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 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 house.” 

“With the increase in legalized medical and recreational marijuana comes an increase in RFI due to electronic grow light ballasts,” Thompson explained on 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 “on” time.

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.

“We’re not concerned about what people are using the grow lights for,” ARRL Media and Public Relations Manager Sean Kutzko, KX9X, told The Coloradoan. “We just want to make sure the manufacturers are in compliance with FCC [rules].”

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 and resolving interference caused by lamp ballasts. It will appear in QST this fall.