Helical Antennas

Basic Information

The Helical antenna is a simple way of obtaining high-gain and a broad band of frequency characteristics. A helical antenna radiates when the circumference of the helix is of the order of one wavelength and radiation along the axis of the helix is found to be the strongest. This antenna is mainly directional. The radiation from a Helical antenna is circularly polarized, that is to say that the Electromagnetic field rotates about the axis of the helix in the direction of the helix turn. Therefore, the radiation is either circularly polarized clockwise or counter-clockwise.

If one were to explore the field from a Helical antenna in the direction of maximum radiation with a simple monopole or dipole antenna, one should find that the strength of the signal will remain the same as long as the dipole is perpendicular to the axis of the Helix. On the side of a Helical antenna, the field is elliptically polarized. Therefore, the horizontal and vertical portions of the signal will not be of equal proportions.

When using Helical Antennas it is very important to make sure that both antennas have the same thread orientation (ie. both clockwise) otherwise the received signal will be significantly decreased. Another very important note is that under the code of Federal Regulations; Title 47, volume 5, Parts 80 to end (47CFR101.117) Fixed Microwave Service: Antenna Polarization, only linear polarization methods may be used. That is to say, if you are using this antenna for any microwave application (928 MHz – 40,000 MHz) helically polarized antennas are not permitted if I properly interpreted provision.

There are at least two good methods for constructing a Helical antenna the first of which is documented in VHF-UHF Manual D.S. Evans (G3RPE) and G.R. Jessop (G6JP) Third Edition printed by RSGP in 1977. Although this is a old book, its design ideas are very quick, easy and strong. They suggest to pre-drill a boom made out of something that is “sufficiently rigid to adequately support the whole structure, and at the same time be of a non-metallic material such as wood, thick-wall plastic tube, or thick-wall fibreglass.”

First off, whatever you use for a center support, make sure it is not metal and, if you are using plastic or PVC, check to see if the dialectic constant is small. If you choose to use PVC or CPVC, try to use PVC. The dialectic constant for PVC is smaller than that of CPVC. Also, minimize the amount of PVC that you use. As will be noted in the next construction method, using too much PVC can slew your signal and cause your antenna to not operate at the proper frequency.

Secondly, the supports for the coil should not be made of metal. Use a good insulator with a low dialectic constant. I would suggest wooden dowels.

Thirdly, the reflector should either be solid, or made of a mesh which has holes no greater than 1/32 λ to 1/125 λ for optimum reception, however; one may use something like metal lath (commonly used to put stucco on buildings) or a heavy window screen and not see a significant degradation in signal quality. Whatever you use, make sure it has a high electrical conductivity.

Finally, run the feed wire as in the diagram above. That is behind the reflector and then through the center of the reflector. Also, the outside sheath of your cable should be connected directly to the reflector, and the center conductor connected to the coil.

Abstract

The helix antenna, invented in the late fourties by John Kraus (W8JK), can be considered as the genious ultimate simplicity as far as antenna design is concerned. Especially for frequencies in the range 2 – 5 GHz this design is very easy, practical, and, non critical. This contribution describes how to produce a helix antenna for frequencies around 2.4 GHz which can be used for e.g. high speed packet radio (S5-PSK, 1.288 Mbit/s), 2.4 GHz wavelans, and, amateur satellite (AO40). Developments in wavelan equipment result in easy possibilities for high speed wireless internet access using the 802.11b (aka WiFi) standard.

Theory in a birds eye view

The helix antenna can be considered as a spring with N turns with a reflector. The circumference (C) of a turn is approximately one wavelength (l), and, the distance (d) between the turns is approx. 0.25C. The size of the reflector (R) is equal to C or l, and can be a circle or a square. The design yields circular polarization (CP), which can be either ‘right hand’ or ‘left hand’ (RHCP or LHCP respectively), depending upon how the helix is wound. To have maximum transfer of energy, both ends of the link must use the same polarization, unless you use a (passive) reflector in the radio path. The gain (G) of the antenna, relative to an isotrope (dBi), can be estimated by:

G = 11.8 + 10 * log {(C/l)^2 * N * d} dBi.

According to Dr. Darrel Emerson (AA7FV) of the National Radio Astronomy Observatory, the results from [1], also known as the ‘Kraus formula’, are 4 – 5 dB too optimistic. Dr. Ray Cross (WK0O) inserted the results from Emerson in an antenna analysis program called ‘ASAP’.

The characteristic impedance (Z) of the resulting ‘transmission line’ empirically seems to be:

Z = 140 * (C/l) Ohm

Type :  Helix
Polarization : Right hand circular
Number of turns : 16
Gain : ~16 dBi*
Total length : 68 cm
Beamwidth (-3dB) : 27 deg.*
Bandwidth : See measured return loss plot

Here is the design of a 2.4 GHz antenna that is ideal for amateur satellite communications. This antenna is easy to assemble because the design itself tolerates inaccuracies in the construction without really affecting performance. And with this antenna, you won’t have any difficulties working all current and future amateur satellites offering the 2.4GHz. band. An exception though is AO-40, which requires a parabolic antenna.

The following drawings and tables describe quite well the construction of this antenna. You can mouse-click on all drawings to get a zoomed view of them.

Here are a few additional notes to consider:

Using a felt tip pen, mark the wire at every 14.75 cm, the equivalent length of one turn of wire. In total, mark the wire 17 times (16 turns). Perform this task on a flat surface such as a table before forming the wire turns.

To form the wire coil, use a cylinder shaped tube with a diameter slightly smaller that the final diameter to achieve. In the end, all the pen markings should line up with each other.

Drill holes whose size matches the hot glue sticks (standoffs) diameter. In this case, a 1.1 cm (7/16″) diameter is required. Drill only one face of the square boom, that is, the face which will receive the standoff.

The hot glue stick standoffs are cut 0.3 cm longer than the optimal length that would maintain the wire coil centered around the boom. Remember to include in your calculations the part of the stick that is inserted inside the boom. For my antenna, the total length was 3.1 cm.

A 0.3 cm deep slot is grooved at the tip of each of the standoffs. The slot depth is calculated in order to maintain the wire coil centered with reference to the square boom. In this case a coil radius of 2.3 cm is targeted. The easiest way to create the slots is to use a heated piece of #12 (AWG) wire and to force it against the standoff tip. This will melt the tip down to the required slot depth.

The standoff tip slots must be oriented with an angle of 12 degrees with the antenna reflector. This will accomodate the wire coil and preserve its shape.
You can apply some hot glue at the standoff extremeties. This will hold the wire in place.

You can paint the antenna parts that are more susceptible to deterioration: the hot glue standoffs, the copper wire and matching plate.