Active antenna arrays for quasi-optical power combining

For 11 years, the group has been working on large scale microwave and millimeter-wave spatial power combiners, both oscillators and amplifiers. The motivation for this work is the fact that the output power of an individual solid-state device is inversely proportional roughly to the square of the frequency. A 20-watt source at 30GHz costs between about $60,000 (Litton) and $110,000 (Amplifier Research). Advantages of spatial (quasi-optical) over more standard corporate combiners are as follows:

• Lower feed loss for large numbers of combined elements
• Power combining efficiency (PCE) remains constant as number of elements increases, above a few tens of elements
• Failure of a single device has minimal effect on operating conditions of other devices
• Graceful degradation as a number of devices fail

Oscillators

Grid-oscillator combiners are radiating periodic structures with a period much smaller than a free-space wavelength, so all elements are tightly coupled and power density is large. As the number of devices increases, the power increases and phase noise decreases, since the power adds coherently and the uncorrelated noise adds incoherently. We have demonstrated 100-transistor combiners at C (5GHz), X (8 and 10GHz) and Ka (31GHz) bands with output powers on the order of 0.5W at X-band. We have developed design methods for these devices components. We have demonstrated quasi-optical VCOs, oscillators with mode selectivity and PLLs. The C-band combiner is shown on the right.

Amplifiers

Amplifier combiners using active antenna arrays with standard half-wavelength spacing have been demonstrated at X (8, 10GHz), Ka (30-33GHz) and V (60GHz) bands. These arrays receive an input wave and amplify and re-radiate an output wave. The output radiation pattern is given by the array design and uniformity of the feed. Patch or slot antennas are used as the radiating elements. We have achieved 10W at 8.4GHz with 70% drain efficiency of each of the 36 elements. At 33 GHz, we have demonstrated a 1-W low-cost manufacturable design using commercial MMIC power amplifiers (the cost is roughly $500/watt). At 60GHz, we have demonstrated the first V-band monolithic array. The array has 36 elements and used a low-noise Martin-Marietta HEMT process. We measured gain at 60GHz, but the project was discontinued due to the MM labs being shut down after merger with Lockheed.

Current work and recent results

Srdjan Pajić is currently working on a high-efficiency switched-mode class-E combining array. He has designed and demonstrated a 4-element subarray with 81% power combining efficiency, over 65% drain efficiency and over 55% power-added efficiency at 10GHz. The devices are Alpha MESFETs with 21-22dBm output power, and the subarray produces 0.5W.