The ETH (electronically tuned helix) antenna

A new class of electrically small tunable antennas that are highly efficient has been developed. Designs for US Army applications include communications antennas in the HF and VHF bands with frequency tuning ranges in excess of 15:1. Designs operating at moderate power levels (10-40 W) and high power (1 kW) exist or are under development. The antennas are low profile, high Q devices inherently suitable for co-site and airborne applications. They exhibit efficiencies at least 10 dB greater than comparable size antennas. The ETH is an electronically tuned normal mode helix. This electrically short (as small as 0.029/spl lambda/) monopole design is tuned by shorting turns of the helix, making the helix resonant in the 1/4 wavelength mode at the operating frequency. At this frequency the input impedance is nominally 50 Ohms. No base tuner is required. Since the inherent RF losses of the ETH are very low, it is a much more efficient antenna than designs of equal size. Antenna tuning is based on the ability to alter the electrical length of the antenna to achieve antenna resonance and to control the current distribution along the antenna surface. For typical communication applications, omnidirectional azimuthal coverage and a broad elevation beam are desired and are readily generated by the ETH antennas. US Army test results on several ETH designs are presented. A 13" tall blade ETH operating at VHF (30-88 MHz) was tested. Additional test results on a vehicular mounted HF ETH antenna operating as low as 1.5 MHz (16 ft. tall antenna) are presented.