interference is most prevalent near densely populated
areas and industrial districts, or within any other region
selectivity at the receivers). Note that, for the purpose
where many transmitters and other interference-
of this discussion, a channel is defined as an assigned
producing equipments are operating simultaneously.
frequency range of a given portion of the RF spectrum
Also included in this category are such ancillary
and may include from 1 to 2700 audio channels.
equipments as electric motors and generators. This
type of interference is grouped into two categories:
narrowband and broadband.
(b ) Harmonics of the transmitted frequency and
other spurious emissions. Harmonics of the transmitted
frequency and other nonharmonic spurious emissions
interference sources consist of cochannel, adjacent-
are always present to some degree in the radiated
channel, harmonic, and other signals at distinct
output of a transmitter. Well-designed transmitters,
frequencies as described below:
maintained in good operating condition and properly
loaded into antennas, will radiate only very low-level
spurious emissions. Improper tuning, loading, and
(a) Cochannel and adjacent-channel operation.
maintenance, and faulty transmitter and antenna
A transmission from a distant station, although fully
elements can cause normally low-level spurious
authorized, may become a source of interference if it is
emissions to increase to extremely high levels. These
unintentionally received along with a desired signal.
signals are primarily generated in the frequency
This situation is usually prevented by assigning
multiplier or RF power amplifier stages of the
cochannel frequencies to equipments only with
In addition, harmonics and other
adequate geographical separation. Adjacent channel
nonharmonic spurious emissions of the transmitter local
operation may require some geographical separation
oscillator (LO) may be radiated.
An example is
also; however, receiver selectivity usually provides
presented in figure 2-2, which shows the transmitter
rejection of undesired adjacent channel signals.
spurious emission measurements on an AN/GRT-22
Conditions which cause cochannel and adjacent channel
Frequently, interference is caused by
interference, then, include unusual propagation
spurious emissions from a transmitter, rather than from
conditions (which cause distant transmitters to appear to
a transmitter operating on the desired frequency.
be local RF sources); frequency spectrum crowding
Frequency allocation plans usually prohibit two
(which may result in adjacent channels being occupied
transmitters operating on the same frequency if the
by transmitters at power levels that override receiver
antenna patterns and service areas are such that
selectivity); use of excessively high, unauthorized power
interference could be caused.
levels; and lack of proper maintenance procedures
(which can result in off-frequency