A quadrifilar helical antenna including four conductive helices having a common central axis, a common direction of turn about said axis, a common pitch, and a common length between opposite ends, the helices being uniformly spaced from each other by 90.degree., and a single dielectric helix concentric with the common axis, lying within and supporting the conductive helices at a nominal diameter. The dielectric helix has opposite ends, a plurality of turns having said common direction of turn, and a second pitch substantially greater than said common pitch. A casing contains the helices and is rotatably fixed to one end of the dielectric helix. A tuning device is fixed to the other end of the dielectric helix and rotatable relative to said casing, so that rotation of the tuning device twists the dielectric helix to alter the common pitch of the conductive helices without substantial variation from said nominal diameter.

An antenna (116) includes an antenna cap (1300), a helical coil (1302), and a bushing (1304). The antenna cap (1300) is sized to fit the helical coil (1302) and forms a bushing threading. The bushing (1304) forms a cylindrical portion (1320) and a cylindrical portion (1322). The cylindrical portion (1320) is sized to receive the helical coil (1302). The cylindrical portion (1322) defines a cap threading and is configured to threadedly receive the antenna cap (1300).

A helical antenna is provided in which a predetermined resonance frequency can be determined at the design stage. The helical antenna includes a conductor which is formed from copper or a copper alloy inside a base in the shape of a rectangular parallelopiped formed from a dielectric material having barium oxide, aluminum oxide and silica as main constituents and which is wound in a helical shape along the length of the base. In such a case, the resonance frequency f0 of the helical antenna and the inductance components L of the conductor satisfy the relationship: ln (L)=A0+A1.times.ln (f0), where ln is a natural logarithm, and A0 and A1 are constants. One end of the conductor is extended onto the surface of the base and forms a power feeding terminal connected to a power feeding terminal formed on the surface of the base for applying a voltage to the conductor, and the other end thereof forms a free end inside the base.

A method and apparatus for providing a high impedance structure or surface comprising at least one electrically conductive wire forming at least one elongate wire spiral, the at least one elongate wire spiral being defined by a plurality of spirals of said at least one wire, the spirals having a pitch and being spaced apart along a major axis of said elongate wire spiral; and an arrangement for varying the pitch of the spirals of said at least one wire to thereby tune the impedance of said tuneable impedance structure. An embodiment useful as an antenna aperture to steer a radio frequency beam having two different polarizations is disclosed.

In an antenna apparatus provided at a mobile station for communication with a satellite, a transmitting antenna element for transmitting a signal having a certain transmitting frequency to the satellite is individually provided separate from a receiving antenna element for receiving a signal having a certain receiving frequency from the satellite, the transmitting frequency and the receiving frequency being different from each other.