Radar System Engineering

SEC. 17.11] GENERAL EQUIPMENT CONSIDERATIONS 717
quencies will be used for such purposes in the future. Until recently it
was not economical to use a-f modulation of microwaves because of the
extreme oscillator-stability requirements necessary to avoid excessive
bandwidth. However, a highly ingenious application of a microwave
discriminator in combination with a feedback amplifier controlling the
oscillator frequency appears to overcome this difficulty and to permit
the use of c-w synchronization methods.
For applications requiring omnidirectional antennas, the ranges so far
obtained at frequencies above 300 Me/see have been rather limited, especially
in situations involving diffraction effects. As greater power becomes
available at the higher frequencies, they will undoubtedly find more and
more applications even where it is not possible to use highly directed
beams. The diffraction difficulty can largely be overcome by the careful
use of diversity antennas.
17.11. General Transmitter and Receiver Considerations.-Although
the specific characteristics desirable in the transmitter and receiver
depend upon the particular application, certain general statements can be
made.
The transmitter should provide sufficient power to ensure clear signals,
free from noise and interference, at the maximum required range. In
common with all components, it must have sufficient bandwidth to accommodate
the band of frequencies present. The receiver should have a
satisfactory noise figure, a proper bandwidth, and in many cases must
provide special means of distinguishing between desired and undesired
signals by methods analogous to the antijamming techniques described
in Sec. 12.8. Automatic gain control is necessary to prevent strong
signals from overloading the receiver, and to insure that signals are
applied to the decoder at the correct level.
Bandwidth varies with the particular characteristics of the radar set
and the type of synchronization used. In general, the video sections will
have a bandwidth between 1 and 3 Me/see, with corresponding r-f and
i-f bandwidths from 2 to 6 Me/see, when normal search radar systems are
used. Since the relay link is only one section of the over-all channel, the
bandwidths of its components must be somewhat greater than would be
necessary if it alone were involved.
An important decision is the choice between amplitude and frequency
modulation. The relative advantages and disadvantages of these two
methods are somewhat different for pulsed and for c-w signals, and
depend also upon the type of interference expected. The principal
advantage of frequency modulation is this: if the carrier power is appreciably
greater than that of an interfering signal, the latter tends to be
1R. V. Pound, “An Electronic Frequency Stabilization System for CW Microwave
Oscillators,” RL Report No. 815, Oct. 1, 1945; Rev. Sci. Inst. 17, 490 (1946).