A four-port magic-T hybrid waveguide junction serves as the central component of a high-efficiency two-way power combiner circuit for transmitting a high-rate phase-modulated digital signal at a carrier frequency in the Ka-band (between 27 and 40 GHz). This power combiner was developed to satisfy a specific requirement to efficiently combine the coherent outputs of two traveling-wave-tube (TWT) amplifiers that are typically characterized by power levels on the order of 100 W or more. In this application, the use of a waveguide-based power combiner (instead of a coaxial-cable- or microstrip-based power combiner, for example) is dictated by requirements for low loss, high power-handling capability, and broadband response. Combiner efficiencies were typically 90 percent or more over both the linear and saturated output power regions of operation of the TWTs.
Figure 1 depicts the basic configuration of the magic-T hybrid junction. The coherent outputs of the two TWTs enter through ports 1 and 4. As a result of the orientations of the electromagnetic fields, which also provides a needed high port-to-port isolation, of these two input signals and the interior design of the magic-T junction, the input powers are divided so as to add in phase at one output port (port 2), and to be opposite in phase and hence cancel each other at the opposite coplanar output port (port 3). The net result is that the output power at port 2 is essentially double that of the output of one TWT, minus the power lost in the magic-T hybrid junction. Optimum performance as a high-efficiency power combiner thus requires a balance of both power and phase at the input ports of the magic-T.
Replicas of this two-way combiner can be arranged in a binary configuration to obtain a 2n-way (where n is an integer) combiner. For example, Figure 2 illustrates the use of three two-way combiners to combine the outputs of four TWTs.
In data transmission tests at the Deep Space Network carrier frequency of 32.05 GHz, it was found that, because of the broad maximum in peak output power at the sum port, a phase-modulated narrow bandwidth data signal could be transmitted at a rate of 8 Mb/s via this power combiner with no observable error. However, a 622-Mb/s data signal (for which a minimum bandwidth of 311 MHz was required) initially could not be transmitted because of a large drop in signal power at the band edges. The large drop was the result of significant phase imbalance at the input ports of the magic-T hybrid junction resulting from a large difference between the rates of change of phase with frequency, which in turn was caused by a large difference between the electrical lengths of the two TWT signal paths. To correct for this disparity in electrical lengths, it was necessary to add a dispersive circuit element to one of the paths, thereby reducing the difference between the rates of change of phase by more than an order of magnitude. This correction made it possible to transmit the 622-Mb/s signal at a very low bit error rate (10–8).
The helical TWTs used in the above demonstration have bandwidths of at least 9 GHz. By maintaining a balance of phase with changes in frequency at the input ports, it is thus possible to extend the operational bandwidth of the magic-T hybrid junction, which was observed to be at least 3 GHz, to that offered by the inherently wide band individual TWTs.
This work was done by Edwin G. Wintucky, Rainee Simons, and Karl R. Vaden of Glenn Research Center; Gary G. Lesny of Alphaport Inc.; and Jeffrey L. Glass of ZIN.
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