The Reflection Cavity

The MMW electrical properties of the cavity and the oscillator at resonance are effectively fixed in their manufacture. This treatment addresses the coupling between those two circuit elements to achieve optimal performance. It considers only the circuits at resonance that is, under the working condition of the spectrometer. Recall that the capacitive and inductive reactances in a tuned circuit at resonance cancel each other yielding a purely resistive circuit element. Nonetheless the reactances both still remain and are manifest in the property Q. The treatment will yield the reflection coefficient p of the coupling interface between the cavity and oscillator in terms of their gs and the mutual inductance coupling coefficient M of the impedance transformer that the interface represents. Optimum performance will be when the two circuit elements are critically coupled to each other and that will be shown to occur when p = 0. 

Tn these high frequency circuits the inductance, resistance and capacitance are distributed throughout the structure. To facilitate interpretation the effects are lumped together and described as discrete components. 

Equation 2.11, whether treated as a function of wM or of R, has a maximum when ZqRc = ((oMf, and this maximum has the value F /4Zq. This is the maximum power available from the source and is delivered to the cavity when the coupling is adjusted to meet this matching criterion. 

Consider now the transmission line behaviour of the system the voltage reflection coefficient p of the cavity is given generally by  

It is the coefficient of coupling between the MMW source transmission line and the cavity. This parameter, which includes the only practicable variable M, is the one that needs to be adjusted experimentally for optimum performance. Substitution with k and then Q yields 

---

For this kind of cavity, there is no form of coupling that leads to zero power reflection as characterises the reflection cavity, but the optimum signal sensitivity still occurs at critical coupling. At this point only 11% of the incident power is... 

---