596 THE GENERATION OF MICROWAVE POWER 



The magnetic field required can be supplied with either an electromagnet 

 or a permanent magnet. If a permanent magnet is used, the magnet weight 

 increases very rapidly with increasing number of cavities and may become 

 impractically large. 



Comparison of Klystron and Magnetron. Compared with a 

 magnetron, a klystron of the same power output will require higher voltage, 

 typically by a factor of 2. Associated with the higher voltage may be a 

 problem of X-ray radiation. Also the required modulator may be heavier 

 and bulkier and present a more severe problem of insulation at high 

 altitude. The efficiency of a klystron is also typically lower than that of 

 the magnetron. For modern power klystrons, the efficiency will typically be 

 in the range of 30 to 45 per cent, while the magnetron will fall in the range of 

 40 to 60 per cent. The klystron will typically also be larger and heavier than 

 the magnetron, especially when a multicavity, nigh-gain klystron is being 

 compared with a magnetron oscillator. 



Power klystrons are capable of producing peak and average powers 

 as great as or greater than the magnetron over most of the microwave 

 frequency range. At the lower frequencies, the klystron is unquestionably 

 superior; at millimeter wavelengths the magnetron has a decided edge. 

 Klystrons have been built that produce peak powers of 30 Mw at fre- 

 quencies of approximately 3000 Mc and peak powers of 1 Mw at 10,000 Mc. 

 Average powers of several kilowatts have been produced at 10,000 Mc, and 

 greater than 10 kw at 3000 Mc. These numbers compare very favorably 

 with magnetron performance. 



The operating life of high-power klystrons appears to be substantially 

 longer than that of magnetrons. A fundamental reason for this is that the 

 cathode is located where it can be operated at a more conservative emission 

 density and is not subject to electron bombardment as is the magnetron 

 cathode. 



A principal difference between the klystron and the magnetron is that 

 the klystron is usually operated as an amplifier, while the magnetron is a 

 self-excited oscillator. The klystron is free from long-line effects that 

 trouble the magnetron oscillator, although the advent of ferrite isolators 

 has made it possible to operate magnetrons into long transmission lines. 



Because the klystron is an amplifier, it can be used to build systems to 

 higher performance specifications and to design more sophisticated systems. 

 The klystron amplifier offers higher frequency stability and lower noise 

 output. A greater variety of modulation techniques can be used. Phase 

 coherence can be maintained over a chain of pulses, making possible a 

 variety of systems that can separate moving targets from fixed targets. 

 Circuit techniques can be used that eliminate the troublesome AFC circuit 

 that controls the local oscillator when a magnetron is used. 



