RIA: A New Approach to Microwave/Millimeter-Wave Transmitters

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There is a need for compact, high-power sources for radar applications in the microwave and millimeter- wave range. The output power levels of solid-state devices in this frequency range are limited. A possible solution to be explored in this Research Initiation Grant is to coherently combine the outputs of hundreds of active devices that load a two- dimensional metal grid with a period equal to a fraction of a free-space wavelength. This active surface is an antenna as well as source. The devices can be MESFETs arranged in a gate-feedback configuration. This type of oscillator has a wide frequency tuning range, as is required in most applications. The oscillating active surface is backed with a tuning mirror that has an electrically variable reflection coefficient. This mirror is another metal grid loaded with varactor diodes. The capacitance of the diodes, and therefore the reflection coefficient of this tuning surface, is bias dependant. The mirror provides positive feedback to the oscillator surface, which is necessary for the devices to lock to a single mode. In a radar, for example, the signal is usually transmitted in pulses. In cases when the transmitter itself cannot be switched on and off at a high rate, a switching active surface can be used. This surface consists of a grid loaded with pin diodes, and it electrically switches the wave transmitted from the oscillator grid. This system is compact and light weight, since it consists of a stack of planar dielectrics loaded with solid-state devices that do not require large power supplies. It is versatile, for example, another active surface of Schottky diodes placed in parallel with the oscillator could have a variable phase shift for beam steering or beam forming applications, or can be used as a multiplier. In addition, the planar structure is suitable for wafer- scale or modular integration.