I park my car in an underground, cinderblock-walled garage. We all (hopefully) learned in high school that FM waves propogate only using line-of-sight, while AM waves can propogate through the ground, or bounce off the ionosphere…so why is it that in my garage, I get FM stations just fine (though there is clearly no line-of-sight to the outside, much less the transmitter) but AM is all static?
Seth
AM works best at much lower frequencies, and lower frequency electromagnetic waves get absorbed by concrete and metal much easier. The higher frequency FM waves penetrate a lot easier.
So there is a direct line of site, it just goes through lots of walls and stuff. 
Or it’s the other way around…don’t remember at the moment.
Size matters: An AM radio wave (at 1 MHz) is 100 times larger than an FM radio wave (at 100 MHz). That’s why AM radio stations fade when you go under an overpass: the AM radio wave is too big to fit.
AM: _ _ _
\ / \ / \ /\
\ / \ / \ / \
\ / \ / \ / \
\_/ \_/ \_/ \_/
FM: /|||\/||\||/|||/\\\////|\\/\///\\/
A “not to scale” representation.
Bizerta and Silo have it right. It’s not that the wave is AM or FM, but the relative wave lengths of where the AM Broadcast Band and FM Broadcast Band have been allocated space in the spectrum. Lower frequency, longer wavelength, signals can bounce from the ionosphere back to earth (skip) while the higher frequency, shorter wavelength, signals do not skip. They just pass right through. Line of sight gemeraly means reception without having to skip off the ionosphere.
When I lived in New York City and rode the subway, an AM broadcast band radio was useless while an FM broadcast band radio would usually work. I guess the tunnels worked as a waveguide.
An AM broadcast receiver will often not work in an office building while a FM broadcast receiver works fine. More “holes” for the signal to find it’s way through at the shorter wavelength.
An AM signal is far more easily compromised than an FM one.
If a signal is weak the gain of the reciever will usually adjust itself to compensate.
If you have a weak and dirty signal the AM reciever will not lock on and increase its gain but the FM reciever can .
You may have flourescant lights in your garage and these often interfere with AM signals but not FM ones.
FM can be odd in that you can recieve it where you think you ought not and vice-versa.This can often be due to signals taking a bounce path route to the antenna off buildings and other structures.If two routes have the same length they can even cancel each other out creating a dead zone of literally inches across.
This is true. Sometimes when the weather is just right, I get AM stations from far off… I’m in central texas, and i’ve gotten radio from mexico… where i’m lucky to get an fm station from san antonio.
This is probably due more to the, uh, lax Mexican broadcast standards than any sort of climate reason. In the U.S. even “clear-channel” AM radio stations are limited to a max signal of 50kW.
South of the border, however, there’s either no such regulations, or they’re very lax. As a result, there’s several incredibly powerful AM stations (somewhere in the range of 200-500kW, IIRC) sitting in border towns like Tijuana and Nuevo Laredo, pumping their signals back into the U.S. Some can be heard across large swaths of the eastern half of the country at night, when AM signals propagate better.
I remember listening to a Tijuana one that was an all-English sports station(!) when I was in Arizona - I believe it was XERC 690, and there’s another powerful one in Monterrey on (I think) 820. Maybe someone who’s got a copy of the World Radio TV Handbook can correct me on those two if I’m wrong…
I was watching Pop-Up Video on VH1 one day when they played that song by Wall of Voodoo. (I think it’s just called “Mexican Radio”, but don’t quote me on that.) One of the little facts that popped up was exactly what scheif said; Mexican radio stations are allowed to broadcast on much more powerful signals than American ones. I thought it also said something about that practice being curtailed in recent years, but apparently some stations are still beaming long-range.
AM signals sure do propagate better at night. When we had the L.A. earthquake in 1994 the only way I could get news
was on the car radio, and the easiest station to tune in
was KNX in San Francisco! I’m sure the absence of electrical interference didn’t hurt, either.