590 THE GENERATION OF MICROWAVE POWER 



minimum power of 225 kw at 0.001 duty cycle and can be operated at 

 5 Msec pulses. At 3000 Mc, magnetrons of 5 Mw output at 0.0007 duty 

 cycle have been developed and are in production. These values probably 

 do not represent performance limits, although problems such as back 

 bombardment of the cathode are sufficiently severe to limit the rate of 

 advancement in peak and average power ratings to a relatively slow pace.' 



Radar System Employing a Magnetron. A simplified block 

 diagram of a typical radar system employing a magnetron oscillator as a 

 transmitter is shown in Fig. 11-15. The magnetron oscillator feeds through 

 the duplexer to the antenna, and the return signal from the target passes 

 from the duplexer to the receiver. An AFC circuit is required to keep the 

 receiver tuned to the frequency of the magnetron oscillator. 



As shown, the system is not capable of distinguishing between fixed and 

 moving targets, although a variety of methods have been devised to refine 

 such a system and give it MTI capabilities (see Chapter 6). Generally, 

 these compare the phase of the received signal from one pulse with the 

 phase of the received signal from the preceding pulse, but phase coherence 

 is not maintained for more than two successive pulses. 



11-2 THE KLYSTRON 



For many years the role of the klystron in airborne radar has been that of 

 local oscillator in the receiver. The reflex klystron oscillator has been 

 almost universally accepted for this role. More recently the virtues of 

 klystrons as high-power transmitting tubes have become more widely 

 recognized, and they are being given serious consideration for this appli- 

 cation by many designers of modern systems. 



Perhaps the most important virtue of high-power klystrons is that they 

 can be operated as amplifiers with high, stable gain. As will be discussed 

 in detail below, this makes it possible to design relatively simple yet 

 sophisticated radar systems such as are not possible with transmitting tubes 

 that are self-excited oscillators. Other virtues of klystrons that cause them 

 to be given serious consideration are relatively long operating life, high 

 stability and low noise, and very high peak and average power capabilities. 



Principles of Operation. A schematic diagram of a klystron ampli- 

 fier is shown in Fig. 11-16. The cathode, focus electrode, and anode are 

 designed so that the electrons emitted by the cathode are focused into a 

 beam which passes through an aperture in the anode. For low- and 

 medium-power tubes a grid may be placed across the aperture in the anode 

 to assist in the formation of the electron beam. i\fter passing through the 

 aperture in the anode, the electron beam, traveling at a constant velocity, 

 passes through the gap of a re-entrant cavity resonator. This gap is shown 



