shield currents.  Adding an inductor in series isn't going to turn the short radials into an effective shield decoupling mechanism.

Zack W1VT

No, but it will help to equalize the current flowing into the radial system and the to inverted L section.  There are two inductors, one being
used to resonate the short elevated verticals, and the other, a common mode choke used to decouple the radials from stray earth return
paths.  I am not quite sure what you are trying to say in your post? 
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K1ZJH
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« Reply #8 on: November 08, 2017, 10:37:59 AM »

Quote from: K1ZJH

There were a few sites that suggested tuned elevated radials would result in minimal voltage
drop across the common mode choke used to decouple the radial system...




That could reduce the power dissipation in the balun, particularly if it were poorly designed.
But overall I think you're better off just measuring the common mode current - that gives
you a better measure of what you really care about, and is simpler to do.

For maximum decoupling, use a pair of feedline chokes a quarter wave apart on the coax.

Feedline is buried hardline,  down 40 inches in conduit.  A second set of chokes is impossible.

Pete
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K1ZJH
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« Reply #9 on: November 08, 2017, 01:43:50 PM »

This one reference that I found, from W8JI's webpage:


Resonate the Counterpoise

"Resonating the counterpoise reduces counterpoise feedpoint-to-earth voltage. This reduces common mode on the feed line, and relaxes current balun or choke requirements.

One way to resonate the counterpoise is to measure counterpoise to earth voltage at the counterpoise center, and adjust for minimum voltage.
"
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W1VT
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« Reply #10 on: November 08, 2017, 02:48:06 PM »

https://www.w8ji.com/counterpoise_systems.htm
Resonance does not directly help the radials or antenna work better. Radials greatly reduce the voltage from counterpoise common point to "earth", and this makes decoupling the feeder much easier.

Adding 600-ohm reactance loading inductors in each radial wire at the feedpoint, and tuning for minimum counterpoise-to-earth feedpoint voltage, reduces 1500-watt counterpoise feedpoint voltage from 5300 volts to just 36 volts. Unfortunately, the voltage null only holds over a small frequency range.
https://www.w8ji.com/counterpoise_systems.htm

It is a difference in viewpoint.  Tuning a counterpoise just to reduce balun stress at small frequency range isn't particularly useful to me, as I want to be able to operate anywhere in the band without the amp tripping out.  I'm also willing to accept a fractional dB loss in signal for the convenience of having an shack mounted autotuner eliminate the need for time consuming precision antenna tuning. 

Zack W1VT
« Last Edit: November 08, 2017, 02:54:50 PM by W1VT » Logged
K1ZJH
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Posts: 3297




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« Reply #11 on: November 08, 2017, 02:59:52 PM »

And no one has bothered to read the original question.  I quit.
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WA1RNE
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« Reply #12 on: November 10, 2017, 07:45:33 AM »

Quote
There were a few sites that suggested tuned elevated radials would result in minimal voltage
drop across the common mode choke used to decouple the radial system from external stray
ground return paths, such as the coax feedline or lightning ground rods.

 I believe some of the answers you're getting are essentially saying, why bother measuring voltage across the choke?

 If the choke is doing it's job, common mode currents will be reduced to the point of being negligible - making common mode current the measurement you need to make to prove choke performance.

Quote
No, but it will help to equalize the current flowing into the radial system and the to inverted L section

The equalization of currents occurs because less current is spilling over onto the feed line.

 But more importantly, proper feed point decoupling via the elevated radials is key - without which the choke becomes a moot piece of equipment.

 ...WA1RNE
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K1ZJH
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« Reply #13 on: November 10, 2017, 08:08:09 AM »

I'm not arguing with the answers I am getting... but several sites, by known authors, have mentioning measuring the RF voltage between the counterpoises or radials and ground... I merely asked how they were doing so. Simple question.

Now if you guys want to keeping arguing that doing so has no value, then explain why the K2AV FCP will NOT work with a conventional ferrite core common choke, without the cores being damaged.  

Here is one such quote, from W8JI's webpages:

Resonate the Counterpoise

Resonating the counterpoise reduces counterpoise feedpoint-to-earth voltage. This reduces common mode on the feed line, and relaxes current balun or choke requirements.

One way to resonate the counterpoise is to measure counterpoise to earth voltage at the counterpoise center, and adjust for minimu
m voltage.[/b][/u][/u]

I am simply asking how Tom would have measured the RF voltage. I know how to use current probes, RF ammeters, etc... but I just want to know what someone would use to measure the RF voltage, which can be considerable.  I'd ask Tom, but he got driven off of here.

Pete
« Last Edit: November 10, 2017, 08:13:48 AM by K1ZJH » Logged
W1VT
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Posts: 2480




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« Reply #14 on: November 10, 2017, 08:56:14 AM »

Perhaps Tom owns an HP 410B?

https://people.ohio.edu/postr/bapix/HP410B.htm
The laboratory grade HP-410B is one of the best VTVMs ever made. It can measure AC voltages with a frequency response to 700 MHz within 1 dB. It is useful to around 3 GHz according to the manual

One way of handling the considerable voltage is start off with low power and carefully raise the power level as necessary.  A QRP transmitter and a step attenuator can provide very low power levels.

Zack W1VT
« Last Edit: November 10, 2017, 09:03:49 AM by W1VT » Logged
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