January 1965 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The stacked halo antenna is a compact configuration for obtaining a nearly omnidirectional radiation pattern with nearly 8 dB of gain. An ideal half-wave dipole antenna provides 2.15 dB, so adding 5 to 6 more decibels by merely stacking two halo antennas (which are essentially curved half-waves) might seem like getting more than the sum of the parts. That extra gain is obtained by concentrating the vertical radiation pattern lower to the horizon as compared to a straight half-wave, even though the horizontal pattern loses a bit of gain contribution from the translation to a nearly omnidirectional nature. There is nowadays a plethora of information available on the Internet regarding stacked halo antennas, but in 1965, this Popular Electronics article was one of only a few readily accessible sources other than college textbooks and scholarly papers (of course the ARRL Antenna Handbook was and still is a prime source).
Easy-to-build high-gain antenna for fixed or mobile use
Coaxial "T" connection ties the transmission line to both halos. Gamma match faces same way and is adjusted for minimum SWR.
By Bob Sargent
Halo antennas are like 1/2-wavelength dipoles in many ways. They are cut to the same size, they are horizontally polarized, and they can be stacked for additional gain. However, the bidirectional characteristic of the dipole changes to an omnidirectional pattern when the dipole is curved to form a halo.
Horizontally polarized antennas favor horizontally polarized signals and are less susceptible to ignition noise and vertically polarized waveforms. Gain of a halo over a 14 wavelength vertical Marconi type is usually about 8 db. A gain of 12 db can be expected from a two-halo stack.
At 2 meters a stacked halo arrangement becomes manageable and suitable for mobile work, since the higher frequencies make it possible to employ smaller size antenna elements. The omnidirectional pattern of the stacked halos is particularly desirable for net control stations and for automobiles facing in different directions.
If rolling tool is not available, let sheet metal shop do the forming. Do not drill mounting hole before the halo is shaped; weakened metal bends easier. Weatherproof with nonconductive paint.
Construction. The halos should be spaced 1/2 wavelength apart, horizontally leveled and oriented in the same direction. See the diagram on the previous page for actual dimensions.
Carefully form the halos to prevent flat spots, kinks and just plain out-of-roundness. There are machines for this purpose, but for a small fee you can get a sheet metal shop to form the halos.
Bolt the halos securely to the mast cutouts as shown in the halo mounting detail diagram. Do not tighten enough to distort the mast or halo tubing, and use lock washers. Connect the halos to each other with 52-ohm coaxial cable. Stranded internal conductor transmission line is preferable to the solid conductor type to reduce breakage from vibration.
Cut slot just wide enough to accommodate halos. Mount pole so that the bottom halo is as high above the ground as possible.
Two lengths of cable, each about 21" long, connect the halos. The center conductor on one end of each cable is attached to the terminal connected to the small mica capacitor on each halo. Connect each outer shield to the adjacent ground terminal. The other end of each cable is terminated in a PL-259 or equivalent type coaxial connector and screwed into an appropriate coaxial "T" fitting. The transmission line from the antenna to the transmitter is also screwed into this fitting.
Gamma Match. To construct the gamma match, install a clamp on each halo at a point 4 1/2" to the right of center. The gamma match on each halo should be located on the same side of the mast.
The capacitors should be shielded from the weather. As a matter of fact, a coat of acrylic paint over the entire antenna and fittings will protect it from the elements. The wire forming the gamma match should follow the outside curvature of the halo. About #13 AWG tinned copper bus wire will do. Place nothing within the center of the halo.
An easy way to tune the antenna is with the aid of an SWR meter or field strength meter. Another method is to connect the halos to a receiver and adjust the gamma match for maximum volume or reading of an "S" meter if available.
Posted June 15, 2018