[pct-l] Radio Propagation (was: Music On The Trail)

[stevesgt at effable.com (Steve Sergeant)]

This seems pretty OT for the PCT-L, since but I can answer this in some
depth...

Mike Saenz typed:
> It's my understanding that AM wavelengths travel further than FM. True?

It's a drastic oversimplification, but it's frequently true.

The system used to transmit AM radio, Amplitude Modulation, encodes the
audio in the strength of the radio signal. So the quality of sound you get
is very directly related to the strength of the signal you're receiving.
Since the AM radio is transmitting lower power when the sound is quieter,
stations process the audio to be as loud as possible all of the time. But
too much of that starts to make the sound, and especially music, less
intelligible, so it's a trade-off.  Also, nobody has yet been able to
build an AM receivers that doesn't pick up some unrelated electrical
noise, such as distant lightning, solar flares, and other natural
phenomenon, as part of the radio signal.

The AM broadcast band's radio waves alternate at a frequency of 510,000 to
1,800,000 Hz. Radio waves travel at the speed of light, 186,000 miles per
second. So the length of a wave of a typical AM radio station's signal, at
say 1,000,000 Hz, would be a little less than 1,000 feet long. Though not
strictly accurate, we can say for the sake of this discussion that a
signal is more likely to be blocked by an object larger than the wave than
it is by an object smaller than the wave.

The system used to transmit FM radio, Frequency Modulation, encodes the
audio as variations in the transmitter's frequency. That means that the
transmitter is sending it's full power at all times. The receiver is
comparatively quite immune to electrical noise, such as lightning, because
it's just trying to follow the wave of the transmitted frequency. The
audio quality of FM radios only degrade when the radio isn't getting
enough of the transmitted frequency to stay "locked" onto it.

The FM radio broadcast band's radio waves  alternate at a frequency of
88,000,000 to 108,000,000 Hz. So the length of a wave of a typical FM
radio station's signal, at say 100,000,000 Hz, would be about 10 feet
long. Much smaller objects can block this wave.

> I've always seemed to get more AM stations than FM stations while
> driving in remote locations...

To further complicate all of this, different layers of the atmosphere can
reflect (bounce) different radio frequencies at different times, or not at
all. Clouds, temperature inversion layers, jet streams, and turbulence in
the outer atmosphere caused by "solar wind", can all contribute to this
effect. This is called "skip."

As another poster mentioned, conditions are often good at night to bounce
the long waves of the AM broadcast band over greater distances. In fact,
this is so common that the FCC requires most AM radio stations to change
their transmit power and antenna directionality at night.

Signals in the FM broadcast band are more likely to travel simply
line-of-sight from the transmitter. But they can bounce too -- I sometimes
get Sacramento or Fresno FM stations in San Jose when the clouds are at
just the right height and density.

FM-band signals can bounce off buildings and mountains as well. While
camping in the western lowlands of Lassen NP, I used a small, low-power,
hand-held ham radio to talk to someone in Davis, CA through a repeater
(relay station) in Chico. My signal was bouncing off Mount Lassen!

If you aim a sensitive antenna correctly, you can sometimes pick-up
FM-band signals that have bounced off the moon.

Is that more than you want to know?