the Radio Set An/GRC-103(V)(*) in this lesson as this is the latest
type of radio set used with pulse code modulation multiplexing
equipment in multichannel radio systems.
Also the frequencies used
in this lesson to illustrate the various frequency allocation plans
will be that of the AN/GRC-103(V)(1)(BAND I 220.0 to 404.5
megahertz).
46. Interference Between Transmitters and Receivers.
a. The purpose of any frequency plan is to minimize the effects
of mutual interference. With a radio set operating full-duplex, the
transmitting power is usually millions of times greater than the
power of the received signal.
The receiver must be able to reject
the transmitted signal and select the weak signal to which it is
tuned.
b. In order to accomplish effective rejection of a transmitted
signal, a receiver must be separated from the transmitter by so many
frequencies according to the type of radio set being used.
c. This type of separation is called a guard band.
When using
the AN/GRC-103(V)(*), the transmitting frequencies at a station must
be well separated from the receiving frequencies to guard against
transmitter-to-receiver interference.
Accomplish this by setting up
a frequency guard band of 16.5 megahertz (33 channels) between the
transmitter and receiver frequency.
d. The receiving frequencies at a station must be well separated
from each other to guard against receiver-to-receiver interference.
Accomplish this by operating receivers so that at least three unused
frequency channels (1.5 megahertz) separate the operating frequency
of any two receivers.
e. Two methods that may be employed to reduce the required
frequency separation from 16.5 megahertz (33 channels) to 5 megahertz
(10 channels) are:
(1) Cross-polarization.
Sometimes antennas on neighboring
antenna may be cross-polarized to advantage.
Cross-polarization
consists of having one antenna polarized in one plane (horizontal or
vertical) and the other antenna polarized in the other plane. Cross-
polarization results in a greater isolation between antennas on the
same mast or neighboring masts and permits the separation between
transmitting and receiving frequencies on these antennas to be much
smaller than with noncross-polarized antennas (figure 17).
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