A radar transmitter in good condition should produce a spectrum curve similar to the curves shown
in view A or B in figure 4-3. Good curves are those in which the two halves are symmetrical and contain
deep, well-defined minimum points (minima) on both sides of the main peak.
Figure 4-3.Comparison of radar spectra.
A curve without well-defined minima, as in the curve shown in view C, indicates that the transmitter
output is being frequency modulated during the pulse. This condition may occur when a pulse without
sufficiently steep sides or a flat peak is applied to the transmitter. It may also occur when a transmitter
tube is unstable or is operated without proper voltage, current, or magnetic field.
An extremely irregular spectrum, as in the curve in view D, is an indication of severe frequency
modulation. This condition usually causes trouble with the receiver automatic frequency control (afc) as
well as a general loss of signal strength. You can often improve a faulty spectrum by adjusting the
transmission line stubs or by replacing the transmitter tube. When the spectrum has two large peaks that
are quite far apart, it indicates that the transmitter tube is DOUBLE MODING (shifting from one
frequency to another). This could be caused by standing waves in the transmission line or a faulty
transmitter tube. Standing waves may be caused by a faulty line connection, a bad antenna rotating joint,
or obstructions in the line. (Standing waves are described in NEETS, Module 10, Introduction to Wave
Propagation, Transmission Lines, and Antennas.)