Transmission analysis on the resonant-cavity combiner of mobile radio transmitters

A general approach to the transmission loss analysis of resonant-cavity combiners of the type used in cellular and in other mobile radio systems is described. A simple closed-form solution is derived for a particular type of combiner in which the cavities are connected in a star configuration via transmission lines that are odd multiples of quarter-wavelength long at the center frequency. This is an exact solution at the center of the combiner band in which one cavity is tuned to this frequency and other cavities are tuned symmetrically in pairs about this frequency. For narrow-band combiners, such as those used in cellular systems, the transmission loss obtained from this solution should be accurate for the entire band. The analysis is valid over wide ranges of combiner parameters: loaded and unloaded Q's of cavities, frequency spacings, and the number of ways it is combined. The comparison of the analytic results with laboratory measurements, although limited in scope, shows that they are in good agreement.