Date of Award
Doctor of Engineering Science in Electrical Engineering
Gerald Martin Whitman
Roy A. Plastock
Felix K. Schwering
Antennas presently being used for vehicular military VHF communications are narrowband. New generation frequency hopping radios, however, require broadband antennas. The antenna must be base isolated in order to reduce undesirable pattern nulls and impedance variations caused by currents induced on the support structure. Physical constraints limit the antenna length to three meters while operating from a frequency of 30 MHz to 88 MHz.
To satisfy the above requirements a model of a broadband, base isolated, cylindrical antenna less than three meters long was analyzed and built. Equations for the current distribution as a function of the physical parameters were formulated and solved. The antenna selected as most optimum, i.e., maximum gain on the horizon, was 2.5 meters in length and fed 1.0 meters above the mounting surface. The most optimum base isolation network consisted of a coaxial cable choke wound on a ferrite toroid. The cable choke was made resonant at 25 MHz with minimum distributed capacity.
The equalizer network for this antenna was located at the feed-point. Several network configuration were examined before selecting a two-pole T-network and autotransformer. The equalizer network reduced the antenna VSWR to 7:1. A 2 dB attenuator is required to reduce this VSWR to an acceptable 3.5:1.
Cunningham, Peter Edward, "Broadband base isolated asymetrically fed VHF antenna" (1983). Dissertations. 1278.