10GHz Gunn Oscillator Power Combiner

Something for Nothing?

What would you say if I told you there is a technique that allows combining the outputs of two 50-mW microwave sources and yields an output of 200 mW? Impossible�can't be done�is the first thought that comes to mind, but under special circumstances, this feat can be performed using a special technique. If two Gunn oscillator sources are combined using the correct method, this apparent "free" power can be realized. Of course, the power has to come from some-where: It comes from an increase in the output from each Gunn oscillator by a synergistic coupling between them.

Exactly why the output of one Gunn oscillator in-creases in the presence of the other is difficult to say. Theories about how this occurs range from harmonics causing parametric conversion processes to the effects of one Gunn oscillator presenting a non-linear load to the other. For the amateur, however, it is important just to know that the technique can work�even if it is not understood! Original work on this >100% coupling efficiency was reported in the March 1983 issue of the Microwave Journal (pp 91-98). Two Gunn oscillator outputs were combined in a dielectric waveguide. Individually, the two oscillators produced 64 and 70 mW, respectively, but produced a combined power output of 235 mW.

The Gunn oscillators used were rather unusual by amateur standards: They were constructed using small cavities in a silicon-resonator block (rather than the more common metal cavity). Amateurs also seldom use the dielectric-waveguide technique to couple the oscillator outputs. The Microwave Journal article noted that both oscillators locked to the same frequency and no spurious outputs were observed. The stability of the combined oscillators was found to be higher than that of the individual oscillators. A recent report in the RSGB Microwave Newsletter (Oct/Nov 1988 issue) mentions work of T. Leiglifield, G3KEU, which more directly relates to amateur practice. He coupled the outputs of two 10-mW oscillators using a waveguide magic tee (discussed later), and obtained a combined power of 38 mW�almost 3 dB more than expected. The Microwave Newsletter article did not mention exactly how the coupling was done.

For those not familiar with the magic tee, a brief description is in order. The magic tee is a four-port hybrid waveguide junction that combines an E-plane T with an H-plane T, as shown in Fig 1. In coaxial or microstrip circuitry, the analogous device is the "rat race"(see Figure 2 below) When all four ports of a magic tee are properly matched, power applied to one port splits equally between two other ports, and the fourth port is isolated. With reference to Fig 1, power applied to port 3 (the H arm) splits equally, and in phase, between ports and 2, and applies no power to a matched load on port 4 (the E arm). Similarly, power applied to port 4 splits equally, but out of phase, between ports 1 and 2, and no power is applied to a matched load at port 3. Power applied to ports 1 or 2 will appear at ports 3 and 4. Because this is a reciprocal device, it can be used to combine (as well as divide) power from two sources. Coherent power sources connected to ports 3 and 4 can deliver power to port I or 2 exclusively, depending on their phase relationship. Likewise, in-phase sources coupled to ports 1 and 2 will deliver their combined output to port 3, while out-of-phase sources will deliver power to port 4. Magic tees have many uses in microwave equipment. One example is as a balanced mixer. Detector diodes are attached to ports 1 and 2, a local oscillator is attached to port 3 and the incoming signal is applied to port 4. Because of the properties of the junctions, at one diode the incoming and LO signals are in phase, and at the other diode the signals are out of phase. By suitably combining the two diode (IF) out-puts, the signal components can be made to add while the noise components of the LO mixing are canceled.

A second use for such a hybrid combiner is as a duplexer. A transmitter is coupled to port 1, a receiver to port 2, an antenna to port 4 and a matched load to port 3. In any application, the individual ports may be tuned using waveguide irises and/or tuning screws. There are a number of ways of coupling two Gunn oscillators using a magic tee. If the two oscillators operate in a phase-locked condition and the arms of the tee are equal in length, and if the oscillators are coupled to ports 1 and 2, the combined output appears at port 3 when port 4 is terminated. Matching screws may be required, and experimentation with arm length (or the use of a phase shifter) may be needed to obtain maximum power output. For those using Gunn oscillators, this technique appears to be a good way to increase power output and get "something for nothing."

Figure2. Rat Race Coupler
A signal input on port 1, will be split between ports 2 and 4, and port 3 will be isolated.