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Why is 89.5 KSBX off the air? The answer is in the atmosphere

KSBX transmitting antenna (right) located on Gibraltar Peak in Santa Barbara.
Flickr/Doc Searls
KSBX transmitting antenna (right) located on Gibraltar Peak in Santa Barbara.

As of today, the Central Coast Public Radio KSBX broadcast in the city of Santa Barbara is off the air. A phenomenon called atmospheric ducting caused far-away radio signals to interfere with ours, rendering KSBX unusable.

Erratic weather and other changes in the atmosphere can cause radio signals to travel much further than usual. In this case, atmospheric ducting has caused a distant radio signal from fellow NPR member station KPBS in San Diego to overlap with ours.

“You get changes in the lower atmosphere when you have these atmospheric ducts, and it changes the way the signals propagate," said Libe Washburn, an oceanographer at UC Santa Barbara’s Marine Science Institute.

Washburn said FM radio signals are normally transmitted in what’s called “line of sight.” This is when a radio transmitter is in a direct line with radio receivers that catch the signal.

ksbx equipment
KSBX transmission equipment on Gibraltar Peak in Santa Barbara.

But he said warm air masses in the lower atmosphere can create these atmospheric ducts, which cause radio signals to bend back towards the Earth’s surface instead of going vertically into space like normal.

This process repeats over and over, which allows the signals to be received by radios far away from the source.

"Normally where the radio waves from at a distant location would be passing way overhead and you wouldn't receive them, you now have a way for the radio waves to follow the Earth's surface," Washburn said. "They can go a lot farther, and I think that's exactly what you're seeing in your radio station from San Diego. They could go hundreds of miles by this process."

Ducting can happen from a variety of factors, and not just over bodies of water but also over land. So while there could be other factors at play in disrupting KCBX’s Santa Barbara signal, Washburn said it makes sense that warmer coastal waters could lead to the San Diego signal overwhelming ours over time.

“The temperature in the ocean would affect the overlying atmosphere, and would change its water vapor in the atmosphere. That really is one of the big controls on how the radio waves propagate, and changing the ocean could change the amount of water vapor in its distribution over the ocean.”

Whether or not climate change is directly causing the ducting issue here, the ocean is projected to keep getting warmer along with the rest of the climate. So, there isn’t a clear way for KCBX or KPBS to fix this.

Even though our broadcast in Santa Barbara is off the air, listeners there can still tune in on digital platforms like the livestream here on kcbx.org or the NPR One app. KCBX News will still cover local news in Santa Barbara, along with the rest of the county where our signal is still going strong, like Santa Maria and Goleta.

Benjamin Purper was News Director of KCBX from May of 2021 to September of 2023. He came from California’s Inland Empire, where he spent three years as a reporter and Morning Edition host at KVCR in San Bernardino. Dozens of his stories have aired on KQED’s California Report, and his work has broadcast on NPR's news magazines, as well. In addition to radio, Ben has worked as a newspaper reporter and freelance writer.
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