Taking ‘pot’ shots at grow light RF interference

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!

Matt Thomas, W1MST, is the managing editor of AmateurRadio.com. Contact him at [email protected].

9 Responses to “Taking ‘pot’ shots at grow light RF interference”

  • Richard KWØU:

    Hey, that’s great. Given that this is going to begin a bigger issue, maybe Tom could commercial market these filters.

  • Phil N4LNE:

    I have to ask why the lights were not reported to the FCC so they could force manufacturer compliance like they do with electric fence chargers and other devices that cause interference. I applaud the solution to the problem but the devices should not be allowed to cause interference in the first place.

    My .02 worth

    Phil N4LNE

  • Michael. N1VSU:

    This interference will be a good way to track illegal pot grower giving up there location .

  • Joe KB3PHL:

    Well, now you just told illegal pot growers how to shield their grow lamps from tracking by law enforcement!

  • Joshua KG7BGZ:

    Smoked ham?

  • Mike KF7VZZ:

    Washington has plenty of hams and new “legal” pot growers. I smell a market for filters! Colorado too.

  • David WB4ONA:

    “He says he built a filter that the grower agreed to use.”

    A very dangerous thing to do! From an all-controlling Government that has grown out of control to greedy Trial Lawyers lurking behind every corner, you DO NOT want to be putting yourself at risk like this.

  • Todd KT0dd:

    Yep, here in Colorado I have just developed this problem with a neighbor somewhere growing pot,I just don’t know who it is. Too bad Pot is legal here, and the FCC is basically useless enforcement wise. The guy may be legal or illegally growing more than 3 plants but I can’t prove it. I just hope he gets his stupid crop done and the noise goes away.

  • Thomas King,W1UKO:

    Here in Mass. we now have home growers for the first time. I am practically dead in the water with the S9 plus rasping 60 hz interference. Very bad on 20 and 40 meters!

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