Thursday, May 29, 2014

Simple Ham Radio Antennas: An Extended Double Zepp for 20 meters. Post #277

How would you like to have a simple dipole-like antenna that would give you almost 3dB over over the classical dipole antenna?  You can if you're willing to build a Double Extended Zepp antenna for your favorite amateur radio band.  

Now that I have enough room at my new home-in-progress in the Puna District of Hawaii Island, I decided to make a simple gain antenna for one of my favorite DX bands--20 meters.

I had a spare MFJ telescoping fiberglass mast available, some leftover #14 AWG house wire in the garage, a spare 4:1 W9INN balun, a 50-ft./15.24 length of 450 ohm ladder line, a sturdy Drake MN-4 transmatch ("the tuner"), and 25-ft./7.62 meters of RG-8X coaxial cable with UHF connectors.

I decided to configure the extended double zepp as an inverted V, knowing that its gain would be a bit less than a horizontal arrangement of a half wavelength dipole.

With school over until mid-August, I thought this simple antenna project would give me something creative to do over the summer, in addition to the usual home repairs, yard maintenance, and the various "honeydos" needed to keep domestic bliss.

Before I built this antenna on Wednesday, I did a bit of research and tried to keep construction simple and fairly inexpensive.

Basically, an extended double zepp is a center-fed dipole type antenna consisting of two collinear 0.64 wavelength (5/8 wavelength) elements fed in phase.  Theoretically, this type of antenna gives an approximate 3dB gain over a conventional horizontal dipole designed for the same frequency.  If you will be using this antenna for only 20 meters, you'll need a 1/4 wavelength matching stub in addition to a current balun and an antenna transmatch ("tuner").  Since I wanted to have multi- band coverage from 20 meters through 10 meters, I decided to use a length of 450 ohm ladder line connected to a 4:1 current balun.  A piece of 50 ohm coaxial cable would would then join the balun to the antenna transmatch in the shack.

Using a general formula for an extended double zepp antenna from William K. Hibbert (WB2UVO), 600/f (MHz)= L (ft), each antenna segment worked out to a length of 42.25 ft/12.88 meters.  The total length of the extended double zepp totaled 84.50 ft/25.76 meters.

MATERIALS:

One hundred feet/30.48 meters of #14 AWG house wire for the antenna elements (two equal lengths of 42.25 ft/12.88 meters).  It doesn't hurt to have more wire than you need.

One 33-ft/10.06 meters MFJ telescoping fiberglass mast.

One 5-ft/1.52 meters wooden support stake for the mast.

Fifty-feet/15.24 meters of 450 ohm ladder line.  This would serve as the antenna feed line.  In an article authored by W2HT and IW5EDI, certain lengths of 450 ohm ladder line were  to be avoided because of matching problems.  These lengths include 32-ft/9.75 meters, 64-ft/19.51 meters, 96-ft/29.26 meters, and 124-ft/37.80 meters.  With 50-ft/15.24 meters of 450 ohm ladder line available, matching problems should be reduced.

One "ladder lock" device to secure the ladder line and attached antenna elements to the top of the mast.

Two ceramic insulators and nylon rope to tie off the sloping antenna segments to nearby tree branches.

Two 50-ft/15.24 meters lengths of nylon rope with fishing sinkers attached.  These ropes would be connected to the end insulators of each antenna segment and  tossed over nearby tree trunks to form a slightly sagging inverted V antenna configuration.  The ropes would be secured to branches approximately 10-ft/3.04 meters above ground.

One W9INN 4:1 current balun.

Twenty-five feet/7.62 meters of RG-8X coaxial cable with UHF connectors.

Station equipment, including a Drake MN-4 antenna transmatch ("tuner"), Ten-Tec Argosy II transceiver, low pass filter, Heathkit Cantenna dummy load, and various tools.

ASSEMBLY:

The antenna was built on the ground.  The ladder lock device was attached to the top of the mast.  Each leg of the ladder line was strung through the ladder lock and secured to an antenna element.  All connections were soldered and wrapped with several layers of vinyl electrical tape.

The ladder line was led down the mast to a point approximately 5-ft/1.52 meters above ground level.  The ladder line was secured with nylon ties.

Each antenna element was attached to a ceramic insulator and to its corresponding nylon rope/sinker combination and shot over nearby tree limbs with a slingshot.

I carefully hoisted the fiberglass mast onto its wooden support stake.

I adjusted each sloping element to form  a uniform V shape and tied off the nylon rope/sinker combination to nearby tree limbs.  The ends of the inverted V were approximately 10-ft/3.04 meters off the ground.

I then led the 450 ohm feed line to the 4:1 balun attached to the garage wall.  The ladder line was not allowed to touch the ground.  The terminal point of the balun was 8-ft/2.43 meters above ground level.  Once the ladder line was attached to the terminals of the balun, I connected a 25-ft/7.62 meters length of RG-8X coaxial cable with UHF connectors to the balun and ran the cable through the patch panel in the window of the shack.  The coax was attached to the Drake MN-4 antenna transmatch.  Short lengths (3-ft/0.91 meters) of RG-8X interconnected the Drake MN-4 to the Ten-Tec Argosy II, the low pass filter, and the Heathkit Cantenna dummy load.

As added insurance against RF entering the shack via the coaxial cable, I attached a "counterpoise bundle" consisting of a 1/4 wavelength of wire for 40, 30, 20, 15, and 10 meters to the ground lug of the Drake MN-4 antenna transmatch.

INITIAL RESULTS:

With the Drake MN-4 in line, I was able to get a 1:1 to 1 SWR across the entire 20 meter band.  The antenna works fairly well on 30, 15, and 10 meters.  With careful adjustment, I was able to get a SWR reading of 1.1 to 1 on 30, 15, and 10 meters.  The antenna also works as a slightly long dipole on 40 meters.  SWR is easily reduced with the use of the Drake MN-4.

Based on a leisurely afternoon of operating, I was able to get multi-band contacts both in Hawaii and the mainland U.S. without any difficulty.  The best band was 20 meters, with CW contacts ranging from 579 to 599 and SSB contacts falling between 56 and 59 with the Argosy II running 50 watts.  Contacts on 15 and 10 meters were a bit spotty because of propagation issues, but even these bands produced solid copy when the bands opened in the early afternoon Hawaii time.

The results from this simple antenna were gratifying.  I had a lot of fun building this "stretched" dipole.  Best of all, I got some modest gain at a reasonable cost.  Most of the materials were in the shack or could have been bought at the nearest hardware or home improvement outlet.

Why not build an extended double zepp for your antenna "farm"?  A 10 meter version will take up less than 50-ft/15.24 meters of horizontal space.  To save space, you could opt for my inverted V design or a sloper tied to a mast or high tree limb.  All you need is a little time, a few feet/meters of wire, some ladder line, a 4:1 balun, a short length of 50 ohm coax, an antenna "tuner", and a tree limb or fiberglass mast.  Have fun!

REFERENCES:

http://www.westmountainradio.com/antenna_calculator_zepp.php.
http://www.iw5edi.com/ham-radio/428/double-extended-zepp-antenna.
http://www.rudys.typepad.com/ant/files/antenna_collinear_zepp.pdf.
http://www.home.frognet/~mcfadden/wd8nf/text/zepp.txt.

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Thanks for joining us today!

Aloha,

Russ (KH6JRM)

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







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