Earlier this week the Wall Street Journal published an article about the growing prevalence of RFI from unlikely sources. They quoted Tom Thompson, WØIVJ, of Boulder, Colorado:
Radio hobbyist Tom Thompson of Boulder, Colo., last year tracked a signal using a homemade contraption. After knocking on the suspect’s door, he traced it to ballasts on marijuana grow-room lights. He says he built a filter that the grower agreed to use.
Tom, a retired NOAA electronic engineer, created an entire web page devoted to the grow light ballast filters. According to his page, these ballasts are switching power supplies for high-pressure sodium or metal-halide lamps.
He goes on to describe the problem in good detail:
Typically the light fixture is removed from the ballast by about 25 to 30 feet. Since this length is about a 1/4 wavelength on 40 meters, it makes a good “antenna” for 40 meters so the RFI may be strongest on 40. I have heard radiations from these systems up to about 1/2 mile away. Also, since the “antenna” is so low to the ground, the RFI is mostly vertically polarized which is important when tracking it down.
Check out his page to see how he built a filter that he says reduces the RFI by about 30dB. That’s some good ol’ ham ingenuity!