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Hi,
Currently trying to design and tune a 1/2 end fed antenna (RF is applied on one end), I am looking for the correct method to measure its impedance. As this one is very high I don't know the right méthode !

The result of this measurement should allow me to create a Stub with SimNEC to tune it.
Thanks in advance for your help

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73 s Christian F8GHE

One way might be to use a transformer. ?Minicircuits has small, inexpensive transformers in a DIP package in ratios up to 16:1 or 25:1.

1:25 CORE & WIRE Transformer, 0.02 - 30 MHz, 50? | TT25-1-X65+ | Mini-Circuits ( )
minicircuits.com ( )

( )

That will get you from 50 ohms to 1250 ohms.

End fed antennas are tricky to measure (and model), because in most cases, the outside of the feedline winds up being part of the antenna.

Jim thanks very much for your advise.
During this afternoon I found these urls and now I don't know what is the best way to measure my end-feed antenna : Your or one of these :
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73 s Christian F8GHE

Some of those techniques are more about measuring a high impedance component, so they wouldn’t work.

You’re making an S11 measurement - it’s just challenging because for end fed antennas, there’s more to the antenna than the wire you’re at the end of.

It’s the same as trying to measure the Z of a top hat loaded vertical or a vertical of any kind - where does the “other terminal” connect?

In the case of an end fed, the “other half of the antennna” is often the feed line.

Thanks Jim, now I understand better.

So, I will build a LC tuning circuit and I will measure afterwards to adjust it.

--
73 s Christian F8GHE

From amfone :;wap2
John K5PRO:
Before the days of plastic antenna analyzers, you either used a GR bridge or a slotted line (for VHF), or just knew the SWR from a cheap wattmeter or Bird, or if you were lucky, a Delta OIB-2. This is the cousin of the famous OIB-1 and OIB-3 used by by broadcast engineers. Freq range is 2-30 MHz, 1 kW thru-power. Reads +/- 500 ohms resistive and up to +/- 800 ohms of reactance, and includes the meter amplifier for precise null

And the modern version using a Hp8753C with a DIY directional coupler , 2 Attenuators , and RF amplifier:


Newbie as I am on NanoVna , it will be beneficial if some can suggest how to use the NanoVna to achieve OIB measurements like the ones mentioned on the paper

The NanoVNA doesn't have a separate Source and receiver ports, so you can't do the external amplifier, coupler approach in the Rackley paper.

Is there a reason you need high power for the test? (that is, can't you just push the mighty milliwatt from the VNA into the antenna)

What you might be able to do is sequentially make S21 measurements, switching the second port (CH1) of the NanoVNA to the various ports of the directional coupler (with attenuators as required). The challenge is that the power amplifier will be "outside the calibration plane", so you'd need to separately characterize it and then do the math to remove its contribution. A complication is that the PA's characteristics probably change with the load impedance.

The techniques in the paper rely on having multiple coherent receivers that are separately accessible, which the NanoVNA doesn't have - two of the three receivers are "dedicated".

Now it's true that you could probably go in and do some surgery on a NanoVNA (they're inexpensive, and you're collecting data into a computer, so you could get the one with the small display) and bring out the different ports you need. (basically, the inputs to the mixers fed from the stimulus, and the bridge on CH0) - you could replace the circuitry with a little board with the same input circuit that CH1 has (which is, I believe, a resistive pad and some coupling capacitors).

You could then use software (Scikit-rf in python for instance, has the libraries) to do the calibrations, etc.; using the modified NanoVNA just as a data collector.

Another approach would be to build up an equivalent system using some inexpensive SDRs all locked to a common reference - A programmable source (the NanoVNA) and 3 or 4 RTL-SDRs might work

The NanoVNA doesn't have a separate Source and receiver ports, so you can't do the external amplifier, coupler approach in the Rackley paper.

Is there a reason you need high power for the test? (that is, can't you just push the mighty milliwatt from the VNA into the antenna)

What you might be able to do is sequentially make S21 measurements, switching the second port (CH1) of the NanoVNA to the various ports of the directional coupler (with attenuators as required). The challenge is that the power amplifier will be "outside the calibration plane", so you'd need to separately characterize it and then do the math to remove its contribution. A complication is that the PA's characteristics probably change with the load impedance.

The techniques in the paper rely on having multiple coherent receivers that are separately accessible, which the NanoVNA doesn't have - two of the three receivers are "dedicated".

Now it's true that you could probably go in and do some surgery on a NanoVNA (they're inexpensive, and you're collecting data into a computer, so you could get the one with the small display) and bring out the different ports you need. (basically, the inputs to the mixers fed from the stimulus, and the bridge on CH0) - you could replace the circuitry with a little board with the same input circuit that CH1 has (which is, I believe, a resistive pad and some coupling capacitors).

You could then use software (Scikit-rf in python for instance, has the libraries) to do the calibrations, etc.; using the modified NanoVNA just as a data collector.

Another approach would be to build up an equivalent system using some inexpensive SDRs all locked to a common reference - A programmable source (the NanoVNA) and 3 or 4 RTL-SDRs might work

Jim, thank you a lot for the detailed information. I will be able to do some surgery on one of the NanoVna gen111.taobao.com firmware, which I updated to 0.3.0m. I will have to check the schematics for it.

Not that I want to deviate to much for the topic of measuring high impedance of vertical, but to bring back a way to update the old technologies like the OIB meter.

"Is there a reason you need high power for the test?"

When you are one of those blessed with a 50KW AM BC nearby and trying to measure the Z of a top hat loaded vertical or inverted V seems a bit of a challenge. After filtering the BC station a lot, I end up with a 10Watts at the end of the feed. Seems like I need some power to fight back.

Patty.