Cellular radio transmitter combiners for narrow co-channel spacing

The use of selective cellular radio transmitter combiners in 900 MHz systems with 600 MHz spacing between channels is well established. The development of compact resonators is likely to overcome the problems that have been found in combining channels covering a total bandwidth greater than 8 MHz, since interconnecting cables between the resonators and throughlines can be significantly shorter. The full 35 MHz that is desirable should now be attainable. To increase the number of users, channel spacings of 150-300 kHz, perhaps as low as 75 kHz are desirable. Very high Q cavities with unloaded quality factor approaching 30000 are necessary to give the desired selectivity. It has been shown that tunable cavities of the required quality are practical and glass versions may be stable but all are relatively large. Dielectric resonator cavities may prove to be as good, giving 900 MHz unloaded Q factors of 24000 whilst still at an early stage of development