MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 465 



C. Noise Due to Cavity Vibrations 



A noise source which can be very troublesome at high modulation 

 fields arises from the currents induced in the walls of the cavity from the 

 modulating field. The interaction of these currents with the dc magnetic 

 field causes mechanical vibrations of the cavity walls. This produces a 

 signal when tuned to the dispersion, but to first order should give no 

 signal when one is tuned to the cavity and sensitive to the absorption. 

 However, any detuning will result in a signal, which, having the right 

 frequency will pass through the narrow band amplifier and lock-in 

 detector. Since this signal is proportional to the magnetic field, it will 

 result in a background signal whose amplitude will ^^ary as the magnetic 

 field is being swept and thus causing a continuous shift in the base line. 

 In a rectangular cavity this effect can be greatly reduced by a proper 

 orientation of the cavity with respect tothedc magnetic field. This is due 

 to the fact that by squeezing the broad face of a rectangular cavity 

 (TEio mode) the frequency decreases, whereas by squeezing the narrow 

 walls of the cavity the frequency increases. Thus in a proper orientation 

 the two effects cancel each other out. We found another way of greatly 

 reducing the effect by using a glass cavity having a silver coating* thick 

 in comparison to a microwave skin depth but small in comparison to the 

 modulation frequency skin depth, thereby decreasing the eddy currents 

 without impairing the mechanical strength of the cavity, 



D. Klystron Noise 



There is very little data available on presently used klystrons. The 

 data quoted by Hamilton, et al* are on a 723A klystron. With an IF of 

 30 mc, bandwidth of 2.5-mc microwave output of 50 mw they obtained 

 a noise power of 5 X 10~ watts. Expressing their results in terms of a 

 noise figure Nk such that the noise power output in the two side bands 

 Pk is given by 



ft = 2N.(kTA.) .: m = I (^) ^^ = .P, (34) 



Substituting their numerical values one obtains for s = 5000 Watt~\ 

 The values for s that we obtained with a 60 mc IF are: 



Higher mode of V-153 klystron s ^ 1,000 Watt"^ 



Lower mode of V-153 klystron s ~ 3,000 Watt"^ 



Higher mode of X-13 klystron s ~ 200 Watt"^ 



Lower mode of X-13 klvstron s ~ 400 Watt"^ 



* We are indebted to A. V. Hollenberg and V. J. DeLucca for the making of 

 the glass cavities and to A. W. Treptow for the excellent silver coatings. 



