32 



MEASURING THE RADIO REFRACTIVE INDEX 



refractonieter [29, 30], as illustrated in figure 2.6, utilizes one sealed cavity 

 as a reference cavity and a ventilated cavity as the sampling element. 

 Each of these cavities serves as a frequency-determining element of a 

 Pound [31] oscillator. Very good cavity resonance is required. The 

 Grain refractonieter is so designed that the cavity resonators operating 

 in the 9400-Mc/s range are slightly different in resonant frequency. The 

 resultant difference frequency is the center frequency from which depar- 

 tures due to changing refractive index are measured. Recent models 

 utilize a 43 Mc/s center frequency. As the refractive index of the con- 

 tents of the sampling cavity changes, the difference frequency at the 

 output of the mixer changes according to 



f — 



Then, since 



AN = An 10^ 

 a change of 1 A^ unit corresponds to a change in the resonant frequency 



SAMPLING 

 CAVITY 



REFERENCE 

 CAVITY 



Figure 2.6. The Crain refractonieter. 



