Chasing RFI: The Workaround

The only thing better than seeing a project through to its conclusion is not having to do the project at all.

I had been reading this document, which does a very good job explaining RFI, while I tried to find some time to hunt down the source of the interference in my house. While I was doing that, the good folks who brought us CHIRP made some improvements to the software that allow me to expand the squelch range on my Baofeng GT3, which meant I could cut out the bursts of static while still hearing actual signals.

"But hold on a sec," you say, "what exactly is this squelch thing you're talking about?" Squelch is a circuit in a radio receiver that turns off ("squelches") the audio when a signal is not being received. When a strong enough signal is detected, the "squelch is opened" - the audio circuit is turned on so you can hear the received signal.

Not all radios have a squelch circuit, but the ones that do (all that I've seen at least,) have an adjustment. This allows you to set the strength of signal that is required before the squelch opens. Ideally, you want to adjust the squelch to be low, so that you don't miss a transmission. However, there are times when you need to turn it up, such as when you're in an area with a lot of RF noise.

The problem with the UV5R series of radios (of which my GT3 is a member), is that the squelch is notoriously useless. Bursts of static will trick the radio into thinking that there's a signal present when there isn't one, and the squelch adjustment is very narrow (there is no noticeable difference between level 1 and level 9).

So, back to CHIRP. The 10 levels of squelch available in the GT3's menu (0 thru 9) are mapped to values inside the radio's programming. In fact, the radio can set the squelch anywhere between 0 and 127. At the factory, the 10 levels in the menu are mapped to these internal values.

Once I installed the latest daily build of CHIRP and looked at the squelch mappings on my radio, I discovered that levels 1 thru 9 (0 is mapped to 0, which disables the squelch circuit altogether) were mapped to values between 17 and 33, in increments of 2. On the advice of this page on the Miklor website, I changed the values to range between 24 and 64, in steps of 5.

After dumping the modified image to my radio, I moved to a part of the house where I previously had static. I turned the squelch up one step at a time until the static stopped. While I was doing this, a couple other folks were having a conversation on the local repeater that I was monitoring. I was able to confirm that my new squelch setting got rid of the static while still letting the signal from the repeater through.

So my time-consuming hunt for RFI sources has been called off. And a good thing too, since I have a bunch of other projects on the go, and I don't really have the time to go chasing RF gremlins.