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Wednesday, August 14, 2013

A 15-Meter Half Square Antenna. Post #221

Now that my 20-Meter half square antenna is performing well, it's time to build its companion for 15-Meters.

The half square antenna is a basic 2-element wire array using two 1/4 wavelength vertical elements connected at the top by a horizontal 1/2 wavelength phasing line.  The antenna is fed in phase and shows bidirectionality, modest gain, and some immunity from noise on its sides.  A ground radial system isn't required.

By feeding the half square in one of the upper corners, we find the current maximum and a fairly good match to 50-ohm coaxial cable.

MATERIALS:

Since my two MFJ fiberglass masts were already being used by the 20-Meter half square antenna, I decided to make two new masts out of those 4-foot/1.21 meters surplus military fiberglass poles you see advertised in the Amateur Radio magazines.  I had enough mast sections to make two, 20-foot/6.09 meters masts.

Two 7-foot/2.13 meters wooden support stakes for the masts.

Number 14 AWG housewire to make the horizontal phasing line and the two 1/4 wavelength vertical elements.  Using the formula 502/f (MHz)=L (ft) for the horizontal element and 249/f (MHz)=L (ft) for the vertical elements, and a chosen 15-Meter frequency of 21.150 MHz, I came up with the following wire lengths:  Horizontal phasing line--23.73 feet/7.23 meters.  Each vertical element--11.77 feet/3.58 meters.

One Budwig HQ-1 coax center connector.

Two ceramic insulators at the bottom of each vertical element.

Fifty feet/15.24 meters of RG-8X coaxial cable with UHF connectors.  You could also use RG-58 or RG-8 coax.  This cable would serve as the antenna feed line.

Basic tools, nylon ties, vinyl electrical tape

Transceiver, dummy load, transmatch, and low pass filter.  Although the 15-Meter half square had a SWR reading below 1.7 to 1 across the band without the transmatch, I decided to leave my trusty Drake MN-4 in line to improve the match.

ASSEMBLY:

The antenna was made on the ground and later hoisted into position.

I attached 11.77 feet/3.58 meters of #14 AWG housewire to each vertical element.  The wires were secured by nylon ties.

I soldered the left hand vertical wire to the - terminal of the Budwig center connector.  I then soldered the horizontal half wavelength phasing line (23.73 feet/7.23 meters) to the + terminal of the Budwig center connector.  Finally, I soldered the end of the phasing line to the top of the vertical wire running down the right hand mast (11.77 feet/3.58 meters).  A ceramic insulator was attached to the end of each vertical segment.

Before I connected the coax to the center connector, I wound a 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 the shack.

Once the antenna was finished, I hoised each mast onto its support stake and adjusted the tension of the half wave phasing line.

The coaxial feed line entered the shack window through a homebrewed wooden patch panel.

INITIAL RESULTS:

With the Drake MN-4 in the antenna system, I had no trouble bringing the SWR below 1.3 to 1 across the 15-Meter band.  Although some adjustments may be needed, I felt the  reading was satisfactory for my purposes.

The 15-Meter band is usable at my location on Hawaii Island between 1000-1700 hours local time.  Sometimes, I can work stations into the early evening if propagation is good.  Most of my mainland U.S. contacts report readings of 569 to 599 for cw and 56 to 59 for SSB.  I generally run between 25 and 50 watts from my old Swan 100-MX.

The half square is easily built, easy to erect, and requires no  radial system. When I'm done for the day, I simply lower the fiberglass masts, disconnect the feed line, and detach the transceiver from the solar powered deep cycle marine battery.

REFERENCES:

http://www.qsl.net/kaidbb/20meterhalfsquare.html.

http://www.antennasbyn6lf.com/2005/04/halfsquare_ante.html.

http://www.hamuniversity.com/wb3agwcurtainantennas.html.

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Thanks for joining us today!

Aloha es 73 from Russ (KH6JRM).

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