588 THE GENERATION OF MICROWAVE POWER 



Effect of Load on Magnetron Performance. The operation of a 

 magnetron will depend upon the load impedance that is presented to the 

 magnetron by the output transmission line. The magnetron is normally- 

 designed to operate into a matched transmission line, and it is specified that 

 the standing wave ratio in the transmission line may not exceed some value, 

 usually VSWR = 1.5 (see Paragraph 10-12). The effects of the load may be 

 established by considering the magnetron as a conventional self-excited 

 power oscillator, with the LC tank circuit inductively coupled to the output 

 transmission line. A mismatch reflected into the tank circuit will affect both 

 output power and frequency of the oscillator. For the magnetron, the 

 voltage-current curve is also shifted by the load impedance, and this shift 

 may under certain conditions lead to moding. 



A phenomenon that is usually more serious is the long-line effect. This 

 effect is not peculiar to magnetrons, but will be found in any oscillator 

 whose output frequency is affected by the output loading. If the magnetron 

 is connected to a relatively long transmission line terminated in a small 

 mismatch, the electrical length of the line (line length measured in wave- 

 lengths) will change rapidly with frequency, and the load impedance 

 presented to the magnetron will also change rapidly with frequency. If the 

 line length is sufficiently long, or the mismatch at the end sufficiently large, 

 the result may be discontinuities in the tuning curve, if the magnetron is 

 tunable, or moding when the discontinuities are approached. 



The critical line length beyond which the problems associated with long 

 lines may be encountered, is given by 



130X/X, 



AF((r2 - 1) 



(11-3) 



where L is the line length in feet, X is the free space wavelength, X^ is the 

 waveguide wavelength, AFis the pulling figure^ of the magnetron (measured 

 at VSWR = 1.5), and o- is the VSWR of the termination at the far end of 

 the transmission line. 



In a practical system, when the inevitable mismatch of antennas, etc. 

 must be located at the end of a long transmission line, the long-line effect 

 may be avoided by using a ferrite isolator at the magnetron output. 



Noise. Noise in magnetrons is important in two different forms. The 

 first is preos dilation noise., or noise that is developed at voltages below that 

 at which the magnetron breaks into oscillation. It is observed that as the 

 voltage approaches the oscillation threshold, the noise output increases 

 rapidly. The mechanism by which this noise is developed is not understood, 

 but the noise itself is important as it affects the buildup of oscillations. 



3The pulling figure is the maximum change in frequency that occurs as a standing wave with 

 a VSWR of 1.5 is presented to the tube and the phase is varied through 360°. 



