A comparison of electrically small HF antennas

Abstract

The performance properties of several electrically small HF antennas are compared. The antennas are made self-resonant near 30 MHz by increasing the total wire length within a fixed spherical volume of ka ap 0.2, where k = 2pi/lambda and a is the radius of an imaginary sphere circumscribing the maximum dimension of the antenna. Three configurations are considered: a single arm spherical helix antenna, a multiple arm folded spherical helix and a top-loaded spherical helix. An impedance match to 50 Ohms is accomplished through several techniques: the use of multiple folded arms, a parallel stub match and an inductively coupled loop. Since all of the configurations are resonant and have a resistance near 50 Ohms, the primary performance properties of interest in this comparison are the antennas' radiation efficiency and bandwidth, which is characterized by the antenna's quality factor, or Q. All of the antennas exhibit reasonable radiation efficiency. The lowest quality factor is exhibited by the multiple folded arm spherical helix configuration