122 BELL SYSTEM TECHNICAL JOURNAL 



output meter could be used instead. The cr>'stal type detector is by far 

 the most convenient but with the power available wouldn't give workable 

 outputs when bad dielectrics were to be measured. With the amplification 

 available in the double detection set, any dielectric could be measured, 

 while retaining the necessary attenuation between generator-resonator and 

 resonator-receiver to keep these elements electrically independent of each 

 other. 



It is necessan.^ to maintain an electrical isolation of this sort to get a high 

 apparatus Q. The equivalent Q of all good dielectrics being high, the 

 measuring apparatus Q must be of the same order to give favorable meas- 

 uring conditions. And, further, unless the generator-resonator couphng is 

 weak, the act of var>'ing the resonator tune will drag the generator fre- 

 quency around and will also vary the generator output amplitude. 



The crystal plus microammeter required something like 80 millivolts for 

 full scale deflection and this could be obtained with the present apparatus 

 with coupUngs giving a resonator Q of 1500, while having enough power in 

 reserve to measure any of the good dielectrics. However, most of the dielec- 

 trics with power factor greater than .01 were measured with the d.d. re- 

 ceiver. All the 10 cm wave-length measurements were made with this re- 

 ceiver. For the latter measurements a shorter tube was substituted for the 

 tubes shown screwed into the two heads in the disassembly photo. 



The cr}-stals were calibrated at 60 cycles by means of a 70-ohm y/l 

 attenuation pad." With full scale deflection this pad was introduced and the 

 new scale deflection read. This \/2 ratio was, as far as was possible to 

 check, maintained in the kilo megacycle range. For calibration the cr}'stal 

 was tapped across 4 ohms in the attenuator pad output. A 15 mf electroly- 

 tic condenser was permanently connected across the meter terminals and, 

 by means of a pair of switches, calibration could be checked in a few seconds, 

 during a measurement run. 



The calibration process, using the d.d. set, was to adjust the output to a 

 convenient meter deflection and then calibrate the meter by throwing in 3 

 db in the IF attenuator. 



The resonator itself constitutes an accurate wave meter when corrected 

 for the change in diameter at the moving plunger. The method of operation 

 was then as follows. The plunger vernier, which allowed reading to 0.01 

 cm., was set at the desired wave-length. The osciUator was then turned on 

 and after it had attained temperature equilibrium, was adjusted if necessary 

 to resonance at this value. This adjustment was infrequently necessary- and 

 always sUght. The apparatus Q was then determined by traversing the 

 plimger across the resonance setting by means of the micrometer. This 



2 Exact, not 3 db. 



