Design of an HF Scimitar Antenna

The Scimitar antenna is a bi-directional radiator constructed as a flat metal plate with a wide end on one side and a narrow point on the other. Once the antenna turns on, it maintains uniform impedance characteristics over a wide range of frequencies, however, the radiation patterns are not stable over the same bandwidth (W. P. Turner, “The Scimitar Antenna,” in The Sixth Symposium on the USAF Antenna Research and Development Program, 1956). When fed at the narrow end and grounded at the wider end, the antenna presents a nominal impedance of about 200 Ω. To match it to the standard 50 Ω one needs a 4: 1 impedance transformer. Moreover, due to the small flare angle at the feed region the fabrication may be somewhat challenging. Thus, increasing the flare angle is a reasonable approach to reduce the Scimitar's nominal impedance (Meinke, H. “Research on Antennas.” Tech. Annu. Summary Report, Munchen, Germany: Air Force Avionics Laboratory, Sept 1963) while providing more attachment real estate. In addition, a wider flare angle reduces the frequency at which desired pattern characteristics manifest and stabilizes the radiation patterns to some degree. Simulations show that when the flare angle is increased from 65° to 130°, the nominal impedance of the antenna is reduced from 200 Ω to 50 Ω, the bandwidth with the stable pattern is increased from 84% to 128%, and the start of the band for a 5.1 m tall scimitar antenna is lowered from 7.65 MHz to 3.75 MHz. The curve describing the outline of the proposed antenna is given in (1), where the values of $k$ and $A$ are 3.65 and 0.2, respectively.