Limitations Of Frequency Assignment Plans.

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

versa.

This

would

practically

eliminate

interconnection

possibilities with ABM plans, if both XY and ABM plans were used in

both broad bands of frequencies; however, interconnections can be

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

often encountered.

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

encountered.

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.

c. The

sensitivity

and

selectivity

and

other

technical

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

critical consideration.

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

and receivers.

Also because of this, proper antenna orientations

will have to be maintained between all antennas.

The required

frequency

separations

between

transmitters

and

receivers

is

not

the

same

for

all

radio

sets.

Also,

when

the

receivers

of

some

multichannel

radio

sets

are

located

94