Sunday, May 12, 2013

Antenna Topics: A 20 meter vertical antenna with tuned counterpoise, post #190

KH6JRM's Amateur Radio Antenna Topics:

A 20 meter vertical antenna with tuned counterpoise.

This weekend has been outstanding on Hawaii Island.  Weather conditions were partly sunny and a bit cooler, thanks to the return of gentle trade winds.  Most of the vog (volcanic smog) has been blown off and most of us can finally breathe normally.  In other words, this was a grand day to build antennas.

In my last post, I described a 40 meter vertical with tuned counterpoise that has performed well on the 7 MHz and 21 MHz amateur radio bands (the 15 meter antenna uses the third harmonic of 40 meters and works as a 3/2 wavelength antenna on 15 meters).  Since I built the antenna for the 7.088 MHz Hawaii Afternoon Net, it functions well around 21.264 MHz in the SSB portion of the 15 meter band.

Now that I've enjoyed some success with this vertical, I decided to make another vertical antenna with a tuned counterpoise for 20 meters, concentrating on a frequency of 14.200 MHz.  Like my 40 meter version, I opted for an elevated tuned counterpoise instead of a radial field to complete the "missing half" of the vertical antenna.  My yard is quite small and an extensive radial field is not possible at this time.

This antenna was built for use without a transmatch or antenna tuner.  My trusty Drake MN-4 is waiting for a general cleanup and maintanance check before I return it to service.

MATERIALS:

A fiberglass, pvc, or wooden mast to support the vertical element.  All I had in the storage room was a homebrew 20-foot (6.09 meters) pvc mast I used for another project.  So, I grabbed that mast and began the project.

A sturdy 6-foot (1.82 meter) wooden or metal fence post to support the mast.  I had a metal fence post that would meet that requirement.

Nylon ties to secure the mast to the support post.

Sufficient wire to make the vertical and counterpoise elements.  Using the general formula 234/f(MHz)=l(ft), I cut a piece of #14 AWG house wire to a length of 16.47 feet (5.02 meters).  This length would make the antenna resonant at 14.200 MHz.  Although many antenna experts recommend that the tuned counterpoise be lengthened by approximately 5%, I decided to keep both vertical and counterpoise elements the same lengths.  So, the counterpoise wire was cut to a length of 16.47 feet (5.02 meters).

Two ceramic or plastic end insulators.  One to support the top of the vertical element and the other to tie off the end of the counterpoise.  A 3-foot (.91 meters) wooden stake would be the tie off point for the counterpoise.  The end insulator would be connected to the wooden stake with a 3-foot (.91 meters) length of dacron rope.

The counterpoise as well as the bottom of the vertical antenna element would be approximately 3-feet (.91 meters) above ground.  This slight elevation would serve to reduce some of the ground losses..

One Budwig HQ-1 center coax connector (available from Fair Radio Sales in Lima, Ohio).  The vertical element would be connected to the + terminal and the counterpoise would be connected to the - terminal.

Fifty feet (15.24 meters) of 50-ohm coaxial cable with UHF connectors.  This would serve as the antenna feed line.  I had some RG-8X coax in the shack for this purpose.  You could also use RG-8 or RG-58.

Before I attached the coax to the center connector, I made a "choke balun" out of 8 turns of the coax measuring approximately 6-inches (15.24 cm) in diameter.  This balun would help keep stray rf off the shield of the cable and prevent rf from entering the radio room.  The "choke balun" was built about 3-inches (7.62 cm) before the center connector.

ASSEMBLY:

The 20 meter vertical was built on the ground.

I attached the center connector to the vertical element and the counterpoise wire.  The connection was made about 3-feet (.91 meters) above ground.  This would help reduce some of the ground losses.  I then attached the top ceramic insulator to the tip of the mast, ran the wire up the mast, attached the wire to the ceramic insulator, and finally secured the vertical element with several nylon ties.  The counterpoise was stretched out along the garden and ran to the end insulator and tie off post.  All connections were soldered and wrapped with several layers of vinyl plastic electrical tape.

I attached the RG-8X coaxial cable (with the "choke balun) to the center connector.  The cable was run a few feet above the level of the lawn to the anti-static unit placed near the radio room.  A short length of RG-8X cable (10-feet/3.04 meters) went from the anti-static discharge unit to the Swan 100-MX transceiver.  I used several short patch cords to connect a dummy load, low pass filter, and the SWR meter to the transceiver.  If the Drake MN-4 was available, the outside cable would have been connected to it first.

When the operating day is done or bad weather approaches, I disconnect the antenna feed line from the anti-static unit, wrap up the coax as far as the antenna mast, and store the unconnected cable in a sealed plastic storage bin at the base of the mast.  The cable will be out of sight and not connected to any station equipment.  I once lost a 33-foot (10.06 meters) pvc mast to a lightning strike.  Thankfully, my feed line was disconnected and grounded at the base of the mast.  The mast took a terrible beating, but none of my equipment was damaged.  As a matter of safety, I always unplug my equipment and ground all antennas when I'm not using them.

PRELIMINARY RESULTS:

The 20 meter vertical with an elevated, tuned counterpoise works very well.  Without the Drake MN-4 in the antenna system, the SWR ranges from 1.5 to 1.7, depending on what portion of 20 meters is being used. A little trimming would help reduce the SWR even further.  I could get better SWR readings if I had installed a few radials, but I chose to avoid that part of the project.  The counterpoise seems to keep things under control.  Once I get the Drake MN-4 back on line, I should be able to cover the entire 20 meter amateur radio band with a SWR of less than 2 to 1.

So far, the antenna has performed as expected. There is no gain, but DX coverage to the mainland U.S.A. and Asia is quite good, with reports of 56 to 59 on most contacts.  I'm running about 50 watts from the old Swan 100-MX.  The rig stays cool and the SWR remains fairly low.

I was fortunate to have most of the antenna materials around the shack.  I've found hardware stores and home improvement outlets a good source of basic antenna materials.  This antenna was an enjoyable experience.  Best of all, it works.

REFERENCES:

http://www.arrl.org/hf-vertical
http://dxzone.com/catalog/Antennas/20M/20_meter_Vertical.
http://www.youtube.com/watch?v=8hvHQuR1CoA.
http://hf-antenna.com/018.

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Thanks for joining us today!

Aloha de Russ (KH6JRM).

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