Marshall Amplification Field Marshall 100 Owner's Manual page 30

Field Marshall Tracking Receiver
Terrain Absorption
As radio waves pass through objects they diminish in strength. The
effect is most noticeable in wooded areas where vegetation saps
the strength of your signal. The further it goes through a forest the
greater the loss. Fog, clouds, snow and rain also absorb radio
waves, lowering your transmitter's range. Your Marshall system,
between 173 and 220 mHz, avoids the absorption problems that
are more pronounced at higher frequencies.
Radio Shadows
The biggest obstacle to radio waves is the earth itself. The range of
a telemetry system is limited first and foremost by the horizon. The
curvature of the earth creates a circular area around the transmitter
where you can pick up the signal, the so-called line-of-sight radius.
The actual radius depends on the elevation of both the transmitter
and the receiver.
what it would be if the earth were smooth. Every kind of terrain pro-
duces "radio shadows". As you move around you can go in and out
of shadows, even picking up a signal much further away from a
spot where you picked up no signal.
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This diagram can give you
an idea of the line-of-sight
distance. To use it compute
the distance for the height
of the transmitter and your
own height separately, then
add them together. For
example, if your transmitter
was on a hill 25 feet high
and your receiver was 8
feet high, the line-of-site
distance would be 11 miles
(7 + 4 miles).
You can see that if your
transmitter is on flat ground
and you are standing on the
ground, the range can be
just a few miles. Irregulari-
ties like hills reduce the line
-of-sight to even less than