#rf interference

I’ve been seeing strong coherent interference showing on the waterfall every night. Each time, at a different hour. For a while I couldn’t figure out where is it coming from. I thought it might be the neighbors turning on some electrical device, but every time it was at a different hour.

I did notice though that on cold cloudy days, it was occurring earlier than on sunny days.. hence my suspicion on electrical heating device.   Well.... mystery is solved!-- (and It’s not a heating device)

While browsing the web randomly a few weeks ago I stumbled on the Joule thief circuit. It uses a single transistor and a Ferrite core inductor which forms an unregulated voltage boost converter to “steal” the remaining charge batteries which seem to be discharged (have reached low enough voltage to be non useful). It does that by oscillating a single transistor, usually around 40KHz. These circuits are used in outdoor solar lighting to exhaust the daily charge as much as possible. Well... putting things together I realized that if you have a square wave oscillator, plus a long wire of LED’s, this can actually act as a transmitter. A transmitter which only starts when..... the battery is almost drained....

Well, here’s the spectrum of the interference:

Here’s after I turned off the switch to our 100′ solar lights: