HF Helical Antennas: Theory and Practice

Resonance Frequency of a Shortened Helical Antenna

 The main question for design of a shortened helical antenna is: What the length of wire should be in the helix that the antenna has resonance in the desired frequency.

My experimenters gave me very simple answer on the question that could be applied to helical antennas with even winding both as for dipole either for vertical ones. The rule gives 10 percent accuracy.

 The shortened helical antenna of the HF range with Shortening Factor of more than 2 and less than 10 has the first resonance at the wavelength in twice less the length compare to the length of the wire used to the helix, if the diameter of the form of the helix is at least 1 centimeter and does not exceed 10 centimeters.

 As you can see, it is simple to find the resonance frequency of a shortened helical antenna. If you have old helical antenna from CB radio and do disassembling the antenna and do measuring the length of the wire in the antenna, you find that the length is near 5 meters. The wavelength of the 27- MHz CB- Band is 11 meter.

 Note: If you have disassembled helical antennas from VHF and UHF devices you may get another length of the wire- but it is only if the antenna works not at the fundamental first harmonic. Some of the antennas of the VHF and UHF devices works on the second or third harmonic or were designed as not lambda/4 antennas- but like lambda/2 or something else. I did not experimented with such antennas... The experimenters are still ahead.

For example, to create a helical antenna for 20 meter band you need wound a helix contained 10 meters of wire. Vertical helical antenna will contain 10 meter wire in the helix and proper grounding. Dipole helical antenna will contain helix with 10 meter wire in each of the parts of the dipole.

To create antenna for 40 meter band you need wound helix contained 20 meters of wire. Vertical helical antenna will contain 20 meter wire in the helix and proper grounding. Dipole helical antenna will contain helix with 20 meter wire in each part of the dipole.

So, it is very simple design and lots room for experimenters.

Input Impedance of a Shortened Helical Antenna

After practical measurement of the input impedance of my experimental helical antennas, I made a plot of Antenna Input Impedance Vs Antenna Shortened Factor. Figure 8 shows the plot. The plot was made for antennas operating in 7- 28 MHz range and coiled on the form in diameter of 2 to 10 cm. Helix has even winding. For the antennas it was used wire more the 0.5- mm in diameter. Of course, it is not strict plot, and it was created in my home laboratory and cannot be perfect. And it was created for my solutions. However, it is explain some of the behavior of the shortened helical antennas.

Figure 8 Antenna Input Impedance Vs Antenna Shortened Factor

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