A cheap 40M DX machine: the twin half-square array

Overview -- the basic half-square

A 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.

Find 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.

The 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 a branches just 37' in the air, if that's what you have to work with.

And if you can't go that high, the bottoms of each leg can be bent inward in an L-shape without any real 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.

I arranged my single half-square so it fired broadside in two directions: the US and JA. It worked very, very well.

Twin half-squares -- doubling up

Following advice from the late LB Cebik W4RNL, I added a second half-square 21 feet away from my original US/JA half-square.

See LB Cebik's article, "Two to One" (free registration required)

The design employs 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.

LB 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.

The 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.

Here's the pattern when pointing the array to the east:

Here's the pattern when pointing the array to the west:

August 2010

In 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.

In 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.

The 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.

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