612 BELL SYSTEM TECHNICAL JOURNAL 



rectifier built to supply bias, a secondary winding is readily provided on the 

 non-linear coil for connection to the linear coil and to the bias rectifier, which 

 can then be maintained with one side at ground potential. 



In either case whenever coupled windings are employed, the inside wind- 

 ing is invariably made to carry the discharge pulse. This provision results 

 in minimum saturation inductance, since the inner winding is brought as 

 close to the magnetic core as the voltage breakdown strength of the inter- 

 vening dielectric permits. This winding is disposed as uniformlyas possible 

 around the core to avoid leakage which would add to the saturation induct- 

 ance, and so limit the rate of current build-up in the pulse. The other 

 winding can then be disposed with generous spacings, and with partial core 

 coverage if desired. 



Pulse Shaping 



The oscillation frequency of the magnetron is determined primarily by 

 its internal structure, although it is to some extent a function of the im- 

 pressed potential. Departure of the driving wave from perfectly rectangu- 

 lar form permits the oscillation frequency to vary during the pulse, to an 

 extent depending upon the size and duration of the departure and upon the 

 characteristics of the magnetron.'* Frequency modulation thus produced 

 disperses energy over the spectrum. With the receiver band width limited 

 to reduce noise and interference, one effect of this spreading of energy over 

 the spectrum is to cut down the strength of the observed echo. For this 

 reason, other things being equal, rectangularity must be approximated well 

 enough to make the wasted energy a small fraction of that usefully employed. 



It is convenient to regard the rectangular wave as synthesized by a series 

 of harmonically related sine waves of appropriate magnitudes. The 

 fundamental component according to this concept has a half period equal 

 to the duration of the pulse, and the other components, progressively smaller 

 in amplitude, have frequencies which are odd harmonics of the fundamental. 

 In the low-power pulser with its rounded discharge wave the harmonic waves 

 are quite small in amplitude. To approach the flat-topped discharge wave 

 necessary in the power pulsers, harmonic components must be built up. 

 This can be done by providing additional resonances in the discharge circuit 

 at the wanted harmonic frequencies. 



With the close spacing between circuit elements and their proximity to 

 the pulser box walls, parasitic capacitances of appreciable magnitude add 

 to those normally present. These involve dielectrics of low loss and, since 

 the circuit elements and connecting wires are firmly fixed in position, they 

 are fairly well reproduced. They can be used, therefore, in conjunction 



4 The Magnetron as a Generator of Centimeter Waves, by Fisk, Hagstrum, and Hart- 

 man, B.S.T.J., April, 1946. 



