4-13
Standing waves that are present on a transmission line can be used to determine the
TRANSMITTER FREQUENCY. Voltage or current peaks are present at half-wavelength intervals. By
measuring the distance between peaks, you can compute frequency mathematically.
TWO-WIRE, PARALLEL TRANSMISSION LINES are usually tested for standing waves with
test devices that are inductively coupled to the line. These test devices vary greatly in their complexity,
ranging from bridge circuits to simple neon lamps.
INSERTION LOSS MEASUREMENTS are performed by injecting a signal of a known amplitude
into a transmission line and then monitoring the signal at the far end of the cable with a power meter.
Loss measurements must be taken at various frequencies to determine if the transmission line is good
across its frequency range.
The most common cause of INTERMODULATION DISTORTION is improper spacing of
transmitters and receivers. CROSS MODULATION is common to equipment that is misaligned.
Intermodulation distortion can be tested by injecting two signals (different frequencies) into a piece of
equipment and then monitoring its output for distortion using a spectrum analyzer. Intermodulation
distortion is usually caused by improper antenna spacing or by poorly shielded components or circuits.
REFERENCES
EIMB, Test Methods and Practices Handbook, NAVSEA 0967-LP-000-0130, Naval Sea Systems
Command, Washington, D.C., 1980.
NEETS, Module 10, Wave Propagation, Transmission Lines, and Antennas, NAVEDTRA 172-10-00-83,
Naval Education Training and Program Development Center, Pensacola, Fla., 1983.
SWR Meter 415E, NAVSHIPS 0969-139-2010, Hewlett-Packard Co., Palo Alto, Calif. 1968.
