Radar System Engineering

408 R-F COMPONENTS [SEC.11.5
Looking toward the receiver, there is a matched line. All the power
goes into the receiver.
In one variation of the basic scheme just outlined, the receiver branch
joins the antenna line in a series rather than in a shunt T. Then the TR
switch must be a half wavelength rather than a quarter wavelength from
the junction. In another variation, advantage is taken of the fact that
the cold (i.e., nonoscillating) impedance of certain types of magnetrons is
such as to be a bad mismatch to the line, so that nearly all the power
coming toward the magnetron is reflected. Then the ATR switch can
be omitted if the line length between the magnetron and T-junction is
chosen correctly. This is known as “ pre-plumbing.” With many types
of magnetron it is not feasible.
The requirements for satisfactory transmission of the outgoing pulse
are rather easily met.
1. The loss in the discharge across the gaps must be a small fraction
of the magnetron power.
2. The line must be matched when the gaps are fired.
From the point of view of the receiver, the requirements are much
more stringent.
1. During the transmitted pulse, the power getting past the TR
switch into the reniver must be less than 0.1 watt or the crystal
may be damaged. This means a minimum attenuation of 60 to
70 db.
2. The TR-tube gap must fire in less than 0.01 psec, or the preignition
“spike” of magnetron energy may burn out the crystal.
3. The gap must deionize in a few microseconds at the end of the magnetron
pulse so that echoes from nearby objects will not be unduly
attenuated. A typical specification would demand less than 3-db
attenuation 6 psec aftqr the pulse.
4. The received signal must see a reasonably good match into the
receiver, and the losses must be kept to a minimum.
Some refinements in the rudimentary system of Fig. 11.14 are necessary
to meet the above requirements. The fired TR-tube gap is not a
perfect short circuit. If the voltage across the arc is V, the leakage power
going to the receiver is V2/Z, where Z is the impedance looking toward
the receiver, measured at the gap terminals. The voltage can be made
smaller by having the discharge take place in a gas at a pressure of only a
few millimeters of mercury. Further reduction of leakage power is
neceseary, however. This may be done by a step-up transformer to the
gap, and an identical step-down transformer to the receiver line. In the
unfired condition, the standard line impedance is maintained on either
side of the TR switch, but in the fired condition the line impedance seen
at the gap appears to be very high and much less power is coupled out
to the receiver.