Figure 4-1.Transmitter spectrum compared with receiver response.
Any frequency, when modulated by another frequency, will produce a base frequency with sideband
frequencies (sum and difference). In other words, the output of a pulsed radar will contain more than one
frequency. The output frequency spectrum of the pulsed radar transmitter does not consist of just a single
frequency that is turned on and off at the pulse-repetition frequency (prf). Consider the spectrum as a base
frequency (carrier) that is modulated by short rectangular pulses occurring at the prf of the radar. Two
distinct modulating components are present: One component consists of the prf and its associated
harmonics; the other component consists of the fundamental and odd-harmonic frequencies that make up
the rectangular modulating pulse.
The distribution of power over the radar frequency spectrum depends on the amount of modulation.
A pulsed radar spectrum is illustrated in figure 4-2. The vertical lines represent the modulation
frequencies produced by the prf and its associated harmonics; the lobes represent the modulation
frequencies produced by the fundamental pulse frequency and its associated harmonics. The amplitude of
the main lobe falls to zero on each side of the carrier. The side lobes are produced by the odd harmonics
of the fundamental pulse frequency. The zero points are produced by the even harmonics of the
fundamental pulse frequency. In an ideal spectrum each frequency above the carrier has its counterpart in
another frequency below the carrier. These frequencies are equally spaced and have equal power.
Therefore, the pattern is symmetrical about the carrier. The main lobe, of course, contains the major
portion of the transmitted rf energy.
Figure 4-2.Spectrum of a pulse-modulated carrier.