HF Helical Antennas: Theory and Practice

Shortened helical antenna cannot provide satisfactory operation above the third harmonic frequency.

Shortened helical antenna that has not harmonic frequencies could work effective only at the resonance frequency. Operation of the antenna on the ranges below or higher the resonance frequency would be unsatisfactory.

Capacitive top loading shortened helical antenna could provide effective operation or (if any presents) at the fundamental resonance frequency either at the second and third harmonic frequency. The antenna has poor operation on the ranges between the resonance frequencies.

In the conclusion I would like add that some ham VHF/UHF transceiver for 144/430- MHz has one helical antenna that work at 144- MHz and on the 433- MHz band. The 433 MHz is third harmonic frequency for the antenna with fundamental resonance frequency 144- MHz.

Example of Calculating a Shortened Helical Antenna for the 40 meter Band

Let's calculate shortened helical antenna for the 40 meter band. Figure 13 shows vertical shortened helical antenna for the 40 meter band. Figure 14 shows dipole shortened helical antenna for the 40 meter band. Lambda for the 7.05 MHz is equal to 41.5 meter. To make the vertical helical antenna for the 40 meter band it required length of wire in 21 meter to wound on dielectric form in any diameter from 10mm to 10-cm.

However, practice shows that the length of the wire should be a bit longer than the required lambda/2. Longer length allows tune the antenna to the resonance frequency by cutting the wire by small pieces. Dipole helical antenna for the 40 meter band will contain two such helixes in the length.

Figure 13 Vertical Shortened Helical Antenna for the 40 meter Band

Figure 14 Dipole Shortened Helical Antenna for the 40 meter Band

www.antentop.org

Page- 23