Measured Impedances of Helical Beam Antennas

Experimentally determined impedance characteristics are presented for helical antennas. At frequencies too low for the axial or beam mode, the terminal impedance changes greatly with small variations in frequency. However, in the frequency range of the fundamental axial mode, the impedance is relatively constant, especially if the number of turns and the pitch angle of the helix are not too small. This characteristic is an advantage in wide frequency-band applications. For example, the resistance variation of a 5-turn, 18-degree helix over a frequency band of about 1.7 to 1 is only 105 to 145 ohms, and the reactance variation only 0±15 ohms. The effect of the pitch angle and number of turns is discussed.