In recent years, active antennas have attracted extensive attention due to their potential applications in compact microwave and millimeter wave systems. Meanwhile, spatial power combining (SPC), which takes advantage of active antennas, has also been introduced as an efficient power combining means in microwave or millimeter wave bands. Besides SPC arrays, active antennas have also found applications in integrated transmitters, mixers/receivers, transponders, and transceivers, etc. at different microwave or millimeter wave frequencies. This dissertation summarizes the author's research efforts in this area.; First, a novel active antenna was proposed. This active antenna has advantages of low spurious radiation, low second harmonic radiation, good heat sinking, ease of integration, etc. The design procedure for this active antenna has been developed and an optimum design has been experimentally achieved.; Second, the frequency domain theory on the modes and their stability of coupled oscillator driven SPC arrays was established. The theory explains all previously published experiments. It is also verified by our own experiments and time domain simulation.; Finally, an integrated Doppler radar transceiver front end using two FET active antennas was proposed. This design has the advantage of lower noise as compared to self-oscillating mixer scheme at low Doppler frequencies. It can be used in low power Doppler radar systems to detect slowly moving objects such as pedestrians with high sensitivity. The design procedure for this integrated Doppler radar transceiver front end is also developed and applied to an example design.
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