Radar System Engineering

44 THE RADAR EQUATION [SEC.2.11
respect to this standard all other modes of indication provide varying
amounts of storage gain, by making use, more or less imperfectly to be
sure, of information gathered on repeated pulse transmissions. Thus we
say that Fig. 2.8 describes the effect of scanning on storage gain.
The number of other independent variables, yet unmentioned, which
affect the amount of gain realized through storage is appallingly large.
The more prominent of these must be discussed briefly, if only to suggest
the scope of the general problem treated comprehensively in Vol. 24.
The video bandwidth b, as distinguished from the i-f bandwidth ~, has
hitherto been assumed to be so great as to allow reproduction of the
detected i-f signal without distortion. This is seldom true in practice
and the question arises, how does the video bandwidth influence the
signal discernibilityy, and how narrow can it be without harm? Making
the video bandwidth b considerably less than I/r amounts to putting the
rectified signals and noise through a long-time-constant filter. This is a
sort of aveTaging, or integration process, in effect, the final output at any
instant being an average over several adjacent intervals, each of the order
of r in duration. It is clearly an integration process operating at a loss
rather than at a gain, in contrast to the sweep-to-sweep integration
discussed earlier, for it includes with the signal an unnecessarily large
amount of noise. The result is an increase in signal power required for
detection: for b r, the spot overlaps several such intervals, or better, several such
intervals contribute to the same spot of light on the tube. Now this is
precisely what happened when the video bandwidth was made too narrow,
and we must expect the same consequences. In effect, the intensitymodulated
cathode-ray tube is a low-pass filter, whose bandwidth is of
the order v/d. For example, a radial sweep covering 50 miles of range
cm a 7-in. PPI tube would be written in with a velocity of about 1.5 X 104
cm/sec. If d is 0.1 cm, the bandwidth of the system is about 150 kc/see.