Tuesday, May 7, 2013

Antenna Topics: A tuned counterpoise antenna for 40-10 meters.

KH6JRM's Amateur Radio Antenna Topics:

A tuned counterpoise antenna for 40-10 meters

In my last post, I related the experience of making a segmented inverted v dipole antenna covering the 20, 15, and 10 meter bands.  The antenna required some adjustment, particularly in the 20 meter segment, which was cut a bit short.  With all the segments connected together with alligator clip leads, I found the 20 meter section approximately 5 inches (12.7 cm) short on each side.  With the addition of some extra wire, the 20 meter section tuned up nicely on 14.200 Mhz.  It pays to cut your antenna elements a few inches (several centimeters) longer than expected.

After using the segmented v for a few hours, I lowered the mast and stored the antenna segments in some plastic storage bins near the radio room.  I can reassemble this antenna again if the need arises.

A NEW ANTENNA

Since my backyard is rather small, I prefer verticals, inverted vees, and loops for most of my HF work.  The only drawback to this scenario is the lack of space to accommodate a radial field.  Vertical antennas need a ground radial system to function efficiently. As I was storing the segmented inverted vee in the garage, I discovered a spare 100-foot (30.48 meters) spool of 450-ohm ladder line.  With the weather mostly sunny and hot today (Tuesday) and most of my antenna materials available, I decided to erect a new vertical antenna using a tuned counterpoise to supply the "missing half" of my vertical antenna.  Normally, a ground radial system would be used in this instance, but I decided to use a counterpoise to save time and space.  I ran across an article in the 14th Edition of the ARRL Antenna Book that gave me hope.

According to the article, "the performance of vertical antennas...depends a great deal on the ground system.  You have no way of knowing whether or not you have a 'good ground' in the rf sense.  If you can eliminate the ground connection as a part of the antenna system, it simplifies things...the system is completed by a wire...of the same length as the antenna.  This makes a balanced system somewhat like the center-fed dipole."

Armed with that advice, my old 33-foot (10.06 meters) MFJ fiberglass mast, enough wire for a 40-10 meter vertical (33-feet/10.06 meters), wire for the 33-foot (10.06 meters) counterpoise, and 70-feet (21.34 meters) of 450-ohm ladder line, I was ready to build a new vertical antenna.

PROCEDURE:

I built the antenna on the ground.

I attached 33-feet (10.06 meters) of #14 AWG housewire to the mast and secured the vertical element with nylon ties.

I laid out another 33-foot (10.06 meters) of #14 AWG housewire along the border of a hedge row.  This wire was attached to a ceramic insulator and tied off on a short stake.

I attached one lead of the ladder line to the vertical element and one lead to the tuned counterpoise.

I hoisted the fiberglass mast onto a support stake and ran the roll of ladder line along a back hedge to my garage.

I connected the ladder line to a 4:1 W9INN balun which was attached to the garage side.

From the garage, a 20-foot length (6.09 meters) of RG-8X ran into the shack.  The coaxial cable was then attached to my Drake MN-4 transmatch (tuner).  From there, short coaxial patch cords connected the tuner to the dummy load, low pass filter, and the Swan 100-MX transceiver.

450-ohm ladder line or 300-ohm TV twin lead should be used for the feed line in this system.  On most bands, the SWR will be quite high.  You will lose a lot of power in the system if you use coax for your feeder.

PRELIMINARY RESULTS:

With the help of the 4:1 balun and my trusty Drake MN-4, I was able to get the SWR below 1.5 to 1 on 40, 20, 15, and 10 meters.

The results have been excellent, with performance on par with my segmented inverted vee.  Depending on the band in use, I've been getting mainland U.S. contacts ranging from 569 to 599 on CW and 57 to 59 on SSB.  I've been running the old Swan 100-MX at the 50 watt level.

So far, this antenna combining vertical and horizontal conductors is doing well.


REFERENCES:

"Combining Vertical and Horizontal Conductors".  The ARRL Antenna Book, 14th Edition.  ARRL, Newington, CT., 06111.  p. 8-8.

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Aloha es 73 de Russ (KH6JRM).

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