Radar System Engineering

SEC. 5.6] SIMPLE DOPPLER SYSTEM 135
simplest possible indication method, a power gain of roughly 15 to 1 over
what might be had by using voltmeter or oscillograph can be achieved.
Aural indication is therefore the logical choice.
This decision, together with the maximum target speed, almost
determines the wavelength, for the doppler frequency must be chosen
to lie in a range for which the ear is sensitive. Thus A = 10 cm gives,
by Eq. (l), 8.9 cps/mph or about 3000 cycles for 300 mph. At the time
this system was designed, faster planes were not common. Even so,
a somewhat longer wavelength might have been desirable, but 10 cm was
chosen because good tubes were available at this wavelength.
This wavelength proved satisfactory, but it sometimes gives doppler
frequencies rather below the frequency region in which the ear is sensitive.
To overcome this difficulty, provision was made to modulate the audio
signal with a 500-cps tone. The modulator was a balanced one, so that
the 500-cPs carrier outp-s zero when the signal was zero. The resulting
variations in amplitude of the tone were quite distinctive, even m the
presence of the noise modulation, and carried the effectiveness of the
system down to frequencies as low as desired.
The wavelength being determined, the antenna size was chosen.
To get as much range as possible, this was taken as large as was feasible
without either (a) making the device impractically large or (b) getting
the beam so sharp that at the specified angular rate of scan the beam
would be on the target too short a time for the listener to hear it. Both
requirements led to a diameter in the neighborhood of 40 in., the value
finally chosen.
It now remained only to estimate the power required. This could
be done either on the basis of experience with similar systems or by
calculations of the sort outlined in Chap. 2. As a result of such considerations,
a power of 10 to 15 watts was chosen.
This completes the discussion of the basic design of the system. A
few of the apparatus details will be given later but first various points
will be discussed which are not mentioned in Chap. 2 but which are of
importance in computing the range. These all relate to the effective
bandwidth, the geometrical factors that determine the transmission
attenuation being the same for pulse and c-w systems.
E~ectioe Bandwidth .—IVhat then, in principle, determines the minimum
bandwidth of a doppler system?
If we use a simple indicator, such as a voltmeter, the bandwidth
must be sufficient to include the doppler frequencies of all targets of
interest. This bandwidth is then determined simply by the wavelength
and the range of target velocities.
If, however, we consider only targets of one radial velocity, or use
some more complex indicator that divides the possible doppler range