A cheap 40M DX machine: the twin half-square array
-- the basic half-square
simple half-square antenna for 40M is cheap and easy to build. For
the CW end of the band (7.050 Mhz), start with one piece of wire
about 36' long. That's the first leg. Then cut a piece 70' long
(the horizontal top wire) plus 36' long for the far leg.
two trees, and hang the wires so they look like a big staple in
the air. Feed it with 50-ohm coax -- the shield connects to the
top of the first leg, and the center conductor to the top horizontal
wire. Check the SWR, make the legs longer or shorter to tune the
antenna, and you're ready to go.
bottoms of each leg can be any distance off the ground -- I've had
mine just a few inches, all the way up to 10 feet off the ground.
The whole thing can fit under branches just 37' in the air, if that's
what you have to work with.
if you can't go that high, the bottoms of each leg can be bent inward
in an L-shape without any significant change in the antenna gain
or pattern. Rudy Severns wrote a
great article about the half-square for the 1996 ARRL Antenna
Compendium, explaining the effects of bending the leg elements.
arranged my single half-square so it fired broadside in two directions:
the US and JA. It worked very, very well.
half-squares -- doubling up
advice from the late LB Cebik W4RNL, I added a second half-square
20.4 feet away from my original US/JA half-square, and used coax
stubs to each antenna to make the whole array reversible.
LB Cebik's article, "Two to One" (free registration
11'5" stubs of 50-ohm coax connected to each antenna (top corner
of each). The stubs are switched with a pair of DPDT relays, so
when one half-square is fed via one of the stubs, the other stub
is shorted at the relay box, turning that antenna into a reflector.
didn't specify how to wire the DPDT relays, but on the left you'll
see how I wired mine. It's not complicated, but it is easy to get
confused with all the wires soldered to your relay terminals. Take
your time and check everything
twice three times.
theoretical gain of the array is better than 6 dBi at 17 degrees
takeoff angle. That gain might not seem like a lot, but compare
it with a single half-square, which is by itself a solid DX antenna
and offers just 3.3 dBi.
The two-element array offers a vertically polarized signal with
nice low angles -- and below about 20 degrees, the gain is actually
better than a two-element yagi (though not a super-high yagi). I
was able to switch this array from the US-Caribbean to JA and Asia.
the pattern when pointing the array to the east:
the pattern when pointing the array to the west:
2010 -- a new location for the half-square array
the spring of 2010, I took down the US/JA half-square array as it
was too close to my SteppIR 40M dipole and 80M 2-element vertical
array. This was the part of a domino effect that began in Jan. 2009,
when I found that a nearby 40M EU raised-vertical array severely
skewed the half-square array's pattern.
an attempt to improve my chances of working Europe on 40M in the
fall contests, in August 2010 I returned to the great twin half-square
concept. This time, the array is down the back yard in the pine
woods. I was fortunate to have trees in the right places to arrange
the elements so they fire at Europe and New Zealand/W6.
location down in the woods is far enough away to avoid interaction
with the SteppIR dipole, though it is actually now between
the vertical elements of my 80M array. Time will tell if that's
a problem, but so far no SWR changes in the big antenna, and the
half-squares are playing well.
2010 -- interaction with 80M vertical array
several weekends of trekking down to the half-square array in the
woods, I have determined that the array doesn't work well in my
location. The reason has nothing to do with the half-squares. It
has everything to do with the 80M wire vertical array beside it.
believe the interaction with the 80M array made the half-square
array, and the twin vertical array before it, poor performers.
the Autek VA-1 RF analyzer, I was able to measure the feedpoint
impedance of the half-square array when pointed either north or
north half-square is bang-on 50 ohms at 7.050 Mhz,
with nearly zero reactance. Pretty darn good piece of wire. If I
removed the south element, I'd have a pretty great but low-gain
single half-square to play with.
20' away to the south, the south half-square, however,
has its horizontal wire just a foot away from one of my 67' tall
80M wire verticals. It has at least TWO resonant frequencies, about
200 khz either side of the 40M CW band. Changing the length of the
half-square's tails does shift the resonant frequency, but not by
the amounts you'd expect (i.e. adding 5" to each leg should
move the resonant frequency down by 100 khz. It doesn't, in my case.)
It is simply too well-coupled to the adjacent 80M wire.
I will have to try yet another location for the half-square array.
But not this winter.
I am moving to the next experiment to get 40M action over the pole:
a 2-element 40M quad, hanging from the same rope that's currently
supporting the half-square array. The quad loops will be quite a
bit further from the 80M verticals, so here's hoping there is far
less interaction for either of the antenna systems.
thoughts on the twin half-square array
you can put the twin half-square array in the air far enough from
clutter and other vertical metal, do it! This may be the cheapest,
lowest, no-radials-required, no fancy networks, way to get great
gain on 40M. If you have some rope, trees, junkbox wire, a couple
of DPDT relays, and some coax laying around, you have all it takes
to build this antenna.
worked extremely well when I had it aimed at the US/JA, and I would
do it again in a heartbeat if that part of the yard was still clear
A roundup look at the antennas currently in use at the
Wire Vertical Phased Array Project
Simple "from the book" phasing lines, a relay switching
box, and two easy 68' wire verticals with raised radials gives me
a competitive contest signal covering most of the horizon on 80M
Rotatable Dipole Project
Using two fiberglass fishing poles (Crappie poles), some wire and
a few feet of TV twinlead, this linear-loaded dipole is less than
40' from tip to tip, cheap and works well for its size.
Hex Beam Project
Read the construction details for a homebrew hex-style beam. This
is a super performer -- if you want all five upper HF bands, or
don't have the wingspan room for a spider beam or other large-format
Beam Group on Yahoo
A spider beam hot spot. See what other homebrewers
(and spider beam kit builders) are doing to get great signals on
20m, 15m and 10m.
Spider Beam Site
Information about the spider beam from its inventor, Con DF4SA.
See how he has used this outstanding antenna to win CQWW contests
from Portugal. The
spider beam is a serious antenna that, in my opinion, is destined
to be one of the most popular homebrewed tribanders.