A three-dimensional (3D) rectangular cavity antenna with an aperture size of 80？mm 80？mm and a length of 16？mm, integrated with a four-MESFET transistor grid-oscillator, is designed and studied experimentally. It is found that the use of 3D antenna resonant cavity in case of small or medium gain microwave active cavity antenna leads to effective and stable power combining and radiation. The lack of lateral cavity diffraction and radiation helps in producing a directive gain of about 17？dB and radiation aperture efficiency bigger than 75% at a resonance frequency of 8.62？GHz. Good DC to RF oscillator efficiency of 26%, effective isotropic radiated power (EIRP) of 5.2？W, and SSB spectral power density of ？82？dBc/Hz are found from the measured data. The 3D antenna cavity serves also as a strong metal container for the solid-state oscillator circuitry. 1. Introduction The miniaturization in antennas is of great significance for the modern mobile and portable aircraft and spacecraft wireless electronic systems. However, in addition to the structural, technological, power, and other limitation factors valid for all electronic devices, the miniaturization in antennas is inherently restricted by the space electromagnetic radiation mechanism. Great antenna size reduction is achieved by using the resonant field phenomenon. The resonant antenna is usually commensurable in size with the design wavelength. If the outer resonant antenna surface is excited in resonance, the antenna is termed exoresonant. Examples of such antennas are the resonant dipoles or monopoles, loops, and slots. By analogy, if some antenna comprises a cavity tuned in resonance it is named endoresonant or simply cavity antenna (CA). Depending on the configuration and inside field mode structure the passive single-cavity antennas are divided into two basic classes: closed or 3D cavity antennas [1–5] and open or beam-waveguide (Fabry-Perot) cavity antennas [6–13]. The classic optical or quasi-optical Fabry-Perot resonator is a laterally open, one-dimensional (1D) resonant cavity with a big transverse dimension and length , where is the resonance cavity design frequency and is an integer number. Frequently in practice, the microwave Fabry-Perot or 1D cavity antennas do not obey the above quasi-optical size condition. The above referenced 1D cavity antennas are based on fundamental-mode cavity resonators with a smaller transverse size of about several wavelengths and a length near to half the wavelength. As a rule, this compromise is on account of the antenna radiation properties. Most of the 1D
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