4-7
Energy is coupled into the cavity from the radar by means of an rf cable connected to the input loop.
Energy is coupled out of the cavity to the rectifier and meter by means of the output loop. You can vary
the amount of coupling between the echo box and the crystal rectifier by changing the position of the
output loop. A schematic diagram of the output circuit is shown in figure 4-4, view B. The energy picked
up by the loop is rectified, filtered, and applied to the meter. The method of connecting the echo box in a
radar system is shown in figure 4-4, view C.
RING TIME MEASUREMENTS
Some of the energy generated by the radar transmitter is picked up by the echo box by means of the
directional coupler. This energy causes oscillations (known as RINGING) within the echo box that persist
for some time after the end of the radar pulse, much in the fashion of an echo that persists in a large room
after a loud noise. As this echo dies down, a part of it is fed back into the radar receiving system, again by
means of the directional coupler. The ringing causes a saturating signal to appear on the radar indicator
(figure 4-6). The longer this ringing extends, the better the performance of the radar.
Figure 4-6.—Ring time saturation of A-scope and ppi.
The length of time the echo box should ring under the particular conditions of the test is called the
EXPECTED RING TIME. You may determine whether or not the radar is performing well by comparing
the expected ring time with the ring time observed.
The ring time to be expected on a good radar depends on the particular type of radar being tested; on
the way the echo box is installed - that is, whether a directional coupler or a pickup dipole is used; on the
length and type of cable used; on the individual ringing ability of the particular echo box in use; on the
frequency of the radar; and on the temperature of the echo box at the time of the test. Corrections are
made for all of these factors according to the procedure given in the technical manual for the echo box
being used.
You may use an echo box without correction to detect a change in the performance of a radar. You
simply log and compare the ring time from day to day. You should recognize that these readings do not
