Figure 2-5. Intermodulation products occurring at a transmitter.

TM 11-490-5

Figure 2-5. Intermodulation products occurring at a transmitter.

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4. Coupling of transmitter signals which result

Higher order frequency intermodulation products are

in cross modulation and intermodulation may occur

usually not detected, even though they are present,

through radiation from one transmitting antenna to

because they are of a very low power level.

another and conduction by means of the normal

(2) Broadband interference.

Broadband

transmission line; through conduction from one

interference is characterized by noise emission which

transmitter to another due to improper grounding or

extends over a wide frequency range. One means by

bonding; or through radiation from a transmitter antenna

which wideband interference is produced is the periodic

or improperly shielded, bonded, or grounded transmitter

opening and closing of a current-carrying circuit, which

to another transmitter which is also improperly shielded.

causes sudden voltage changes (signal transients

Frequently, it has been found that poor or rusting

containing steep wavefronts). These transients are

connections on guy wires for antenna supports or power

composed of many frequencies that are even multiples

lines can cause serious interference problems. As has

(harmonically related) of the occurrence rate of closing

been shown, intermodulation products may be produced

and opening the circuit. Undesirable responses appear

at many sum and difference frequencies when the

in a receiver throughout a broad tuning range when

fundamental frequencies of two transmitters are present

signals at these frequencies are radiated and detected.

in the same nonlinear element. Interference may also

Interference sources of this type are discussed in (a)

occur when three signals mix in a nonlinear element.

through (h ) below.

The sum frequency of two of the signals may produce a

(a) Powerline. Powerline interference is a result

difference frequency with a third signal. Of importance

are the intermodulation products which fall in the same

of any sudden change of voltage which may accompany

surges, corona, sparking, or the interruption of an

or adjacent channel of the band in which the desired link

electric power circuit. Typical sources of powerline

equipment is operating.

These are the products

resulting from such combinations as:

interference are: insulators

2-8