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RFI Spectrum Hunter Loop

rfi spectrum hunter

By combining TinySA with my direction finder, I may be able to create a lightweight RFI spectrum hunter loop. Just the ticket for tracking down noisy neighbors.

A few years ago, I built a passive tuned loop for tracking down sources of RFI in my neighborhood. My RFI loop, shown above, uses capacitors to tune signals from 1 to 20 MHz. It also contains an attenuator.

By hooking up the loop output into the external antenna connector on my old Grundig YB-400 receiver, I am able to direction find many noise sources. But holding the top-heavy loop in one hand, and trying to tune the receiver with my other hand was a challenge. At one point, I thought of using my older SDRplay RSP2 with a PC for my portable direction finding.

I tried this with an Asus clamshell and an Huawei dual-boot tablet. My approach worked, but was even more cumbersome. At one point, I thought of “borrowing” a shopping cart from Walmart to support my man-portable RFI station.

But now, it strikes me that my new TinySA might do the job as a very portable RFI spectrum hunter. Shown above is my TinySA spectrum analyzer sitting on RFI direction finding loop. I think it would be easy to 3D print a mounting bracket to attach the TinySA to the loop’s wooden handle, and will try that next.

Naturally, a tuned DF loop is narrow bandwidth device. It is moderate Q so provides a sampling signal bandwidth of maybe 100 kHz. This means that I would have to set the scanning range on the TinySA to match the loop’s tuned frequency. But that should be possible, even if one handed.

RFI Spectrum Hunter Loop Sensitivity

My biggest concern is whether or not the TinySA is sensitive enough for the job. According to the specifications, TinySA has a minimum discernable signal of between -90 to -100 dBm. This is equivalent to S4 to S6, or a reasonably strong signal. So, I suspect that my tuned loop Q might provide a usable output for worst case RFI, but I would have to be reasonably close to the source.

It’s worth a try.

By the way, the TinySA comes with a small whip antenna, shown above. This works pretty well at VHF and above, but is useless at HF and below.

One comment

  1. Chris says:

    Hi John, Thanks for all the great posts on this blog, I’m very excited to have found it. I just enjoyed your discussion of diversity reception on the recent Ham Radio Workbench podcast (Oct 18, 2021). I live in an urban part of the SF Bay Area, and have S6-9 noise on most HF bands at most times, so some day I hope to develop a diversity reception setup to help with that. But right now I’m just getting started and would like to see if I can find and hopefully reduce some of the worst noise sources in my house and neighborhood. You mentioned using this DIY loop antenna with a portable SW radio, which seems like a great initial solution for me. Do you have any more info on how the loop is designed and wired, and the specs for the capacitor and attenuator? Any pointers to more details or instructions would be fantastic!
    Thanks and 73! -Chris KJ6WEG

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