A comparison of electrically small HF antennas

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