HF Helical Antennas: Theory and Practice

What is the Shortened Helical Antenna

The idea of creating a shortened helical antenna is simple. Figure 1 shows the idea. Supposed we have a dipole antenna that have first fundamental resonance on the frequency F. It is a classical dipole antenna with input impedance 75 Ohm and physical length lambda/2. Current distribution on the antenna is shown in red line. Then just take antenna wire and forming it in helix as it shown in Figure 1.

Let's build such antenna that has the first fundamental resonance on the desired frequency F. The current distribution on the helical antenna is shown in red line. It is may supposed that the helix antenna will have physical length much less the initiative antenna has and almost the same directivity pattern (from the current distribution) as parent dipole antenna. However, the input impedance of the antenna would be not the same like in the full length dipole antenna.

Ratio L1 (length of the resonant lambda/2 dipole antenna) to length L2 (length of shortened dipole antenna having the same resonance frequency as the lambda/2 dipole antenna) is a Shortening Factor of the helical antenna. Shortening Factor of the helical antenna may apply not only to dipole antenna it is may apply to any parent antenna- vertical or frame antenna. As it will be shown later the helical antenna may have Shortening Factor equal to 2- 10, so, the physical length of the helical antenna may be in 2- 10 times less the length of the usual wire (in our case lambda/2 dipole) antenna. 

Below we take a look on historical helical antennas.

Nicola Tesla's Shortened Helical Antenna

View of the shortened vertical helical antenna used for the "magnifying transmitter" in the laboratory of Nikola Tesla on Long Island, near New York, USA, is shown on the Picture 5. Picture 6 shows the inside design of the helical antenna as it is shown on the patent description. Tesla believes that the antenna from "magnifying transmitter" could transfer energy around the globe.

Figure 1 Shortened Dipole Antenna

The antenna is a classical shortened helical antenna. Upper hat (capacitive top load) of the antenna (or what Tesla named as Cupola) has 21 meter in the diameter and now the hat is known as capacitive load. Coil (helical antenna itself) with hat (capacitive load) was tuned to frequency (it assuming from some data providing by Tesla) 150- kHz. Lower part of the coil (helical antenna) was grounded. There are some different descriptions of the grounded system- it may be iron plates buried under the ground or rank of metal tubes pushed into the earth... but nevertheless it would be very effective grounding. The antenna fed through exiting coil placed at the lower end of the spiral of the helical antenna.

The overall height of the Tesla helical antenna was 57 meter. It is assumed that the resonance frequency of the helical antenna was 150 kHz. The wavelength in free space for the frequency of 150 kHz is 2000 meters.

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