Radar System Engineering

SEC.17.10] THE RADIATION PATH 713
niques has given satisfactory results even under very severe interference
conditions. It can, of course, follow sector, as well as continuous,
scanning. Its principal drawback is its relative complexity, but much
of the complication in Fig. 17.14 arises from extreme precautions against
interference and from the necessity of operating a servomechanism.
Were fixed-coil PPI’s used, or were it possible to operate a servomechanism
satisfactorily from d-c signals, the equipment would be considerably
simplified. A far simpler model operating fixed-coil PPI’s and designed
for less severe interference conditions has seen considerable successful
service.
The phase-shifted puke method is quite comparable to the sinecosine
method in effectiveness; its method of converting to mechanical
motion is somewhat simpler, and it has considerably fewer adjustments.
Since more data are transmitted per radar cycle, greater protection against
interference is probably afforded. Although this method is of more
recent origin than the sine-cosine method, has not been so highly developed,
and has not had such thorough test% it appears to be mtisfactory.
The sine-cosine method using multiple a-f signals is in principle
quite simple and should prove satisfactory. Although it has been
successfully operated in an actual system (not at the Radiation Laboratory),
the author is not aware of any extensive tests in the presence of
interference. This method requires more complicated and wider-band
transmitting and receiving equipment than do the pulsed methods which
involve time sharing.
THE RADIO-FREQUENCY EQUIPMENT
In selecting the radio-frequency equipment, the requirements to be met
and the operational conditions must be carefully considered. Among the
important factors are the station locations (land-, water-, or air-based),
the maximum range required, the types of interference to be met, and the
nature of the data to be transmitted. These factors affect many of the
variables of the design, such as the types of antennas chosen, the r-f power
necessary, the carrier frequency most desirable, and the mechanical
construction of the equipment.
17.10. Antennas, Frequencies, and the Radiation Path.-The antenna
gain should be as high as practicable at both stations. For a given
transmitted power, the signal discernibility is always directly proportional
to the gain of the transmitting antenna. At the receiving station the
signal-t o-internal-noise ratio is also proportional to the antenna gain.
and considerable interference reduction is accomplished by the directionality
that accompanies high gain. Such antennas therefore permit the
use of much lower power to achieve a given result than would be required
with omnidirectional antennas.
Highly directional antennas can always be used for transmkaion