A Simple Combining System for Two High-Power HF Transmitters

A symmetrical, linear, four-wire system in a special hybrid configuration provides a simple method for combining the balanced output of two transmitters operating on the same frequency and feeding the combined output power to a common balanced load. Due to its simplicity of operation, economy of construction, power capability, and broad-band characteristics, this combiner provides significant advantages over the conventional lumped-constant combiners that are commonly used for combining high-power transmitters. It provides complete protection in the event of failure of either transmitter without affecting the input impedance or operation of the other; and, given a constant resistive load equal to Z4/2, it will take power at any frequency up to about 30 MHz. If symmetry is realized in practice, the two tuning adjustments may be mechanically coupled to reduce the operating controls to one. An analysis of the system demonstrates that the distributed voltages and currents for two 50-kW transmitters fall well within practical limits, and that with careful design, it is applicable for combining the outputs of transmitters as large as are presently being built. Such a combiner has been constructed for two high-frequency, 50-kW, broadcast transmitters at the Radio Free Europe transmitter plant near Biblis, Germany, and its performance characteristics confinn the theoretical expectations.