Why You Might Have Problems Hearing KUT's Radio Signal

The FM Broadcast band, being VHF, is characterized by line-of-sight propagation. This means the best signals are received in places that can “see” the antenna, meaning there are no significant obstructions in the path. Austin has somewhat rugged terrain, with deep valleys and tall hills. These terrain features mean there are some places within the city limits where KUT’s signal isn’t very good.

This map shows the visual horizon from KUT’s antenna. Areas shaded green have an unobstructed view of our antenna and will receive a very strong signal. There will be some signal in the un-shaded areas, but it can be weaker. In some cases, much weaker.   

Here is a more representative map of the predicted coverage of KUT’s transmitter over the Austin metro area. The yellow shading represents areas where the signal strength is greater than 70 dBu, which is a very strong signal. This is what the Federal Communications Commission considers a “city-grade” signal. What is interesting in this map are areas where significant obstructions exist. If you live in one of these areas, it is likely you have trouble picking up KUT on clock radios or portable radios, which have less than optimum antennas.   

This map shows the predicted coverage at 60 dBu, which is generally considered the listenable area for car radios and radios with outside antennas.

Anomalous Propagation, or why you sometimes hear that religious station from Houston in the morning instead of KUT.

While VHF propagation is normally characterized by line-of-sight, sometimes the weather conditions are such that signals travel much further than normal. The most common phenomenon that causes this is tropospheric ducting.

Signals that come in via tropospheric ducting can be as strong, or stronger, than the local signal, and due to the capture effect of FM receivers, the stronger signal will be the one heard. These effects normally only last a few hours, and once the atmosphere warms up, the ducts cease to exist and propagation returns to normal.

Tropospheric ducting is most evident on spring and fall mornings, but can happen anytime weather conditions exist that permit the ducts to form in the atmosphere.

Picket-fencing and multipath, or why you hear that “swishy sound” when driving around downtown.

Radio signals can be reflected by buildings and other objects in the vicinity of the receiver. Because of the different path lengths taken by the direct and reflected signals, the signals arrive at different times. The relationship between these times causes the signal to be stronger or weaker depending on whether the waves arrive in-phase or out-of-phase. This phenomenon is called “multipath.” There is really nothing that can be done about it, it’s just one of those things that happens when radio waves encounter reflective objects.