Miscellaneous Systems
313
Figure 17.2: A block diagram for a plan-position indicator (PPI) type of radar system
the duration that is required, and these pulses are applied to the magnetron by way of a modulator. The pulse of microwaves passes through the TR/ATR tube, which separates the transmitting circuits from the receiving circuits. This is an essential part of any radar in which a single antenna is used for both transmission and reception, because with typical transmitter pulse power of 1 MW and typical received signal of a few microwatts, the receiver would be vaporized if the transmitter signal reached it. The antenna consists of a dish similar to the type that is so familiar for satellite reception (which typically uses around 11 GHz frequencies). Radar dishes are usually larger because space is not usually a problem, and frequencies lower than 11 GHz are being used. In addition, the dish rotates so that it aims its microwave beam in a slightly different direction for each pulse. When the pulse of microwaves has been transmitted, the TR/ATR tubes close off the transmitter circuits from the antenna and open the receiver circuits. The returning echo is received on the dish and its signal is passed to the receiver which, as usual, is a superhet design. At the demodulator, the signal consists of a pulse identical in width to the original pulse. This signal is then gated by another pulse that is derived from the clock circuit. The purpose of this is to keep unwanted signals from the display by allowing signals to pass only for a specified time after sending out a pulse. The classic type of display consists of the CRT with a scan that is radial, deflecting the beam from the center of the tube out to the edges. At one time, the scanning coils were rotated in step with the rotation of the antenna dish, but by the 1950s it was more usual to keep the scanning coils steady and alter the phasing of the scanning signals so that the direction of deflection of the beam rotates in step with the rotating antenna dish. The received pulse is used to brighten the beam, providing a target display, and a graticule (a transparent scale) can be used to read direction and distance.