Radar System Engineering

260 RADAR BEACONS [SEC. 8.6
lower than that of the radar by a factor of 20 or so. Either the beamwidth
of the antenna for receiving replies from the beacon is made very
broad or this antenna is made omnidirec bional. Microwave angular
discrimination is still provided by the sharpness of the interrogating
beam—again only for beacons at ranges beyond that for which side lobes
begin to cause broadening of the reply. Figure 8.9 shows a whole set of
replies from such a beacon, exposures having been made every few sweeps
of the PPI. The interrogations were at 10 cm, the replies at 1.5 m. The
principal reason for using such a beacon rather than one that gives a
microwave reply is the relative simplicity of the low-frequency transmitter
and antenna of the beaeon. Also, by placing several simple
antennas and receivers for the interrogation at different places on an aircraft,
it is possible to achieve a good approximation to an effectively
omnidirectional pattern for the interrogation link. This is almost
impossible to achieve with single simple microwave antennas because of
interference effects. It is necessary to put the outputs of such multiple
receivers in parallel after detection so that there will not be a combination
of r-f signals in varying phases to give rise to interference maxima and
minima.
The foregoing discussion implies that the broadening of the arcs for
beacon replies is greater than that for radar signals. This is not the case,
however, if the maximum range for both is the same. If G~U and G(,oh)
are the respective gains of the antenna at the center of the pattern and at
the maximum of the first side lobe and R! and R;[l.h) the corresponding
maximum ranges for interrogation of the beacon (to give minimum
triggering power), from Eq. (5) we get GJ(R:) 2 = G(l.~)/R~O.~).
The same line of argument followed through with the radar equation
gives P-J(R:) 4 = G2(]~~)/R&~), which gives the same value of R,(,ok)
when R: is the same. In practice, however, the broadening of the beacon
signals is likely to be greater since i~ is customary and usually desirable
to set the sensitivity of the beacon receiver so that the maximum freespace
range is considerably greater than the radar range to an average
target. This is done partly to ensure increased range and partly to allow
for the considerable unavoidable variations between the receivers of
different beacons as installed and operated.
8.6. Frequency Considerations. General.4ince beacon replies are
much like radar echoes, the considerations involved in choosing the frequency
region for a system in order to get the desired resolution are much
the same. In general, the higher the frequency, the better the azimuth
discrimination for a given tolerable size of antenna system. This
advantage of improved angular separation with increased frequency is
somewhat offset by a tendency toward increased size and weight of transmitters
needed for a given range performance.