b. If adjacent broad bands of frequencies are divided into six
blocks each, it is necessary to avoid transmitter-to-receiver
interference between blocks VI and VII (VII is block I of the
adjacent group). A station using way X in one of the board bands of
frequencies should use way Y in the adjacent broad band, and vice
possibilities with ABM plans, if both XY and ABM plans were used in
made by placing a guard between the two broad bands of frequencies
and by using way X at the same stations in both bands, or by using
the XY plan in only one of the two broad bands of frequencies.
51. Limitations Of Frequency Assignment Plans.
a. The frequency assignment plans that have been discussed in
this lesson have their limitations.
Generally, a simple two-block
frequency plan will work for any simple isolated system, regardless
of the type of multichannel radio sets used; however, when
frequencies are needed for complex network with more than two
transmitters and receivers at the same location, new problems are
b. The ABM plan, double ABM plan, and the XY plan are not the
answer to minimizing all of the mutual interference problems
encountered within all complex networks.
When the frequencies used
with some radio sets are broken down into blocks according to the ABM
and other plans, and the required frequency separations for the
particular type of multichannel radio sets are maintained, problems
with harmonics (multiples of a base frequency) are sometimes
This problem can become even more complex in a
sophisticated frequency diversity troposcatter network, where the
baseband signal is applied to two or more transmitters operating on
different frequencies and is received on two or more receivers.
characteristics of multichannel radio sets differ from set to set.
When two or more systems utilize the same site, frequency separation
between transmit and receive antennas on different masts becomes a
A receiver acted upon by certain base or
harmonic frequencies from a transmitter located at the same location
will encounter mutual interference.
Because of this, a required
frequency separation will have to be maintained between transmitter
Also because of this, proper antenna orientations
will have to be maintained between all antennas.