A microwave oscillator is a device that converts direct-current (dc) energy into alternating-current (ac) energy (1). In modern wireless systems, microwave oscillators are commonly used to generate carrier signals for radio transmitters. At RF receivers, oscillators are usually associated with mixers and phase-locked loops for extracting message signals. For decades, active devices such as Gunn diodes, impact ionization avalanche transit time (IMPATT) devices, and resonant tunneling diodes (RTDs) have been used broadly to perform this important dc-to-ac conversion. Bipolar-junction transistors (BJT), metal–semiconductor field-effect transistors (MESFET), metal-oxide-semiconductor field-effect transistors (MOSFET), and high-electron-mobility transistors (HEMT) (2) are among the modern transistors in common use in the design of various oscillator circuits.
In past decades, oscillators were integrated with antennas mainly for spatial power combining (1, 3). This is because a microstrip is extremely lossy in the millimeter-wave ranges, while, on the other hand, a spatial power combining has relatively lower loss. An oscillating antenna, also called an antenna oscillator, is a multifunctional circuit that can be designed easily by employing an antenna simultaneously as the resonator, the feedback element, and as the load of an oscillator. In this chapter, design examples are given for all the aforementioned cases.