Demonic Radio Possession

(Friday, July 14, 2023)

Tags: blogs science weather

It was a typical Friday afternoon outing, into a slightly more urban area to run an errand. It was hot (95F) and a little humid, near the tolerable maximum in a moving car. But it's been significantly worse to the south in Texas. On the drive, I was listening to a local NPR station in Norman, OK, listening to the latest goings on concerning tribal land management.

Over the last few days during this nasty bit of heat, the station had not always come in clearly. Atmospheric conditions or somesuch. But today, as I neared my destination, it suddenly set a new bar. It wasn't just interference...a completely different station was playing loud and clear. Clear enough that the info in the carrier wave could be parsed and displayed.

SMOOTH, it said, to the rhythms of mild jazz.

Curious, I looked it up. I knew that KGOU was roughly 20 miles away. At that moment, it was 23.5 miles (39.5km) away. This other station (KRNB) was...142 miles (229km) away? Knowing a bit of the relevant physics didn't stop this from surprising me. The radio was...with extreme clarity...picking up a station six times farther away. And doing it so well it was as if the local station didn't exist.

The "physics" in the case was a temperature inversion, which in turn causes tropospheric propagation. Both of these articles are interesting reads, but the short version is that a temperature difference at different altitudes can cause the line between those temperature layers to act like a mirror to radiation. This causes more of the radiation (radio waves in this case) to reflect back toward the ground instead of into space. The reflection effectively increases the range of that signal.

Wikipedia handily provides the station class for both stations. KGOU is Class A with 6,000 watts (the weakest allowed for FM broadcast), while KRNB is Class C with 93,000 watts. The FCC provides a handy chart of the ranges based on class. This gives KGOU an optimal range between 10 and 17.6 miles (16.2 and 28.3km), and KRNB between 42 and 57 miles (67.7 and 91.8km). Attenuation is a beast! Note that radio waves travel and can be detected well beyond these values. From the FCC:

A service (or interfering) contour may be visualized by imagining a rough circle surrounding a transmitter site at some distance, where the circle represents a certain field strength value, with greater electric field strengths inside, and lesser electric field strengths outside. FCC

For both stations I was outside the contour, but am still always "in range" of the closer station. The signal was weaker, but still adequate for the radio to tune. What we had here was a case where...mostly due to horizon and line of sight...a stronger signal I normally could not receive came through loud and clear and simply overpowered the local one.

The heat dome that's making people here miserable at least is scientifically interesting.