Narrowband interference

TM 11-490-5

interference is most prevalent near densely populated

operation through transmitter frequency drift and lack of

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

(1) Narrowband

interference.

Narrowband

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

transmitter.

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

transmitter.

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

2-3