There seems to be some renewed interest in the Big Wheel
Antenna. It's a really great omni antenna for the 2M band. The design is such
that it can be scaled to make smaller (or larger) Wheels for use on other
VHF/UHF bands. The RMVHF+ Group is using them on the four bands of beacons;
2M, 1.35M, 70cm and 33cm. I use one when I'm the Net Control station on the
‘groups’ 2M Monday evening net, in conjunction with my 36 element array (4 X
9M2’s). I can hear the check-ins quite well with it from Cheyenne to Pagosa
Springs and Grand Junction to Smith Center, KS but the copy is considerably
better with the array. The omni is great for out to about a 100 mile radius.
Time has gotten past us for any recent publication of the construction of the
Wheel antennas. The ARRL’s last mention of them was in the early ‘70’s
“Handbook”. Information on them can be found in some of the old ARRL VHF
Manuals (late 60’s early 70’s). I guess no one believes in using an omni for
local VHF work anymore. Everyone must be working all that DX on 2M with their
long yagis! I find it interesting when I see some enormously long 2M yagi on a
roof or tower that is vertically polarized, what a waste! Ya’ know they are
using it to work some FM repeater maybe 30 miles away that could just as
easily be worked with 10 watts and a ground plane (stinger). Trying to work
SSB on 2M with those long yagis vertically polarized, they might as well be
using a wet noodle. The polarization loss will be on the order of 22 dB.
Please, when operating SSB on the 2M and higher bands use horizontal polarized
yagis and omnis. But that’s not the intent of this paper. Let me get into the
“Rerolling of the Wheel”.
Although I have designed and manufactured hundreds of Wheels for as low in
frequency as TV Chan 2 and as high as the 5.7 GHz ISM band, I am not the
original designer. W1FVY and W1IJD were the developers of the Big Wheel (for
the 2 meter band) back in the 60’s. I first started building and using the Big
Wheel in the early 70’s for mobile 2M terrestrial and satellite work as the
Wheel is circularly polarized above and below its horizontal plane. In the
horizontal plane the Wheel exhibits a gain of a couple of dB more than the
common resonate dipole (and even the Turnstile). These antennas can be stacked
as a pair to obtain 5 dBd and as a quad array of four for ~8 dBd.
Those of you that have never seen a Wheel antenna let me explain, they look
like a shamrock or three leaf clover when looking down on them, or up at one
toward the bottom of it. The elements (of which three are required) can be
constructed of aluminum rod or tubing (3/8” to 1/2” dia.) and 80” in length.
Mark 20” in from each end and bend them at those points with a 6” radius to
form one of the elements which will look like the skeleton of a single clover
leaf. The roundness of the 40” portion that is in between the 20” sections can
be formed by hand. I’ve been accused of and found guilty of having ‘educated
fingers’ in constructing the elements for Wheels at the higher frequencies.
I've never used a jig to do the forming. In each element end drive in an
appropriate diameter length of hard wood dowel rod, about an inch or so. Drill
a #8 hole perpendicular to the plane of the element a half inch back from the
end. Do this to both ends of all three elements.
The elements are attached to a hub which consists of a pair of 4" aluminum,
1/8” thick, discs. They will be spaced1.5" apart using a shorting strap. This
shorting strap will need to be fashioned from a 5” length of 1” wide, 1/8”
thick aluminum stock. Drill a #8 hole at each end of it, on center and 1/4” in
from the ends. Bend this strap into a “U” shape so that the open ends are
approximately 1-3/4” apart.
Lay out the pair of discs by placing 3, #8 holes equally spaced around the
periphery of the discs and a half inch in from the edge. Then between two of
the #8 holes, drill another #8 hole but in from the edge only 1/4". These
holes are to be used to hold the "U" shaped shorting strap. Then in the center
of one of the discs drill a #8 hole. In the center of the other disc drill or
punch a hole to fit a coax connector of your choice. This will be referred to
as the bottom disc. From the center conductor of this coax connector solder a
length of #8, brass ‘all thread’ or a long #8, copper or brass screw that
will be secured to the top disc with the appropriate brass or copper nuts, one
on each side of the disc.
The elements can now be installed on this hub. Place one of the ends of an
element on the bottom disc and secure it to that disc at one of the holes
provided. The other end of this element will be secured to the top disc. The
first end of the second element will be secure to the bottom disc right below
the second end of the first element. Then the second end of the second element
will attach to the top disc at the hole provided. Follow suite with the third
element.
An “L” bracket can be fashioned from 1” X 8” X 1/8” aluminum strap (bend to
90° at the 4” point) and secured to the bottom disc using any of the screws
for mounting, and hose clamp to a pole or mast.
That is basically all there is to the construction or the ‘Rolling of a
Wheel’. Testing and adjusting is accomplished by mounting the Wheel about 6’
in the air attach a convenient length of coax to the Wheels’ coax connector
and to an antenna analyzer such as an MFJ. Find where the Wheel is resonate.
You’d like it to be in the vicinity of 144.2. If the resonate point is below
this frequency, take an element in both hands near the bottom disc and bend it
down slightly. Do this to all three elements. Check the resonate point again.
If it is still low in frequency, take an element that is attached to the top
disc, again with both hands, and bend it up slightly. The Wheel is fairly
broad and should not require too much tuning. If the frequency is higher than
144.2, bend the elements so that the spokes are closer together in the
vertical plane. The tuning can also be done with a transmitter and VSWR
bridge. Once the VSWR is 1.5:1 or less at 144 or 144.2, leave it alone! There
is nothing to be gained by getting the antenna any closer to a 1.0:1 VSWR.