SDR Antenna Analyzer

Discuss commercial and home made antennas.
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apl
Posts: 2
Joined: Tue Dec 19, 2017 4:22 pm

SDR Antenna Analyzer

Post by apl » Tue Dec 19, 2017 4:28 pm

So I'd seen this:

https://www.rtl-sdr.com/using-the-airsp ... -antennas/

...which is fun, especially since I'm working on an antenna anyway.

Then I found this:

http://qrpguys.apps-1and1.com/k7qo-noise-bridge

...and this fits the function of noise source and the splitter in the article.

I'm getting that for noise bridge for Christmas, and I'll post something once I get an antenna up and analyzed.

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: SDR Antenna Analyzer

Post by rtlsdrblog » Wed Dec 20, 2017 3:30 am

Nice, but whats the top end frequency that it can be used at? Looks like it's designed for ham use at about up to 30 MHz, so not sure it would work on say a 1090 MHz ADS-B antenna.

apl
Posts: 2
Joined: Tue Dec 19, 2017 4:22 pm

Re: SDR Antenna Analyzer

Post by apl » Wed Dec 20, 2017 2:49 pm

Good point!

Per the product website, the noise source "generates a wide spectrum of white noise up to several hundred MHz." One could probably use the SDR to see at what frequency the source output gets too close to the noise floor of the system to give meaningful characterization of the antenna. Maybe this could be done by intentionally imbalancing the bridge with a >100Ω dummy load on the antenna connection. I don't see why leaving it open would not work. If there's a hard limit on the max frequency, it's probably where the toroid gets too leaky.

I'd have to think about how to do it, but the idea here is to basically get a frequency response function of the system using the random excitation of the source as the input and the reflected noise as the output. Normally I would think of it the opposite way; the output is what's leaving the antenna, not what's being bounced back. But by conservation of energy, anything that doesn't come back must have left through the antenna.

One of the parameters of a frequency response test like this is the coherence, which is a characterization of the linearity and time invariance of the system's response to the input. If the input is too low relative to the noise floor of the input or output, then the response will not cohere with the input. The frequency where the coherence starts rolling off would be where the source is not giving enough power to give meaningful analysis.

I will test this when I get the source kit up and running.

Thanks again for the excellent thoughts.

snn47
Posts: 186
Joined: Tue Dec 27, 2016 11:00 pm

Re: SDR Antenna Analyzer

Post by snn47 » Sun Dec 24, 2017 5:06 am

What frequencies do you want to use it for?
RTL-SDR is correct, that is the basic design used in many short wave HAM Antenna Noise Bridges, that use a receiver as detector.
The noise of a zehner diode will decrease with frequency. The trick was to find a zehner diode that produced high noise to start with. Luckily I was allowed to meassure a roll with a few hundred zehner diodes to select the few producing a lot of noise at the time. :roll:
Then there a designs either using transistors or diodes designed to generate a low but wideband noise, which requires sufficient amplification. However that is only the noise generator.
Other designs use tracking VCO instead of wide aband noise.

You ned also the couplers that wort at the frequency you are interested in. Desings for upt to a few hundred MHz require lot's of ferrites to improves performance and are based on an older HP reference design.
You might also look for VNA in the HAM literature to find more inspiration.

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