Abstract: Piezoelectric crystals are arranged to function as variable resonators. In one embodiment, three crystals are arranged as an array, with a center crystal between two outer crystals. An electrical excitation of the outer crystals causes a mechanical displacement in the crystals along a common axis. This displacement squeezes the center crystal, resulting in a shift of the crystal's resonant frequency. By this arrangement, signals input to the outer crystals vary the resonant properties of the inner crystal.

Abstract: A filter including an enclosure, a dielectric material within the enclosure, at least two microstrip antennas within the enclosure, and at least one frequency selective surface including a metallic pattern. The frequency selective surface is utilized to filter an electromagnetic signal propagated within the enclosure. The geometry of the antennas and the frequency selective surfaces as well as the resonant frequencies of the frequency selective surfaces determine whether the filter is a bandpass, bandstop, notched, or combination filter. If the frequency selective surface is omitted, the combination acts as a delay circuit for delaying the electromagnetic signal, where the time delay is a function of the dielectric constant of the dielectric material.

Abstract: A stationary, lightweight, easily transportatable antenna capable of full duplex operation, i.e., portions of the antenna can transmit and receive simultaneously in the same frequency band. The antenna has a near omnidirectional pattern in the azimuth plane for both transmit and receive. The receive portion (2) consists of four antenna elements (20), each having a beamwidth in the azimuth plane slightly greater than 90.degree.. The receive antennas (20) are arranged symmetrically about a midpoint (93) that lies in the azimuth plane. The beams of the four receive antennas (20) face outwardly away from the midpoint (93) and thereby cover the full azimuth plane. The transmit portion (1) of the antenna is a colinear set of dipole elements (12) arranged within a cylinder (11) that is orthogonal to the azimuth plane and centered on said midpoint (93).