Low Loss Multi-Channel Communication Combiner for Cellular Applications

The increasing demand for cellular communication channels calls for multichannel solutions. Using one antenna for each channel results in a high density of antennas at the compact cellular station front, which generates inter-channel interference. This may result in distractive interference and significant losses at the output and input of the channels. To overcome these drawbacks, we propose a technology that combines several channels into a single antenna. We implemented a frequency-tuned, multi-channel phase control system with a phase shifter connected to each channel, with low insertion loss and low return loss. The phase shifter is composed of an ultra-wideband coupler and a computer-controlled capacitor. To obtain low insertion loss of the phase shifter, typically 1.5 dB, we designed a planar tandem hybrid coupler for the desired frequency range. We used a capacitor bank of varactor diodes for frequency tuning. The capacitance of these diodes was controlled by the applied reverse bias voltage generated by a D/A converter. The control computer received the digital input to the converter through a serial communication line. The channels were simultaneously phase matched to minimize channel losses. We performed an extensive theoretical analysis of a multi-channel frequency combiner. Based on the simulation results, a three-channel multi-coupler was implemented. The high performance of the system was demonstrated experimentally.