Tuesday, July 9, 2013

A modified 20 meter extended double zepp antenna. Post #210

How would you like to boost your 20 meter signal by 3 dB with only 84.5 feet/25.76 meters of wire, two supporting masts, some 450-ohm ladder line, ordinary 50-ohm coaxial cable, and a few miscellaneous parts?

Today, I ran across a fascinating article by Paul E. Fuller (N8ITF), who designed a simple data sheet to help you build what is called "an extended double zepp antenna."  The double extended zepp is a dipole type of antenna consisting of two collinear 0.64 wavelength elements fed in phase, providing approximately 3 dB gain over a dipole on its intended frequency.  By following Fuller's advice, you can build an antenna that will give you some gain and more DX in the process.

Although my backyard is a bit cramped, there are numerous tall Norfolk Pine trees in an adjacent lot which could serve as a temporary support system for the 20 meter extended double zepp antenna.

With Fuller's article in mind, plus a few other ideas from several amateur radio operators, I set out to build this intriguing antenna.


Using Fuller's data table, I measured out two dipole elements, each segment being 42.25 feet/12.88 meters long.  I used #14 AWG housewire for the antenna elements.

Two supports for the antenna.  I had a 31 foot/9.53 meters jackite pvc mast already in place (it was supporting a 40 meter inverted vee) and found a nearby Norfolk Pine tree with a suitable branch at the 35 foot/10.67 meters) level. The horizontal portion of the antenna system would not be perfectly level, but, for my purposes, the available supports would do nicely.

A 4:1 current balun on the end of a 450-ohm ladder line stub.  Fuller says the 4:1 current balun should be capable of handling "at least twice the operating voltage to prevent  the balun from burning up for the swr mismatch on frequencies other than what the antenna was designed for."

For 20 meters, Fuller recommends a 450-ohm ladder line stub of 7.25 feet/2.21 meters.  For other bands, the length of the ladder line stub can be found by using the general formula 103/desired frequency in megahertz.

A convenient length of 50-ohm coaxial cable.  I had 75 feet of RG-8X with UHF connectors on both ends in the storage room.

An antenna transmatch, just in case.  I decided to use my trusty Drake MN-4 to take care of mismatches in the antenna system.

A suitable transceiver.  In my case, I used my qrp rig--the 1970s vintage Yaesu FT-7.  The rig would be run off solar charged deep cycle marine batteries.


Construction of the antenna was fairly simple.

I used a slingshot, fishing line, and a small "sinker" to launch one antenna element into the tree branch, which was 35 feet/10.67 meters) above ground.  Once I knew there was enough line to pull one dipole segment up to the tree, I lowered that element to ground level.

I lowered the jackite pvc mast to the ground.  With both antenna segments on the ground, I soldered the 450-ohm ladder line stub to each antenna segment.  Each connection was covered with clear fingernail polish and several layers of vinyl electrical tape.

A 4:1 current balun was attached to the ladder line stub.  The 75 foot/22.86 meter piece of RG-8X was connected to the 4:1 balun.

I first hoisted the jackite mast onto its mounting stake.  I then pulled on the fishing line to raise the remaining antenna element to its full height (between 30 feet/9.14 meters and 35 feet/10.67 meters).  The fishing line was tied off at a nearby wooden post. Ceramic insulators were used to keep the antenna elements separated from the pvc mast and the tree.   I left a little slack in the line to accommodate the wind and the swaying of the tree.

The coaxial cable was run into the shack through a plexiglass panel in the window frame of the shack.

Without the Drake MN-4 in the antenna system, my initial swr on 20 meters measured 1.6 to 1--not too bad.  I decided to leave the Drake MN-4 in the system in case I wanted to operate on other bands.  The old Drake transmatch kept the swr below 1.3 to 1 on 20 meters.  I will have to adjust the element lengths a bit to get a better swr, but, for now, I'm satisfied the antenna works.  My first few SSB reports in the late afternoon ranged between 56 and 59 with the little Yaesu FT-7 running around 10 watts output.

I briefly tried the antenna on 15 and 10 meters.  The tuning was a bit tricky, but I could get contacts without stressing the transmatch or the Yaesu FT-7. Rather than risk ruining the old rig or damaging the Drake MN-4, I decided to keep this antenna strictly for 20 meters.

The 20 meter extended double zepp was an enjoyable project that gave my qrp signal a needed boost.



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Aloha es 73 de Russ (KH6JRM)

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