RF Millivoltmeter

[ramuna]

Hi,
I would like to investigate some electrically short antennas in the HF to VHF region (5 - 200 MHz). Specifically, I would like to measure the antenna impedances as accurately as possible, compare the results to the NEC models etc. One crucial element for which I have very little knowledge of, is accurately measuring RF voltages. Basically, I'm looking for an RF millivoltmeter design. Bear in mind that these electrically short antennas will have a high to very high, absolute susceptance to conductance ratio, which makes accurate voltage measurement more important than otherwise. The methodology I am thinking of using is outlined here:
**broken link removed**
The above link, is part of webpage where Jim Tregellas VK5JST, gives details of his Aerial Analyser: **broken link removed**
In answer to the obvious question (why don't I just duplicate Jim's circuit ?): Jim's voltage measurement accuracy relies on (1) relatively high antenna conductances & low susceptances compared to my antennas (2) using relatively high feed voltages such that diode voltage drops are insignificant; I would like to get similar results with lower drive voltages.

Thanks in advance for advice & assistance in this quest.

 

[ronsimpson]

Why do you want to test the antenna at low voltage? It seems high voltage is simple.

 

[ramuna]

RonSimpson asked: Why do you want to test the antenna at low voltage? It seems high voltage is simple.
Hi Ron,
Take the example of a short wire antenna with a true impedance of 1.5 -3500j at 50 MHz. Suppose we construct a voltage divider with the antenna in series with a pure resistance of 50 ohms, and feed 10V (peak_to_peak) RF across it. Attached is a screendump of the LTSpice simulation for this situation, with the antenna at the earthy end of the potential divider. The graph above shows the voltage between the antenna and ground. As you can see, its very low. The reason I want to be able to measure low RF voltages is not because I don't want to feed high RF voltages into the setup, but because the voltage across the antenna is so low. It would be overwhelmed by diode drop for example.

 

[KeepItSimpleStupid]

Check analog devices: https://www.analog.com/en/rfif-components/detectors/products/index.html#RMS_Detectors

I bought one in this https://www.google.com/url?sa=t&rct...JkMrjEwvF7Z-h-43dL6zF6A&bvm=bv.69837884,d.aWw series, forget which one from the bay with the IEEE interface, but it's never been out of the box.

I got a good deal.

 

[ronsimpson]

I won't say what I think of 1.5 ohm antennas.
BUT
The 0.9pF cap is much of the problem. At 50mhz there is a great deal of loss in the cap. You have a low frequency block filter where 50mhz is considered low frequency.

OK..... I will say something about 1.5 ohm antennas. You don't connect then to a 50 ohm source.

A 50:1.5 ohm mismatch coupled with a cap that will block 50mhz gets you from volts to almost nothing.

 

[ramuna]

@KISS: Thanx for the links Kiss...most interesting ! I'll prob'ly spend the w/e reading thru the AD RF detectors datasheets...and only say something once I've learnt something about them. That gear looks most impressive, 180uV to 700V wow!

ronsimpson: Ron, I told you I was looking at electrically short antennas. Try running a lambda / 10 monopole thru an NEC program and look at the impedance that it spits out. It would be something like 1.5 - 3500j (ie short wire lengths look to the RF source as capacitors, their radiation resistances are miniscule relative to their susceptances. Instead of signals getting radiated into the far-field, you get energy trapped in the static near-field because the susceptances are so high. The 0.9pF cap in the LTSpice simulation was my way of simulating the antenna susceptance of -3500j @ 50MHz. I agree that 50R is too high to put into a voltage divider with such an antenna impedance. An ideal real resistance would be 5R. But that combination would require too much power from a typical instrumentation wideband RF source, because the total impedance is so low.