Why is the KTBB 97.5 FM signal interrupted by static and other stations?

The problem has nothing to do with KTBB’s signal strength. Our signal strength, like all licensed radio stations, is a function of fixed values that are set forth in the operating parameters section of our license, which is issued by the Federal Communications Commission. No station has the capacity or the authority to unilaterally alter its operating parameters.

Nominally, the FCC sets those parameters so as to provide interference-free service over a station’s city of license. Under normal conditions, KTBB 97.5 FM puts a “city grade” signal over Tyler and Longview and the surrounding communities.

The problem stems from an atmospheric phenomenon called “tropospheric propagation,” — commonly just called “ducting.” Here’s a Wikipedia article on it:

https://en.wikipedia.org/wiki/Tropospheric_propagation

The phenomenon tends to be particularly acute in this part of the world during the summer.

What follows is a greatly simplified explanation.

Tropospheric propagation, or ducting, most commonly occurs when air aloft is warmer than air on the surface – what the weathermen call a “temperature inversion.” Normally, air gets cooler as you gain altitude away from the heat reflected by the Earth’s surface. But when you have a large high pressure dome overhead, (which typically accounts for our miserably hot temperatures in the summer), it is common for air temperatures to warm as you gain altitude. Eventually, as you climb, the air temperatures will start to cool. But in the summer, when winds are light and high pressure systems tend to stall over a particular area and stay in one place, there is frequently a layer of air – a few thousand feet thick — that is warmer than the air on the Earth’s surface – particularly in the morning (more on that in a moment).

This area of warm air aloft creates what amounts to a boundary for VHF radio signals. FM radio broadcasting occurs in the VHF portion of the radio spectrum. Rather than leave the transmitting antenna and radiate straight out across the horizon and into space, radio frequency energy comes into contact with this warm air boundary and is refracted or bent, just as a lens bends light — causing the signal to follow the curvature of the Earth. What is created is effectively a “duct” through which a VHF signal can travel a great distance.

The result is that a radio station from far away will interfere with a station close to home. The interference you typically hear in this area is either from KFNC, the all-sports station at 97.5 FM near Houston, or KLAK, an adult contemporary station at 97.5 FM in Grayson County, near Sherman, Texas. While those stations are interfering with us, our signal is most likely interfering with them. It is a two-way street. The effect on your individual listening depends almost entirely on where you happen to be at any particular moment. When the phenomenon is occurring, it affects nearly every FM station at one place or another within its coverage area.

The phenomenon typically decreases as it gets later in the day and the Earth’s surface warms. When surface temperatures rise, the differential between surface air and air aloft disappears and thus the warm air boundary disappears. When that boundary is gone, the “duct” no longer exists and the radio frequency energy resumes radiating straight out across the horizon and into space.

Ducting is extremely annoying and, unfortunately, there isn’t anything we can do about it other than — in the particular case of KTBB — jump over to the AM band and listen at 600 AM.

Thank you for listening.