CO Ham Tracks,Resolves Interference from Pot Cultivators' 'Grow Lights':
The ARRL Letter
June 12, 2014
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Colorado Ham Tracks Down, Resolves Interference from Pot Cultivators' 'Grow Lights':
The ARRL already has complained
to the FCC that so-called "grow light" ballasts can generate severe
interference on the HF bands. According to a recent article in The
Coloradoan, retired electrical engineer Tom Thompson, W0IVJ, first
noticed interference on 40 meters at his location in Boulder a couple
of years ago. So, he coupled his own portable receiving loop
http://tomthompson.com/radio/ReceivingLoop/loop.html with a
direct-conversion receiver that he could use to walk around his
neighborhood and pin down noise sources. In at least one instance, the
problem emanated from a domestic marijuana-growing operation -- a "grow
"With the increase in legalized medical and recreational marijuana
comes an increase in RFI due to electronic grow light ballasts,"
his website, where he describes how he constructed a filter that
considerably reduced interference from the devices. "These ballasts are
usually switching power supplies, capable of lighting 600 to 1000 W
high-pressure sodium or metal halide lamps," Thompson said. "The
switching frequency is usually 50 to 70 kHz and is rich in harmonics."
Thompson said that because the light fixture is separated from the
ballast by about 25 or 30 feet of wire -- approximately a quarterwave
on 40 meters -- the RFI may be strongest on that band. "I have heard
radiations from these systems up to about one-half mile away," he said.
"When the [marijuana] plants are young, the lights are on 24/7. After
about 2 weeks, the lamps are on for 12 hours, and off for 12 hours."
Thompson said that since most systems are on a timer, it's possible to
predict when the RFI will start, once you have determined the initial
Thompson said one of the interfering growers was nice enough to loan
him a lamp ballast for testing, and he was able to get a used lamp for
free from a local grow shop. He gives away the common-mode choke
filters to owners of offending lighting systems.
As the article in The Coloradoan pointed out, with 22 states and the
District of Columbia now allowing medical marijuana, and Colorado and
Washington permitting its recreational use, "there's been an explosion
in the number of people growing their own pot, much of it indoors." The
noise problems are reported to be worst in Colorado and California.
Thompson told The Coloradoan, "If I can track this down, anybody can
track this down. If I listen long enough, I can tell when they turn the
lights off...you can tell exactly when the harvest is."
Thompson has written an article on the topic of tracking down and
resolving such interference. It is scheduled to appear this fall in
The ARRL Letter
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