Wednesday, July 16, 2014

Simple Ham Radio Antennas. A 5-band vertical dipole. Post #286

How would you like to have a 5-band vertical antenna that covers 20, 17, 15, 12, and 10 meters that requires no ground radial system and can be built from materials from your junk box or from the nearest hardware store?

You can, if you build a simple Vertical Dipole and feed it with 300 ohm television twin lead or 450 ohm ladder line terminated into a balanced antenna "tuner." If your "tuner" has no provision for balanced feeders, you can insert the twin lead/ladder lead into a 4:1 current balun and use a short piece of 50 ohm coaxial cable to connect the antenna to your "tuner."

I got this idea from an article in "Simple and Fun Antennas for Hams" by Chuck Hutchinson (K8CH) and Dean Straw (N6BV).  Hutchinson and Straw conducted a series of antenna experiments using 15 meter vertical dipoles in a space-restricted urban area. The results of their work were impressive, so I decided to use some of their suggestions and make my own version of this versatile vertical antenna suitable for limited real estate.

Like Hutchinson and Straw, I decided to pattern my design for the 15 meter phone band (21.200 Mhz). With 450 ohm ladder line, a 4:1 balun, and my trusty Drake MN-4 antenna transmatch, I could get decent multiband performance from one antenna.  I recently acquired an Elecraft K-3, so I could test the antenna on 17 and 12 meters as well.

MATERIALS:

I found all of my materials in the garage storage room at my new home (under construction) in the Puna District of Hawaii Island.

Feed Line.  I had a spare 50-ft/15.24 meters roll of 450 ohm ladder line.

Antenna "tuner".  My trusty Drake MN-4.

Balun.  I had a spare W9INN 4:1 balun in the shack.

Coaxial cable.  One 3-ft/0.91 meters piece of RG-8X with UHF connectors.  This piece would run from the 4:1 balun to the patch panel in the shack window.  One 10-ft/3.04 meter piece of RG-8X with UHF connectors.  This segment would run from the patch panel to the Drake MN-4 transmatch.

One 33-ft/10.06 meters MFJ telescoping fiberglass mast. This would support the top vertical element and the bottom vertical element.

One "Ladder Lock" center connector to support the center of the vertical dipole and connect each segment to its respective leg of the ladder line.

A nearby tree limb to support the feed line above ground as it leaves the midpoint of the vertical dipole (more on that later).

One 5-ft/1.52 meter wooden garden stake to support the fiberglass mast.

#12 AWG house wire.  Using the general formula for a dipole, 468/f (MHz)=L (ft), and a chosen frequency of 21.200 MHz, the total dipole length before cutting the wire into two equal segments came to 22.07-ft/6.73 meters.  Rounding things off to the nearest foot/meter, each segment of the vertical dipole came out to 11-ft/3.35 meters.

Fifty feet/15.24 meters of nylon rope tied to a fishing sinker. A slingshot was used to shoot the free end of the ladder line over a nearby tree limb to maintain a near 90-degree departure angle from the center of the vertical dipole.

Basic tools, soldering gun, nylon ties, two ceramic insulators, and vinyl electrical tape.

ASSEMBLY:

The vertical dipole was built in the garage car port.  A nearby electrical outlet provided power for the soldering gun.

A ceramic insulator (you could use plastic, teflon, wood, or even glass) was attached to top end of the upper segment and to the bottom end of the lower segment.

The top insulator was secured to the metal loop at the top of the 33-ft/10.06 meter fiberglass mast and the top segment was led down the mast and secured with nylon ties.  The bottom segment was left unattached until I attached the "Ladder Lock" to the mast (approximately 22-ft/6.70 meters above the base of the mast).  

I then soldered both top and bottom elements to their respective legs of the 450 ohm ladder line.  The connection was wrapped with several layers of vinyl electrical tape.  The bottom element was then secured to the mast with nylon ties.  The bottom of the lower vertical element was approximately 11-ft/3.35 meters above the base of the mast.

Once the both antenna elements were attached to the fiberglass mast, I hoisted the mast onto its wooden support stake.  The mast was approximately 33-ft/10.06 meters from a nearby tree.  I was able to shoot the free end of the ladder line over a branch about 20-ft/6.09 meters above ground using a slingshot, nylon rope, and a fishing sinker.  I was able to maintain an almost 90-degree departure angle from the center of the vertical dipole (22-ft/6.70 meters above the base of the mast).  The ladder line was nearly horizontal from the mast to the tree limb. The remaining 17-ft/5.18 meters of ladder line was terminated in the W9INN 4:1 balun.  Coaxial cable (RG-8X) went from the balun to the patch panel (3-ft/0.91 meters).  A 10-ft/3.04 meters length of RG-8X went from the patch panel to the Drake MN-4 transmatch.

Three-foot/0.91 meters pieces of RG-8X connected the Elecraft K3, the Heathkit Dummy Load, and a Low Pass Filter to the Drake MN-4.

I also attached a 5-band "counterpoise bundle" to the ground lug of the Drake transmatch.

INITIAL RESULTS.

With the help of the Drake MN-4 transmatch, I was able to get a 1:1 SWR reading on all amateur bands from 20 to 10 meters.  Twenty meters was fairly "touchy" owing to the short length of the vertical elements, but I was able to get a good match on this band.  The 12, 15, 17, and 10 meter bands were easily adjusted for a 1:1 SWR.

I was able to make both Hawaii and mainland U.S. contacts on all bands except 10 meters, which was very "noisy" in my location.  Fifteen and 17 meters did very well from 1100 to 1700 hrs local time(0100-0700 UTC).  Using my backup solar- charged deep cycle marine battery as a power source, I ran the Elecraft K3 at 10 watts and had no problem establishing contact. CW reports to the mainland U.S. (California) ranged from 559 to 599 and SSB contacts (California) varied between 54 to 57--enough for an enjoyable QSO.

Other than the original purchase of wire from garage sales and a few items collected at the local recycling center, I spent spent no money on this simple antenna project.  Of course, my wallet will be a bit thinner once I replace the ladder line and coax used for this project.  I can't get ladder line on Hawaii Island.  The coax sold locally is not satisfactory. With the possible exception of RG-6 cable, which I sometimes get as "remainders" from the local cable and satellite dish installers, I usually order coax and ladder line  through a mainland U.S. vendor. Don't dismiss RG-6.  With adapters bought from Radio Shack, I can use RG-6 for patch cords and even feed lines for dipoles.  The mismatch (75 ohms vs 50 ohms for ordinary coax) is easily handled by my Drake MN-4.  I also use RG-6 for making matching sections for loops. This was a fun and educational project.

If you have limited space and a little time, you can have a multiband HF antenna that will get you on the air with a respectable signal.  Best of all, you won't need a ground radial system to make this vertical work.

REFERENCES:

Hutchinson, Chuck (K8CH) and Straw, Dean (N6BV).  Simple and Fun Antennas for Hams. ARRL, Newington, CT, 06111. Copyright 2002-2005, First Edition, Second Printing.  Pages 9-1 to 9-9 and 10-1 to 10-4.

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Until next time,

Russ (KH6JRM).