HF Helical Antennas: Theory and Practice

The optimal shortening factor for capacitive top loading helical is from 2 to 5. With larger shortening factor the top loading helical antenna is critical to tuning to resonance.

Umbrella of the helical antenna should have at least 3 wires. The optimal wires may have length near half of the height of the helix. The wires may be placed at 90 to 45 degree to the helix.

Umbrella consisting from two wires going into appositive direction as well works. Length of the wires may be more the height of the helix.

And remember that capacitive top loading helical antennas are capricious in tuning and operation compare to helical antennas without capacitive top loading. Always install RF choke at the feeding terminals of the antenna.

Harmonic Frequencies of the Helical Antennas

When I did measuring the resonance frequencies of the shortened helical antennas I found that the antennas, as well as the linear wire antennas, have resonances at high harmonic frequencies. Most of the tested antennas have second and third frequencies for harmonic resonance. Some antennas have ever fourth frequency of the harmonic resonance. As a rule, capacitive top loading helical antennas do not have clear frequencies for harmonic resonance at all or the harmonic frequencies are hard to predict. So we stopped by only to harmonic frequencies of the usual shortened helical antennas.

In linear wires antennas their harmonic frequencies are practically multiples of the main resonance frequency. It is not so in the shortened helical antennas. During my experiments I found a simple law that explains how it is possible found the higher harmonics.

As a rule, the second harmonic of the shortened helical antenna is at frequency that is slightly higher than the doubled frequency of the first resonance of the antenna. For example, if the antenna has the primary resonance at frequency 7.00 MHz, the second resonance of the antenna should be detected at frequency of approximately 14.50 MHz.

However, some types of the helical shortened antennas may have significant difference in the second resonance frequency.

It is:

Helical shortened antennas with shortened factor more the 10.

Helical shortened antennas with small pitch in the helix.

Helical shortened antennas wound by thin wire with diameter less the 0.3- mm.

Helical shortened antennas wound on the form with high dielectric permeability.

Shortened helical antennas wound on the form with small diameter- less the 16- mm.

If the shortened helical antenna is wound by thick wire (with diameter of more than 1- mm) on hollow dielectric form of large diameter (for example, on form made of empty plastic bottles), and has a small shortening factor (for example, equal to 2 - 4), then the frequency of the second resonance of the antenna will have minimum deviation from the doubled frequency of the first resonance. For example, if the antenna has the first resonance at a frequency of 7.00 MHz, then the second resonance of the antenna may be found at approximately frequency 14.150 MHz.

Typically, the third harmonic of the shortened helical antenna may be detected at frequency slightly higher than the sum of the frequency of the first resonance of the antenna and the frequency of the second resonance of the antenna. For example, if the antenna has a main resonance at a frequency of 7.00 MHz, and the second resonance is at frequency 14.500 MHz, then the third resonance of the antenna may be detected at frequency of 23.0 MHz.

The significant difference in the frequency of the third resonance from the sum of the frequencies of the main and second resonance of the antenna is observed in the following types and designs of the shortened helical antennas:

Helical shortened antennas with shortened factor more the 10.

Helical shortened antennas with small pitch in the helix.

Helical shortened antennas wound by thin wire with diameter less the 0.3- mm.

Helical shortened antennas wound on the form with high dielectric permeability.

Shortened helical antennas wound on the form with small diameter- less the 16- mm.

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