Trevor Hughes

USA TODAY

BOULDER, Colo. — A few years ago, retired electrical engineer Tom Thompson noticed it was getting harder and harder to hear his friends across the country talking to him on their ham radio sets.

So Thompson built a portable antenna system so he could walk his neighborhood and track down whatever was interfering with his radio transmission. The culprit? Marijuana grow operations, whose powerful grow lights can emit interference blocking radio broadcasts on the ham and AM spectrums.

The first grower he encountered wasn't pleased to know Thompson, now 73, could tell exactly what was going on. "He said 'what are you going to do, call the cops?' " Thompson said. "And I said, well no, it's a federal matter."

With 22 states and the District of Columbia allowing medical marijuana, and Colorado and Washington permitting recreational use, there's been an explosion in the number of people growing their own pot, much of it indoors. With that growth has come increasing interference from the grow lights, which suck down huge amounts of electricity to shine upon budding marijuana plants. Growing pot indoors is usually more secure and gives the grower more control over light, water and insects, which results in higher-quality plants commanding a premium price.

The interference problems from one type of system have gotten so bad that the amateur radio association, ARRL, filed a formal federal complaint on behalf of the country's 720,000 licensed ham operators. The problems are worst in Colorado and California, said Sean Kutzko, an ARRL spokesman.

The interference is caused by what are known as "ballasts," electronic systems controlling the grow lights. Unless they're properly shielded, the ballasts can throw off a wide range of interference. For ham radio operators in the area, it's like trying to have a conversation during an intense thunderstorm.

"We're not concerned about what people are using the grow lights for," Kutzko said. "But we're seeing numerous cases ... and that's causing us a problem. We just want to make sure the manufacturers are in compliance with FCC laws."

The Federal Communications Commission has the power to regulate anything that interferes with licensed radio transmissions, such as ham sets, but also cell phones and AM radios. It often sends letters to people suspected of causing interference, and also can send agents out to knock on doors, Kutzko said.

In a statement, FCC spokeswoman Kim Hart said she couldn't address the specific complaint filed by AARL, but said the FCC is aware of the problems caused by certain grow lights.Thompson said he's also tracked down interference from traditional halogen lamps and even a neighbor's camcorder.

Thompson said he recognizes that federal regulators probably have better things to do than force marijuana growers to change their lights, so he found his own solution: He created a $20 cable shield he gives out to anyone whose operation is interfering with his radio.

"If I can track this down, anybody can track this down," he said. "If I listen long enough, I can tell when they turn the lights off ... you can tell exactly when the harvest is."