Author William Blair lamented in a 1958 issue of Radio & TV News magazine that helix (aka helical) antennas had not yet been widely adopted by amateur radio operators despite the advantages they can provide. Helix antennas are used for transmitting and receiving circularly polarized electromagnetic waves. An advantage of using a circularly polarized antenna for receiving is that it is able to make use of wavefronts arriving at any polarization angle along the propagation axis, thereby accommodating transmissions at any polarization angle. Theoretically, an ideal antenna with a particular polarization would not receive a signal arriving at an angle perpendicular to it, and the strength of any signal would be proportional to the cosine of the angle of impingement; e.g., cos 0° = 1, cos 30° = 0.866, cos 45° = 0.707, cos 60° = 0.5, cos 90° = 0. Satellite communications is a popular use of helix antennas because radio signals can undergo an unpredictable amount of rotation as they travel through the atmosphere. Long distance terrestrial communications can also benefit from circular polarization since signals can be effectively rotated due to reflection or even changes in local atmospheric densities. For instance, signals reflected off of water often assume horizontal polarization regardless of the original orientation. Intentional radiation in a circularly polarized mode is done with either a right-hand or left-hand rotation, depending on whether the spiral radiator is wound clockwise or counterclockwise. Opposite handedness of transmit and receive helical antennas are mutually exclusive in the same manner as vertical and horizontal transmit and receive antennas. If helical antennas are so great, then why aren't they used more often? The physical size required for efficient operation is large compared to other antenna formats.
Putting the Helix to Work
Examples of helical antennas for microwave frequencies.
For information on a 108-mc. helix, see the article "Listening to the Satellites" on page 44 of this issue.
By William S. Blair, K6QXW
HRB San Diego Laboratories, Solana Beach, California
The simplicity and effectiveness of this proven antenna make it ideal for u.h.f. TV and the higher ham bands.
The helix is one type of antenna which has not become too popular with amateurs thus far. This is so because, for frequencies of 144 mc. and below, its dimensions become large and the antenna is usually unwieldy to handle. (Editor's Note: For information on a 108-mc. helix, see the article "Listening to the Satellites" on page 44 of this issue.) However, as more and more amateurs explore the higher frequencies, the helix may well become "old reliable" in a relatively short period of time.
In planning for operations in the higher amateur bands, enthusiasm mounts quickly when one begins to list the advantages of the helical antenna. For example: (1) it has almost uniform characteristics over a broad band (maximum to minimum frequency ratio approaching 1.8 to 1); (2) gains of 15 db over this band are readily obtainable; (3) the radiation pattern is a single major lobe with typical half-power beamwidth of 30 degrees; (4) the dimensions of the helix are not at all critical; (5) it intercepts either horizontally or vertically polarized waves equally well; and (6) the antenna is backed by a ground plane and is ideally fed with coaxial cable.
An excellent theoretical discussion on helical antennas is included in the book "Antennas" by John D. Kraus. However, only the general design equations are necessary for the construction of practical antennas.
Fig. 1 - (A) Dimensions and radiation pattern. (B) Pitch angle. (C) Charge distribution. side and (D) end views.
Fig. 2 - Antenna gain and beamwidth shown as a function of the number of turns, for a helix having a circumference equal to one wavelength along with a pitch angle of 12.5 degrees.
Fig. 1A indicates the important dimensions of a helix and its ground plane. D is the diameter of the helix, L the length of one turn, S the distance between turns, n the number of turns, and d the diameter of the ground plane. If one imagines that one turn has been unwrapped and laid out flat, the triangle of Fig. 1B results. The pitch angle α (alpha) is the angle whose tangent is S divided by the circumference of the helix, πD.
Figs. 1C and 1D show side and end views, respectively, of the helix. The plus and minus signs indicate an instantaneous distribution of charge for the axial mode of operation. This is the mode most commonly used and is one which results in a highly directional radiation pattern. In order that the charge be so distributed, the circumference must be on the order of one wavelength at the desired operating frequency. As examples of the latitude permissible in the dimensions, this axial mode of operation is readily obtained with circumferences between 3/4, and 4/3 of a wavelength, pitch angles from 12 to 15 degrees, and any number of turns greater than 3.
