Radar System Engineering

SEC.85] INDEPENDENCE OF INTERROGATION AND REPLY 257
Here P~i is the available pulse power at the beacon for any relative position
of the interrogator and the beacon. The gain of the transmitting
antenna, G~,, is a variable that depends upon the angular position of the
antenna with respect to the line from it to the beacon. The received
power is a maximum only when the interrogator is really looking at the
beacon. Similarly GEi, the gain of the beacon receiving antenna, is a
variable depending on the orientation of the antenna with respect to the
line from the beacon to the interrogator.
Let us consider, as an example, a ground radar set scanning around
through the complete azimuth circle and interrogating an airborne beacon.
Curve a of Fig. 8.7 represents schematically a polar diagram of the
logarithm of the power recwived at
the beacon as a function of the angular
position of the radar antenna.
The line to the beacon is assumed
to be the upward vertical. In the
ideal case the curve is the same as
the antenna pattern of the radar; in
actual cases it is often modified by
reflections from near-by buildings,
hills, etc. If circle b represe,,ts the
threshold power for triggering, the
beacon will be triggered only through
the sector AOA’. If, however, the
values of received power should all
be increased by 12 db or so (by decreasing
the range to the beacon),
the beacon would also be interrogated
FIG.S.7.—Schematiclogarithmicpolar
by side lobes of the antenna. Figure
diagram of the antenna pattern of a
8.8 shows this effect. It is clear that microwave radar. The triggering of a
if we consider interrogation that beacon occurs when the energy radiated
in a given direction is in excess of some
begins when the separation is first necessaryminimum amount.
large — and then decreases, the beacon
is first interrogated at maximum range through a very narrow angular
region. As the range decreases, the sector of interrogation increases, but
remains of the order of magnitude of the half-power beamwidth of the
antenna until a range about one tenth of maximum is reached. There,
assuming the side lobes of the antenna pattern to be about 20 db
down, side-lobe interrogation begins and grows with decreasing range.
Finally, in practical cases, a range is reached for which the beacon is
interrogated no matter which way the antenna is pointed. It is also clear
that reduction of the radiated power would produce a narrowing of the
sector of interrogation of any particular beacon, but would lead to failure
to interrogate those at greatest range.