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Author Topic: How to measure RF voltage across RF antenna choke?  (Read 4129 times)

Posts: 3623

« Reply #15 on: November 10, 2017, 09:09:50 AM »

I doubt it.  That instrument only goes to 300 VAC, and being an AC line powered device, it would add alternate RF paths into the mix.  Regardless, I believe the idea is to show whether the voltage is increasing or decreasing, not exact voltage measurements.  Common point radial voltage on the K2AV can approach 2400 volts.

I own a few Boonton RF Millivolt meters, but that is overkill.  QRP power levels is good suggestion--one of the shop signal generators might work. Hadn't thought of that.

My thoughts would be to add a winding on the balun for a diode detector, that should show what is going on, but I am not sure that is the best method. So far, I surmise no one but Tom and few others have bothered using this trick to tune radicals or CPs.
« Last Edit: November 10, 2017, 09:18:10 AM by K1ZJH » Logged

Posts: 2430

« Reply #16 on: November 10, 2017, 02:27:10 PM »

So far, I surmise no one but Tom and few others have bothered using this trick to tune radicals or CPs.

I suspect you are correct.

I have been trying to figure out an adjustment procedure for doing this but have not been very successful (at least at finding a reasonable procedure).  Problem, is when you adjust the length of the radials, the resonant frequency of the antenna changes.  So then you have to readjust the vertical part to bring the resonant frequency back to where you want it.  Then you can test the voltage to ground.  Repeat that procedure until you arrive a minimum voltage.  After about one iteration of that. most people will say to heck with this, looks good enough for me.

The minimum voltage will be at only one frequency and will vary across the band.  You can also come up with a combination where minimum voltage occurs above the resonant frequency of the antenna (which is what usually happens when you start with radials that are resonant at the operating frequency)..

Jerry, K4SAV
« Last Edit: November 10, 2017, 02:30:56 PM by K4SAV » Logged

Posts: 147

« Reply #17 on: November 12, 2017, 06:15:55 PM »

OK ill take a crack at it. cuz your probably close.
. Using a multimeter set to dc connect one leg of the meter to the SO239/pl259  entering the choke ,  connect the other leg to the output of a half wave bridge rectifier,that has its input connected to the SO239/pl259 Exiting the choke . The cap goes from one pl259 to the other .

Else you can use stripped rg8x and core .

I would think though an RF current clamp meter on the feed line and tuning for current would be the same. 

point of this is to keep the feed line from radiating . which if its 40 in under the soil i doubt that you even need the choke .

Posts: 175

« Reply #18 on: November 13, 2017, 07:56:46 AM »

I think that this could be measured in an uncalibrated way by using a single turn through the choke ferrite if that is what you are using for the common mode choke.

And this single turn could go to an oscilloscope.  I prefer an oscilloscope for making these kinds of measurements especially if you later want to look at more than one channel at a time or compare phase.

If you want a calibrated measurement, I would think a resistive divider made up of series resistors able to withstand the anticipated voltage and a high enough impedance that it does not lower the expected choking impedance across the feedline choke.  And then connect this resistive divider to an oscilloscope.

Not that I would necessarily measure it this way myself as others have already opined.

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