Abstract
It is shown that the minimum detectable absorption coefficient for an isolated spectral line as measured in a matched wave guide of optimum length is [4kTNΔf/(P0/e)]αc,where N is the noise factor and Δf the band width of the receiver, αc is the attenuation constant for the wave guide, and P0/e is the average microwave power available at the receiver. The equivalence between cavity and wave guide methods is developed. The Stark modulation method of Hughes and Wilson is analyzed, assuming a quadratic Stark effect, and expressions are developed for the modulation factor of a microwave at twice and four times the frequency of the Stark field. For a matched guide the modulation factor is αgl/4. Also the shift in line position is calculated, and finally, the directions in which improvement in sensitivity may be expected are indicated.
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