Test Setup to Measure Antenna Impedance Transformer

[FlyingHacker]

Hi,

I would like to measure the frequency response of some transformers I am winding to connect various wire antennas to various coax lines (50 Ohm or 75 Ohm, maybe even 100 Ohm CAT5 wiring). I would like to test various transformer core materials, winding methods, turn counts, etc.

The one I am testing now is designed to match an approximately 470 Ohm loop to 50 Ohm coax. I have a Keysight 1000X scope with the Frequency Response Analysis feature. This is similar to a Bode plot, but sadly nowhere near a VNA. Nevertheless, this is what I have to work with at the moment.

So I have a choice of having the wave gen built into the scope output 50 Ohms or Hi-Z. Since I want to put the wave gen into the 470 Ohm side I was using Hi-Z and using a 1X probe as my signal injection connection. Then I was connecting a 10X probe To the same terminals on the 470 Ohm side as my input sense connection (Ch1). (When I say terminals I mean my custom box has screw terminals for the 470 Ohm connection and an SO-239 for the 50 Ohm coax.)

I then connected a 50 Ohm RG-58  to the output of my transformer (it is in a box with an SO-239). I connected this to the scope as the output sense (Ch2). The scope does not have a 50 Ohm input, so I used a terminator at the scope end.

With the Ch 2 coax terminated I did not get much of a result curve. Un-terminated I saw a curve that looked fairly similar to the impedance curve of the torroid I used. I am also wondering if this isn’t a signal level issue. What signal level should I be using out of the wave gen? I know receive signals are very weak, but the wave gen will only go down to 2mV P-P. Using a higher level may give me a better response curve, no? How accurate would it be vs. actual received signals, though? I am using good sized torroid, so I don’t think I have to worry about saturation.

Any thoughts on how to best optimize this test setup for experimenting with different windings and cores? Is using Hi-Z out of the wave gen the correct approach? Do I need to put a 470 Ohm resistor at the input to get the source impedance correct? I am foggy on this.

Thanks in advance.

[tautech]

Sounds like a fun project. 

The thing that sticks out to me wrong with your setup is the use of a scope probe in the signal source path for the FRA test.
When we first got the new 4ch Siglents that can do Bode plots I had a go at using probes but was never satisfied with the results so got a few BNC to croc clip leads just for this purpose. At the comparatively low frequencies that a DSO can do FRA cheap croc to BNC leads shouldn't have much affect on results unlike using a VNA.

How high BW are you sweeping to ?

[FlyingHacker]

Thanks for the reply, Tautech.

I am sweeping up to 20MHz. I am am mainly concerned with 160m through 20m (~1.8-14.35MHz). These are frequencies I use the most, and the lower the frequency the greater the noise. The antenna I am trying to optimize is a loop on the ground, which reduces overall signals a fair amount, but increases the signal to noise for both man made noise and lightning crashes. It is a receive only antenna. So we are dealing with very low power levels. As of now it is not amplified.

I do have at least one or two BNC to alligator clip cables. I could give that a try.

What about the impedance of the wave gen vs. the antenna side of the transformer impedance? I am not sure the best approach to use there.

Thanks.

[PA0PBZ]

What about the impedance of the wave gen vs. the antenna side of the transformer impedance? I am not sure the best approach to use there.

I think the best approach would be to put the wave gen output to 50 Ohm and then adding 420 Ohm series resistance, that will give you a 470 Ohm output.

[tautech]

I'm no RF expert at all FH, even though I've just got the new Siglent SVA SA/VNA combo.
I've been working on much the same as you for a receive antenna for 315 MHz but I'm working on 50 Ohm feeder and the reciprocity principle that a TX ~= RX antenna and while I'm guru I think 50 ohm is where it's at for RF.
Let's hope somebody really clued up on this chimes in.

My new toy is with hendorog for a bit as he's my local VNA guru. Last fiddle I had is in the SVA1015X thread and that was sweeping it with my SA tuning the resonant frequency of a new antenna I built.
It all gets pretty complex if you want to do the job fully pimped with SWR's, Smith charts and so on, let alone the proximity sensitivities of working close to the antenna you're working on.

You can follow my stuff ups here and there's a bit of help offered from those more experienced members that you might also benefit from.
https://www.eevblog.com/forum/testgear/siglent-sva1015x-1-5ghz-spectrum-vector-network-analyzer-(coming)/msg1686137/#msg1686137