High-efficiency (greater than 90 percent) microwave power combining and high-data-rate transmission(622 megabits/sec) were both successfully demonstrated for the first time at the NASA Glenn Research Center using a novel two-way power-combiner waveguide circuit with two 100-W state-of-the-art space traveling-wave tubes (TWTs) The TWTs were designed specifically for efficient operation (greater than 55 percent) over the Deep Space Network Ka-band frequency range of 31.8 to 32.3 GHz (refs. 1 to 3). The power-combining testbed for the two TWTs (SN101 and SN102, Boeing Model 999H, Boeing Company, Chicago, IL) is shown in the photograph.
Left: Power-combiner testbed for 100-W space TWTs (serial numbers 101 and 102). Right: Two-way power-combiner test circuit.
Long description of left figure.
The two-way power-combining circuit, shown schematically in the preceding diagram, was based on a four-port hybrid junction known as a Magic Tee, the basic operation of which is illustrated in the following figure. The TWT input powers (ports 1 and 4) add in phase at the output port (port 2) and cancel in phase at the opposite coplanar port (port 3). For optimum combining efficiency, the powers and phases of the two TWTs at the input ports must be balanced, which ideally results in double the power at the sum port (assuming no power losses in the junction).
Wave-guide hybrid Magic Tee showing input and output port functions.
The geometries of the Magic Tee and alternative hybrid junction were modeled by computer, leading to designs with improved efficiency and power handling (ref. 4). Two designs, which potentially offer combining efficiencies above 95 percent, are shown in the final figure. Successful transmission through the Magic Tee of the 622-megabits/second digital signal, which was quaternary phase-shift-keying (QPSK) modulated, required maintaining a close phase balance over the 311-MHz signal bandwidth. It was thus necessary to appropriately modify the waveguide circuitry to compensate for the widely disparate rates of change of phase with frequency of the two TWTs and corresponding circuit paths. Initially, the phase differences were about 1°/MHz with no signal synchronization. This was subsequently reduced by a factor of more than 6, resulting in an observed low bit error rate of 2.4×10-8. An additional benefit of the circuit modification was an extension of the useful bandwidth of the Magic Tee to more than 3 GHz.
Computer-modeled hybrid junctions. Left: Magic Tee matched by inductive windows. Right: Folded E-plane tee with split colinear arms.
Future NASA deep-space exploration missions may, in some cases, require telecommunication systems capable of operating at very high data rates (potentially 1 gigabit/sec or more) for efficiently transmitting large volumes of scientific data back to Earth. This would require high-frequency microwave transmitters (Ka-band) with large bandwidth. The large distances and the use of practical antenna sizes would dictate the need for transmitter power as high as 1 kW and possibly more. High electrical efficiency would also be a requirement. The high-efficiency, wide-band two-way combiner approach verified here, which can readily be extended to combining 2n TWTs, where n is an integer, also demonstrates the feasibility of meeting these high power, data rate, and efficiency requirements.
Last updated: September 24, 2006
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