Friday, August 9, 2013

A Simple 40-Meter Half Square Antenna. Post #219

On Friday, 09 August 2013, I built my first 40-Meter Half Square Antenna.  Several hams I know have used this simple antenna to increase their contacts on 80 and 160 meters.  The antenna provides some gain over a single vertical antenna and offers some signal rejection off the sides.

According to my preliminary research, the half square is a wire antenna with 2 vertical elements fed in phase.  One quarter-wave vertical is fed at a  top corner where its attached by a coaxial center connector (such as the Budwig HQ-1).  The other end of the coaxial connector is attached to a horizontal half-wave phasing line and then connected to another quarter-wave vertical aiming down to the ground.  Both vertical segments are insulated from ground.  The vertical elements are supported by masts or other objects such as trees.  With the antenna being fed at a top corner, the current portion of the antenna is high with a good match to 50-ohm coaxial cable.  No ground radial system is required.

According to some antenna experts, the half square is essentially broadsided, bidirectional, and exhibits between 4 to 5 dB of gain over a single vertical radiator.  The antenna can be built at low cost and offers a low angle of radiation--perfect for DX activity.

MATERIALS:

Two supporting masts.  I had two 33-foot/10.06 meter telescoping MFJ masts that would work for this project.

Support stakes for the masts.

One Budwig coaxial center connector.

Fifty-feet/15.24 meters of RG-8X coaxial cable for the antenna feed line.

Two ceramic insulators.  These insulators would be attached to the bottom portion of each vertical element and insulate the radiators from the ground.

Sufficient #14 AWG house wire for two vertical elements and one half-wave phasing line.  Using the general formula 468/f (MHz)=L (feet) and the chosen frequency of 7.088 MHz, I cut wire measuring 66.02 feet/20.13 meters.  This length of wire would be the half-wave phasing line.  Each quarter-wave  vertical element would be half of this amount, or 33.01-feet/10.06 meters.  There are formulas specifically designed for the half square antenna. These formulas would make each of my elements a bit longer.  But, in the interest of simplicity and testing, I decided to use the standard dipole and vertical equations.

An antenna transmatch to compensate for the small amount of SWR found in my system.  If other, more specific equations are used, the 40-Meter half square can be used without a transmatch.  

Transceiver and associated equipment.  In my case, I used my old Swan 100MX, a dummy load, and a low pass filter.

ASSEMBLY:

I built the antenna on the ground.  Before I attached the RG-8X coax to the Budwig coax connector on the top of the left mast, I wound a choke balun out of the coax near the connecting point.  The choke would cut off any stray rf running down the coax shield.

I secured the wire to the first vertical element (33.01-feet/10.06 meters) and soldered the coax connector to the wire.  A ceramic insulator was attached to the bottom of this element to keep it off the ground..

I next connected the half-wave phasing line (66.02 feet/20.13 meters) to the other end of the Budwig coax connector.  That connection was soldered.

I finished the antenna by attaching the final 33.01-feett/10.06 meters) to the horizontal phasing line and passing that line down to the bottom of the second mast.  As with the first mast, the vertical element was secured by nylon ties to the MFJ fiberglass mast.  A ceramic insulator was attached to this vertical element to isolate it from ground.

Once the antenna was hooked up, I hoisted each mast onto its wooden support stake, adjusted tension on the horizontal phasing line, and ran the coax to my Swan 100-MX.

INITIAL RESULTS:

Considering the liberties I took with the general dipole formula, the antenna proved remarkably successful.  With two vertical elements fed in phase, I experienced very little noise coming in from the side of the antenna.  I oriented the half square antenna NW to SE, with the main lobe of radiation aimed at the mainland U.S. and other lobe in the general direction of Australia.  I received contacts from both areas during the early evening hours.  Using about 50 watts output from the old Swan 100 MX, I got 569 to 599 reports on CW and 55 to 59 on SSB.  Received audio was about 1 to 2 "S" units louder than my 40-Meter Inverted Vee.

I kept the Drake MN-4 transmatch in the antenna system to take care of the small SWR expected from this experimental antenna.  With the Drake MN-4 in line, I was able to keep SWR 1.3 to 1 over the entire 40-Meter band.  I probably should have made the antenna elements and horizontal phasing line a bit longer, but, for my purposes, the antenna was satisfactory.

I had fun building this antenna.  The half square can improve your signal with very little financial expense.

REFERENCES:

http://www.hamuniverse.com/wb3aywcurtainantennas.html.

http://www.angelfire.com/electronic/hypower/webdoc7.html.

http://www.va7st.ca/home.com/ant/files/antenna_halfsquare_broadband.pdf.

rudys.typepad.com/ant/files/antenna_halfsquare_array.pdf.

McCoy, Lew (W1ICP).  Lew McCoy On Antennas--Pull up a chair and learn From The Master.  Second Printing, 1997.  CQ Communications, inc., Hicksville, NY, 11801. pp. 54-55.

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Aloha es 73 de Russ (KH6JRM).

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