Radar System Engineering

SEC.56] SIMPLE DOPPLER SYSTEM 133
that the returned signal will be 10–7 or 10–’ times the transmitted signal
in voltage, it follows that amplitude modulation of the transmitter
would have to be held below this value in order not to be obtrusive.
Though perhaps not impossible this would certainly be difficult.
The easiest way to keep transmitter power out of the receiver is to
use separate transmitting and receiving antennas, and this was the course
adopted. With a moderate amount of work on the antennas, the leakage
from transmitter to receiver can be made to be of the order l(FS or lees
in power. Another power of 10 is canceled by an adjustable leakage
path from transmitter to receiver. Further than this it does not pay to
go since reflection from nearby ground objects contributes a leakage of
this same order.
It has often been suggested that a single antenna would be satisfactory
if a bridge-like system were used similar to that used in two-way
telephone repeaters. Ordinarily however the single antenna is not
satisfactory., For one thing, the increased antenna gain resulting from
greater available dish area is lost because of the power used by the
‘‘ artificial” antenna which balances the real one. More important, since
very slight mechanical changes will spoil a 60-db balance between two
equal voltages, such bridge systems tend to be highly microphonics.
The second modification relates to the intermediate frequency of the
crystal mixer. The reader will have observed that the system is well
described as a superheterodyne with zero intermediate frequency, the
leakage from the transmitter constituting the local oscillator power and
the modulation frequency being the doppler frequency. The only
unconventional feature is that the signal is single sideband. I Of course,
the absolute sensitivityy limit of such a system depends on kT and the
bandwidth of the amplifier. Experimentally, however, this limit is not
even remotely approached because a crystal detector, when passing
current, generates a noise analogous to carbon microphone hiss. This
noise increases with decreasing frequency and is enormous compared
to thermal noise for audio frequencies. To avoid this excess noise a
local oscillator is introduced and amplification done at some normal
intermediate frequency, 30 Me/see for example. At this frequency the
excess noise is negligible. In the i-f amplifier a strong component is
found due to the beat between leakage from the transmitter and power
from the local oscillator, and a much weaker component, due to the
target, displaced by the doppler frequency. After suitable amplification
these two frequencies are passed into a second detector whose output
signal consists of a d-c component associated with leakage from the
1It is not difficultto makesystemsthat determineon whichsideof the carrierthe
sideband lies, and are thus able to discriminatebetween approachingand receding
targets.