1. Technical Field
The present invention relates to the field of radio antennas, and to wide frequency coverage parasitic array antennas. More particularly, the present invention relates to adjustable antenna elements, and to antenna systems employing two such adjustable elements.
2. Description of the Prior Art
Antenna systems employing a single antenna having adjustable-length elements providing excellent performance over a wide frequency range are known in the art. Examples of such antenna systems are the antenna systems manufactured and sold as Steppir Antennas by Fluid Motion, Inc., of Issaquah, Wash., and include dipole, vertical, and yagi antennas.
A limiting factor in prior-art antennas is that, as the frequency of operation of the antenna becomes lower, the physical length of the antenna element must increase to allow it to resonate at the selected frequency. For example, in the case of a conventional yagi antenna having two or more elements extending outward from a boom support arm, element lengths of up to 70 feet are necessary for operation at frequencies in the 40-meter band (7.0 through 7.3 MHz). For operation in the 80 meter band (3.5 through 4.0 MHz) element lengths are up to 140 feet. Of course, elements such as loading coils can be used to shorten the physical lengths of the antenna elements, but they degrade the performance of the antenna.
Mechanical considerations for constructing such antennas become more complicated as the element lengths increase as the operating frequency decreases. Considerations such as mechanical stress and wind survivability make the design of such adjustable antenna systems more challenging when long element lengths are necessary. In addition, space considerations, rather than operating requirements, often dictate the maximum size of an antenna.