Monday, August 12, 2013

A simple 20-Meter half square antenna. Post #220.

I thoroughly enjoyed my first half square antenna during this past weekend.  Built for 40-meters, the easily assembled antenna exhibited a bidirectional pattern, offered some gain, required no ground system, and was fairly immune to noise and qrm from the sides.  I disassembled the antenna on Monday and stored it for future use.

Now, I wanted to build a similar antenna for 20-Meters, one of my favorite DX bands.  I generally followed the pattern of the earlier half square with a few modifications.

In general terms, the half square antenna is a basic 2-element wire array fed in phase using two 1/4 wavelength verticals connected by a 1/2 wavelength horizontal phasing line running from the top of each vertical element.  According to Rudy Stevens (N6LF), "the theoretical gain over a single vertical is 3.8dBi."  The half square is fed at the top of one of the verticals, where the current is at a maximum.  This arrangement is a good match for a 50-ohm coaxial feed line.

Although I used the general dipole formula (468/f (MHz)=L (ft) and the general vertical formula (234/f (MHz)=L (ft) for my first half square, I elected to use slightly different formulas suggested by WB3AYW for the 20-Meter half square.  For the 1/4 wavelength vertical sections, I used 249/f (MHz)=L (ft) and for the horizontal 1/2 wavelength phasing line, I used 502/f (MHz)=L (ft).

For my chosen resonant frequency, I selected 14.200 MHz.  Using the appropriate formulas, my vertical antenna elements worked out to be 17.53 ft/10.77 meters.  The horizontal phasing line came out to 35.35 ft/10.77 meters.

I reused the two MFJ fiberglass masts (33-ft/10.06 meters), wooden support stakes, insulators, Budwig HQ-1 center coax connector,and  RG-8X coaxial cable from my previous half square antenna.


I built the antenna on the ground and later hoisted each fiberglass mast onto its support stakes.

From the top of each mast, I ran 17.53 ft/10.77 meters of #14 AWG wire down the mast to a little more than halfway down the mast.  A ceramic insulator was attached to each vertical segment.  The wire was secured to each mast with nylon ties.

At the top of the left hand mast, I attached a Budwig HQ-1 coax center connector.  The connector was secured to the top of the mast with nylon ties.  The vertical element was soldered to the - terminal of the coax connector.

I then soldered the horizontal 1/2 wavelength phasing line (35.35 ft/10.77 meters) to the + terminal of the coax connector.  The horizontal line was secured to the top of the right hand mast with nylon ties.

The horizontal phasing line was soldered to the remaining vertical element.

Before I attached the coaxial feed line to the coax connector, I made a simple choke balun out of the coax just before the UHF connector.  The balun would help keep rf off the coax shield and hopefully out of my shack.  I then attached the coax to the Budwig HQ-1 connector.

I led the RG-8X coax away from the antenna at a 90-degree angle.  The coax entered the shack via a homebrew patch panel in the shack window.  MFJ sells a commercial version of a patch panel that can fit into your shack window.


Although the antenna can be used without a transmatch on 20-Meters, I preferred to leave my trusty Drake MN-4 in the line.  I was able to get the SWR down to 1.1 to 1 across the entire 20-Meter band.

Results were most satisfactory.  With the half square positioned NW to SE, most of my 50 watts from the Swan 100-MX was aimed for the mainland U.S. and Australia (the antenna is bidirectional).  My receive signal from both areas was about 1 to 2 "S" units above my low-slung dipole and multiband inverted Vee antennas.  Transmitted signals also showed some improvement.  CW reports varied from 569 to 599, with SSB contacts reporting 55 to 59.

When I'm done for the day, I just lower the masts and disconnect the RG-8X coax feed line.  I'm pleased with this simple, inexpensive antenna.  Best of all, I don't have to spend time establishing an extensive ground system.

Later, I plan to build half squares for 15- and 10-Meters.

If you want to build an antenna with some gain over a vertical or a dipole, the half square may be worth trying.


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Aloha es 73 de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.