Thursday, March 02, 2023

The AM radio band has unique propogation characteristics

At the base of KWSU AM radio tower near Pullman. Out in the Palouse wheat fields west of town.

Someone recently posted that some car manufacturers are not including the AM band in new car radios. Folks are worried because AM might be the most reliable source of information in an emergency.

I got to thinking about the AM band. It's at the low end of the frequency spectrum, just above the obscure longwave band. It's the only band that lots of old radios could get as FM didn't get started till the 1940s. AM dominated clear up to the 1970s.

One big advantage of AM is that the signal is less confined to "line of sight" propagation. AM radio waves can bend more easily over hills, around buildings and into shadow areas. It can reach areas that are often missed by the higher frequencies of FM radio, cellphone or satellite.

At night, it can bounce off the ionosphere for distances of over a thousand miles without needing either a satellite or the internet.

Most people use the higher frequency, more line of sight, signals like FM or cellphone. If there aren't cellphone towers in the area, or one is in a shadow, the higher frequency signals can drop out. AM is a bit more stable, in that regard, but it has it's disadvantages as well.

The AM band's low frequency signals don't carry as much information as higher frequency channels. They are lower "bandwidth" so lower fidelity of sound, plus less ability to send more things. Some digital FM stations offer several subchannel programs. Maybe one station, on the dial, offers more than one program, such as "KUOW, 1" and "KUOW 2," for people with special "HD" radios.

Another big problem is the static. FM is modulated differently than AM so it picks up less static. These days, there is a lot of static from all the electronics in homes. AM often can't be heard over the static. Even phone and power lines can make static along the road.

Another problem is that most of the AM stations are owned by just a few corporations and the programming tends to offer little variety. A few conservative networks tend to dominate the market.

Many radio stations don't bother to produce their own local programming. To save money they just repeat network stuff that's on hundreds of stations coast to coast, or they just have automated music formats.

Pullman's KWSU is one of the few college educational stations on the AM band. Most educational stations use FM. These days, even KWSU is mostly just stuff off a network as well; The NPR Radio Network.

I like NPR, compared to the mostly conservative stuff on commercial stations, but it's still just from the national network. I listen to NPR from FM stations, here in Bellingham, one of them basically repeats what's on KWSU from Pullman. It's on FM, here in Bellingham; The "News and Information Service of Northwest Public Broadcasting."

Some NPR stations do produce a few of their own local shows. For instance KQED in San Francisco. I listen to some of those shows on the internet.

AM towers don't need to be in high places as the signals can follow contours of the land better than higher frequency signals. AM towers are often out in fields where there is plenty of room. The tower may be tall, but the whole tower is usually the radiator for those longer wave signals.

FM and TV towers can be tall, or on high places such as mountains, for longer reach with line of sight. The radiating antenna is usually just at the top of the tower.

Cellphone towers tend to be shorter as the signals don't have to reach that far. The cellphone system just passes one off to the next tower if one gets out of range. There are usually lots of celltowers in any given area, or coverage becomes spotty.

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