Fig. 2 shows how the antenna gain and beam width vary as a function of the number of turns in the helix. These curves are computed for an ideal helix with a circumference equal to one free-space wavelength and a pitch angle equal to 12.5 degrees. From these two values, Fig. 1B shows that the spacing between turns is equal to 0.22 free-space wavelength. It is interesting to note that doubling the number of turns in the helix results in 3 db greater gain and a reduction in beamwidth by a factor of 0.707. Generally even for high-frequency antennas, it is not too practical to build helices with more than about 15 turns since the gain does not increase very rapidly above this number and support of the structure becomes more of a problem.
In the event that the dimensions of a particular helix are not those used in calculating the curves, the following approximate equations may be used to determine gain and beamwidth:
In these equations, Cλ and Sλ are the circumference and turns spacing, respectively, in wavelengths. They apply only for the axial mode of operation where Cλ is between 3/4 and 4/3, α is between 12 and 15 degrees, and n is greater than 3. Gain in db may be determined by taking ten times the logarithm of the number obtained from the first equation. In this equation k: is the radiation efficiency factor. For the ideal loss-less antenna assumed in the curves, k = 1, but where losses are present, k is always somewhat less than 1. In approximating the gain of a particular antenna, the value obtained from the curve should be reduced by 2 to 3 db in order to account for this factor.
Table 1 gives the dimensions of helical antennas suitable for several of the higher frequency amateur bands, as well as for the u.h.f. television band. In television, this type of antenna is especially effective because of its high gain over such a broad bandwidth. Some amateurs have built helical antennas for the lower ham bands, but the size of the structures becomes larger as the frequency goes down and rotation becomes a problem. However, for propagation in one fixed direction this type of antenna holds out real possibilities. In this case the ground plane would be fabricated of large mesh screening in order to reduce wind resistance.
The three antennas shown in the photographs were constructed for operation in the microwave bands. From the largest to the smallest, the center frequencies are: 1250 mc., 3000 mc., and 10,000 mc. respectively. The two smaller ones were wound on wooden dowels while the largest one had a hollow 3" cardboard tube for the helix support. In the latter case, wooden plugs cut from 1/2" plywood were inserted in the ends of the tube for rigidity.
Table 1 - Dimensions of helical antennas for various amateur and television bands.
The three helical microwave antennas shown in the lead photo are displayed here before being mounted on the mast.
In each instance the ground plane was made by fastening sheet aluminum to 1/2" plywood with wood screws. The diameter of the ground plane is not critical as long as it is equal to or greater than one-half wavelength at the operating frequency. A type UG-58/U coax connector was mounted behind the ground plane by means of machine screws which passed through the plywood and aluminum sheet. Lock washers should be used under both the heads of the screws and the nuts in order to insure good electrical contact between the coax shield and the ground plane.
The largest helix was wound with No. 14 solid copper wire while No. 16 bus wire was used for the two smaller units. A uniform spacing of the turns is easily obtained by cutting a strip of cardboard as wide as the desired spacing and then winding it with the wire between turns. If the strip of card-board is not long enough for the whole helix, it may be advanced as the winding progresses.
The end of the winding is anchored in place by fashioning a small bend in the end of the wire and inserting it in a small hole appropriately placed at the end of the tube or dowel. For protection from the weather several coats of Krylon spray-on plastic and a final coat of paint were applied.
An empirical equation for the antenna impedance, in ohms, is:
R = 140 x circumference in wavelengths
For the preceding examples of helices with circumferences equal to one wavelength, a 93-ohm coaxial transmission line could be used for a good match. However, for the antennas pictured, a 75-ohm line was used with excellent results.
Because of the configuration of the helical antenna, the radiation is, in general, elliptically polarized or, for certain dimensions, circularly polarized. The direction of rotation of the polarization is either right- or left-handed, depending upon the direction of the helix winding. This characteristic is fortunate when the helix is used as a receiving antenna since it will intercept either horizontally or vertically polarized signals.
For communication between two locations both using helical antennas, it is necessary that both antennas be wound in the same direction. An electromagnetic wave possessing one direction of polarization rotation will have a minimum interaction with an antenna designed for the opposite direction of polarization rotation. It is recommended that a right-hand sense convention be used for uniformity.
In general, it may be said that the helical antenna is one of the simplest and most effective beam-type antennas it is possible to make. So, when the day finally arrives when you get around to firing up those klystrons that came in your big surplus "buy" - think of the "old reliable" helix and put it to work!
Posted November 6, 2019