Thursday, May 2, 2013

Antenna Topics: A simple 15 meter vertical dipole, post #185

KH6JRM's Amateur Radio Antenna Topics:

A Simple 15 meter vertical dipole.

Like many of my fellow amateur radio operators I'm "horizontally challenged" by a small lot without many trees or tall structures to support a half-wavelength, flat top dipole.  So, I've usually resorted to inverted vees, small loops, and random length wires.  All of these antennas have performed well, considering the lack of space available.  I've also tried verticals, with uneven results.  With a good radial or counterpoise system, vertical antennas can do a good job.  Perhaps, I'm getting old, but I don't get excited putting radial wire in my backyard.  Although I've used elevated counterpoise wires with some success, I just don't enjoy running wire all around my back yard, especially with neighbor children and pets running around.  Besides, the ground conductivity in my area is very poor.  The thought of "planting" a large number of radials in my small backyard just doesn't appeal to me.

With all this in mind, I decided to attach  dipole elements  to a vertical fiberglass or pvc mast and run the antenna structure as a vertical dipole.  Hopefully, I could get a workable antenna without ground radials that would perform as well as my earlier flat top antenna projects.  Last weekend, I assembled a test vertical dipole antenna for 20 meters.  It works very well, is fairly inconspicuous, and, most of all, it's cheap because I had all of the material on hand.  What you don't have around your QTH can be bought at the nearest home improvement or hardware store.

MATERIALS:

A 33-foot pvc or fiberglass mast (10.06 meters).
A Budwig HQ-1 coax center connector (available from Fair Radio Sales).
33-feet (10.06 meters) of hookup wire.  I usually use #14 AWG housewire for antennas, but all I had in the shack was a 50-foot (15.24 meters) spool of #22 AWG hookup wire.  So, I used that spool for the antenna wire.
Two ceramic or plastic insulators to tie off each dipole element.
Fifty-feet (15.24 meters) of 50-ohm coaxial cable for the feed line.  I had some RG-8X cable in the shack for this purpose.  You could also use RG-58 or RG-8.
A wooden stake to support the mast.  I had a spare MFJ 33-foot (10.06 meters) mast in the garage.  The mast will slip over the stake.  The stake should be at least 5-feet (1.52 meters) long to provide enough support for the flexible mast.
Nylon ties and vinyl electrical tape to secure the vertical elements to the mast.

CONSTRUCTION:

The antenna was built on the ground and then hoisted over the support stake.

For the 20 meter vertical dipole, I selected a frequency of 14.200 MHz.  Using the general formula 234/f (MHz)=L (ft) and allowing for a few inches of overlap, I cut each dipole element to a length of 16.6 feet (5.06 meters).
The top half of the 20 meter dipole was attached and soldered to the + terminal of the Budwig connector.  The bottom half of the dipole was attached and soldered to the - terminal of the Budwig connector.
I attached a ceramic insulator to the end of each dipole element.
The top insulator was tied off with nylon ties and vinyl tape.  The bottom insulator was similarly tied off.
Each vertical element was secured to the fiberglass mast with several nylon ties and vinyl electrical tape.
Before I attached the RG-8X to the Budwig center connector, I wound several turns of the coax into a "choke balun" to prevent rf from running down the coax to the shack.
The RG-8X feedline was attached to the center connector at the mid-point of the mast (approximately 16-feet (4.87 meters) above ground.
The coax feed line was led to a plastic hook on my garage.  The hook was 16-feet (4.87 meters) above ground.  The cable had a little sag, but was nearly perpendicular to the mast.  Hopefully, this step would keep the radiation pattern from being skewed.
The feed line was attached to a static discharge system and copper ground rod just below the shack window.
A 10-foot (3.04 meters) length of RG-8X from my Drake MN-4 match box (tuner) was threaded through a window space and attached to the static discharge system.  Small patch cords connected the MN-4 to the dummy load, low pass filter, and the old Swan 100-MX transceiver.

PRELIMINARY RESULTS:

Results have been favorable.  Without the Drake MN-4 in the line, my SWR measured 1.7 to 1, indicating that some trimming may have to be done.  With the MN-4 in the line, I was able to keep SWR below 1.3 to 1 over the entire 20 meter band.  Not perfect, but it works.  The vertical dipole seems to work as well as my 40-10 meter inverted vee fed with 450-ohm ladder line.  Of course, the 20 meter antenna is strictly a one-band affair, while the inverted vee can be used on other bands from 40 through 10 meters.

While this vertical dipole has its limitations, it has the advantage of taking up little space, requires no radial system, and is inexpensive to make.  I've painted the fiberglass mast a dull green, which blends well into the surrounding vegetation.  The thin antenna wire can't be seen from the street running past my house.

Since my mast is collapsable, the antenna is suitable for portable or emergency use.

I'm enjoying this "homebrewed" antenna.  It's brought me many hours of enjoyable contacts with Japan and the mainland United States.

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Thanks for being with us today!

Aloha from the Big Island of Hawaii,

Russ Roberts, KH6JRM, BK29jx15.