Wednesday, May 22, 2013

Antenna Topics: A simple 15 meter vertical dipole. Post #194

A simple 15 meter vertical dipole.

One of my favorite amateur radio bands is 15 meters.  The band is usually open during daylight hours and provides plenty of DX for those of us operating from the central Pacific.  Although most of my antennas are designed to be used on several bands (using ladder line, a 4:1 balun, and an antenna matchbox or "tuner"), I sometimes want to use an antenna specifically designed for a single band using whatever coaxial cable I have around the shack.

In my location, I prefer using verticals, inverted vees, and loops because my backyard is small and room for a decent radial field is non-existent.  If I can't use a horizontal flat top dipole, I can always go up using a vertical dipole.  These antennas are simple to build, erect, and take down.  They are ideal for portable use with a telescoping fiberglass or pvc mast.  Once I finish testing a vertical dipole for a chosen band, I keep the pre-assembled antenna in a plastic storage box for future emergency and portable use.

Since I was't teaching today, I decided to use the sunny and somewhat voggy (volcanic smog) morning to build a portable 15 meter vertical dipole.  All of the antenna materials were found in the storage room near the garage.  Feel free to substitute materials you have available.  Most of the wire and mast materials can be obtained from a hardware store or home improvement center.

MATERIALS:

One 33-foot (10.06 meters) MFJ telescoping fiberglass mast.  You can also use pvc pipe, surplus military mast sections, or even wood.

Approximately 30-feet (9.14 meters) of #14 AWG house wire.  You could use any wire you have available, since the antenna elements will be secured to the mast.

One Budwig HQ-1 center coax connector.  You could also strip a piece of 50 or 75-ohm coaxial cable and use the pigtail to connect the upper and lower sections of the vertical dipole to the feed line.  I happened to have an extra HQ-1 center connector in the shack.  Fair Radio Sales of Lima, Ohio sells these connectors.

Fifty-feet (15.24 meters) of 50 or 75-ohm coaxial cable.  If you prefer 50-ohm coax, you can use RG-8, RG-8X, or RG-58 coaxial cable.  I used 75-ohm RG-6 coaxial cable for this project because I had some cable left over from a studio remodeling job at my former radio station.  I found "F" to "UHF" connectors at the Hilo, Hawaii Radio Shack store--this allowed me to use the RG-6 without cutting off the "F" connector and soldering on a "UHF" connector.

A 5-foot (1.52 meters) wooden stake to support the MFJ fiberglass mast.

Nylon ties and vinyl electrical tape to secure the antenna elements to the mast.

ASSEMBLY:

As is my usual practice, I built the antenna on the ground.

Using the general formula 468/f (MHz)=l (ft), I cut the dipole to a length of 22.02 feet (6.71 meters).  This would put antenna resonance at 21.250 MHz in the 15 meter band.  Each antenna element would then measure 11.01 feet (3.35 meters).

I attached each antenna element to the Budwig HQ-1 center insulator.  The top half of the vertical dipole was connected to the + side of the connector.  The bottom half of the vertical dipole was connected to the - side of the connector.  Each connection was soldered and wrapped with several layers of vinyl electrical tape to protect the connection from wind, salt air, and rain.

The center connector was secured at the 16-foot (4.87 meters) point of the fiberglass mast.  Each element was run along the mast and secured with nylon ties and vinyl electrical tape.

I wound a "choke balun" out of several turns of the RG-6 coax before I attached the coax to the center connector.  The "choke balun" would keep unwanted rf out of the shack.

Once I finished building the antenna and securing the elements to the mast, I hoisted the mast onto a wooden stake.

The RG-6 coax was led from the mast at a 90-degree angel to a plastic hook under the garage roof.  The cable maintained a 16-foot (4.87 meters) height above ground until it was attached to the support hook.  At this height, the feed line presented little risk to people, pets, or vehicles that might pass near the mast.

The RG-6 coax was led down to the anti-static unit near the shack window.  A 10-foot (3.04 meters) piece of RG-6 coax was attached to the anti-static unit and led through the shack window to the Drake MN-4 antenna matchbox ("tuner").  Small patch cords made from RG-6 connected the Swan 100 MX transceiver to the Drake MN-4, the low pass filter, and the dummy load.

PRELIMINARY RESULTS:

This antenna has been a joy to use.  With the Drake MN-4 in line, I've kept SWR below 1.3 to 1 across the 15 meter band.  Using 50 watts or less, I've been able to get CW reports of 569 to 599 and SSB reports of 55 to 59, depending on propagation.

This antenna requires only a few minutes to erect or take down, making it perfect for emergency or portable use.  I'm in the process of building similar vertical dipoles for 20 and 10 meters. All of these antennas will be tested and kept in storage for future use.

So, if you have a few hours and want to build an effective, cheap, and efficient mono-band antenna, consider erecting a vertical dipole.  Best of all, this antenna doesn't require a ground system.

Have fun!

REFERENCES:

http://www.hamuniverse.com/kl7jrverticaldipole.html.
http://www.arrl.org/files/Technology/info/pdf/9106023.pdf.
http://www.arrl.org/hf-vertical.
http://www.wolfington,net/articles/dipole/

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Until next time,

Aloha es 73 de Russ (KH6JRM).

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