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
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
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
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