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Why is there so much interference at night in the signal of WKXB 99.9 FM from a Raleigh station?

John Peaspanen
StarNews

Q. Why is there so much interference at night in the signal of WKXB 99.9 FM from a sports station in Raleigh? Around Hampstead, the Raleigh station is overpowering our local station at night. Are the two stations on the same frequency? It appears the bleed in of the signal from WCMC-FM, Boiling Springs Lake, NC, into the signal space of WKXB-FM is seasonal and unintentional.

A. It appears the bleed in of the signal from WCMC 99.9 FM, of Holly Springs (near Raleigh), into the signal space of Wilmington’s WKXB 99.9 FM is seasonal and unintentional.

According to Brian White of WKXB, the curvature of the Earth and current temperature patterns allow signals from a distance to be picked up locally.

“It has to do with the temperature inversion recently,” said White. “We’re at a peak time for it, with warmer days and cooler nights. The closer you are to the coast, the more it will happen.”

White explained that a FM radio signal travels “line of sight” or straight. The curve of the planet sometimes causes a signal to climb and suddenly drop, instead of making an arching trajectory. Strong radio signals will occasionally get caught in the cooler atmosphere of 30,000-40,000 feet and suddenly drop into areas where the air is significantly warmer close to the ground.

“The signals get trapped in the airstreams,” he said. “It drops down and overtakes the local station.”

White added the phenomenon is a two-way street, as WKXB has been drowning out WCMC 99.9 FM in the mornings. The good news is that this issue is pretty much seasonal and should abate when nighttime temperatures settle into a higher range.

According to the Federal Communications Commission, neither WKXB nor WCMC have made any changes in frequency or signal strength recently. If one station was intentionally broadcasting into the protected service area of the other, an investigation by FCC engineers might ensue to alleviate any infractions. The FCC reports both stations are authorized to operate on the same channel and are of the same class (C2, which is a powerful signal).

RELATED LINKS:

Why are local radio stations 94.5, 103.7 and 104.5 not coming in as clearly as they once did?

When will other local radio stations (besides WHQR) broadcast in HD?

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