Omnidirectional transmitter combining antenna

Historically, it has been difficult to combine transmitters which are closely spaced in frequency onto the same omnidirectional antenna. Two principal techniques have been used: cavity combining and transmission line hybrid combining. When using cavities, the minimum separation is limited by the amount of insertion loss that is acceptable and by the frequency stability of the cavities. In the 800- MHz land mobile frequency band, cavity combining has been used to combine transmitters as closely spaced as 0.5 MHz with 3 dB of insertion loss. When combining transmitters separated less than 0.5 MHz, hybrid combining has been used. When two transmitters are combined using this technique, half of the power of each is dissipated into a matched load. Further, each time the number of transmitters being combined is doubled, an additional 3 dB is added to the insertion loss. A new technique has been developed which utilizes transmission line hybrids to combine the transmitters, but which does not suffer from large insertion loss. The power that was previously dissipated in the resistive load is radiated in a manner that produces an omnidirectional pattern. The antenna and network that accomplish this combine signals with 90° phase shifts. Measurements show that it is possible to combine eight transmitters arbitrarily close in frequency with 35 dB of isolation between adjacent channels, less than 0.5 dB insertion loss, and with horizontal pattern circularity better than ±3 dB. Additionally, this technique can be combined with cavity combining to maintain high isolation between additional tranmitters.