834 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1957 



than for a tuned coaxial cavity, because of the lower RF gap impedance 

 of the former. To compensate for this drawback, a sensitive noise re- 

 ceiver is employed, similar in principle to the radiometer invented by 

 R. H. Dicke. The input noise power is replaced periodically by a 

 matched load at room temperature by pulsing the beam on and off 

 with a 1,000 cps scjuare wave, and placing an isolator in front of the re- 

 ceiver. A synchronous detector eliminates gain-fluctuation noise and 

 converts the receiver output to a dc voltage. 



Noise power variations at various microwave frerjuencies are measured 

 in terms of the changes of attenuation, between probe and receiver, re- 

 quired to keep the receiver output constant. These rapid adjustments in 

 attenuation are performed by a servo amplifier-motor loop, and recorded 

 on a chart, whose speed (1| inches per minute) is synchronized with that 

 of the moving probe. In the same way, records of collector current as a 

 function of probe position can be obtained, and correlated with those of 

 noise power. The probe can be moved a distance of about 17 inches, its 

 position nearest the gun (2 = inches) corresponding to a distance of 

 0.95 inch between the anode and the input plane of the RF gap. 



VACUUM 

 '''ENVELOPE 



50 OHM 

 COPPER COAXIAL 



DRIFT TUBE ^LINE 



. ,,, V/////////A, I 



,, COLLECTOR / -O^O DIA.^ \ 

 /'I \ i .050 DIAn \ \ 



"^ top DIA I ( 



,531 R 



Fig. 1 — Cress-section of experimental tube, showing electron gun, probe, and 

 two solenoids. The isolated current-collector electrode serves as inner conductor 

 of a coaxial line. The induced RF power can be measured over a wide range of 

 frequencies. 



