close together with reference to the unit under test. The output of the system under test (an amplifier,
receiver, or transmitter) is monitored on a spectrum analyzer that is comparable in characteristics to the
suspect system. The resultant display should be an exact reproduction of the input frequencies. If not,
some form of intermodulation distortion is present. To determine if external sources are causing the
intermodulation distortion, you can use a single-frequency signal. If the display on the spectrum analyzer
does not show the single frequency, then intermodulation distortion is present.
Intermodulation distortion cannot be entirely suppressed, but it can be minimized by shielding
components and circuitry, parasitic suppression circuitry, and antenna spacing. These factors are
incorporated in the design of the system and are tested during production. Any shields or parasitic
suppressors that are removed by the technician must be replaced before troubleshooting and/or repair can
be effective. Antenna locations also pose a consideration when installing a new system. Ship alteration
specifications must be observed when new antenna systems are being installed.
When you are testing a piece of equipment for intermodulation distortion, what should the
output of the equipment look like?
The important points of this chapter are summarized in the following paragraphs:
STANDING WAVES are the result of an impedance mismatch between a transmission line and its
load. If a transmission line is not properly terminated, it will cause a percentage of the transmitter power
to be reflected back to the source. The reflected wave or standing wave will increase in magnitude as the
mismatch becomes greater.
VSWR refers to the voltage ratio of the incident wave (that which is transmitted to the load) and the
reflected wave (that which is reflected by the load back to the transmitter). An ideal vswr is considered to
be 1 to 1.