Monday, May 6, 2013

Antenna Topics: A 20, 15, and 10 meter segmented inverted vee dipole. Post #187

KH6JRM's Amateur Radio Antenna Topics:

A 20, 15, and 10 meter segmented inverted vee dipole.

In my last post, I described a 40/15 meter inverted vee dipole using aligator clip leads and short pieces of wire to improve the SWR on 15 meters.  The short additional length of wire on each end of the inverted vee lowered the SWR for 15 meters and provided a slightly better match for the RG-8X coax used as the feed line.  When I wanted to use 40 meters, I just unclipped the short lead.  By selecting the correct length of wire for each band and using the clip leads to add or subtract a small amount of wire length, I was able to use this antenna without an antenna match or tuner.

This past weekend, I was able to modify this idea to get an inverted vee that would cover 20, 15, and 10 meters without a tuner.  As in the case with the 40/15 meter inverted vee, I incorporated some ideas from Edward M. Noll (W3FQJ) and Larry Barry (NV5I) to make a homebrew tri-bander inverted vee that has proven to be effective from my location on the Island of Hawaii (the island with the volcanoes).

MATERIALS:

One 33-foot (10.06 meters) fiberglass, pvc, or wooden mast.

Approximately 35-feet (10.67 meters) of antenna wire.  I used some #14 AWG housewire stored in the garage.

One Budwig HQ-1 center coax connector (available from Fair Radio Sales in Lima, Ohio).

Three 5-foot (1.52 meters) wooden or metal stakes.  One will support the mast and two will be used to tie off the ends of the inverted vee.

Eight alligator clip leads.

Six ceramic or plastic insulators.  These will be used to segment the antenna elements.

Fifty feet (15.24 meters) of RG-8X, RG-8, or RG-58 coaxial cable with UHF connectors.  This will be your feed line.

Ten feet (3.04 meters) of RG-8X or other 50-ohm feed line to connect your station to the main antenna system.  My antenna feed line is connected to a static discharge system below the shack window.  This small length of cable will connect the static discharge system and antenna feed line to the shack's equipment.

ASSEMBLY:

I built the antenna on the ground.

First, measure and cut the dipole elements for 10 meters.  Using the general formula 468/f(Mhz)=L(feet) and a frequency of 28.4 Mhz, the 10 meter dipole was cut to a length of 16.47 feet (5.02 meters).  Each dipole element was cut to a length of 8.23 feet (2.51 meters).  I attached each dipole element to the ends of the Budwig HQ-1 center coax connector.  Before tied off each end of the dipole elements to its insulator, I attached an alligator clip lead.  This lead would be connected to the next segment of the antenna element--the 15 meter portion.

Using the above formula for a frequency of 21.250 Mhz, the full-length dipole would measure 22.02 feet (6.71 meters). Each dipole element would then be 11.01 feet (3.35 meters).  Since I already had a 10 meter antenna length of 8.23 feet (2.51 meters) for each side, all I had to do was clip a small length of wire to the 10 meter clip lead on each side to make a full length 15 meter dipole. In this case, the additional wire length was 2.78 feet (0.85 meters).  I attached an alligator clip lead to the top and bottom of each 15 meter segment before I secured the 15 meter portion to the bottom of the 10 meter insulator.

Using the above formula for a frequency of 14.200 Mhz, the full-length 20 meter dipole would measure 32.95 feet (10.04 meters).  Each dipole element would then be 16.47 feet (5.02 meters).  As in the above examples, all I would need to complete the full 20 meter antenna element for each side is a short piece of wire added to the clipped together 10 and 15 meter elements.  In this case, the additional wire length was 4.97 feet (1.51 meters) for each side of the inverted vee.

I attached an alligator clip to the top of each 20 meter segment and tied off the ends at the final insulator.

The total length of the connected elements came to 15.98 feet (4.87 meters) for each side of the inverted vee.  By connecting the appropriate segment, I would be able to select any one of the bands (20, 15, or 10 meters) without encountering excessive SWR.  Later, I had to adjust the length of the 20 meter segment because the chosen length was a bit short (about 2.5 inches/6.25 cm) per side.

After I attached the RG-8X to the center connector, I wound approximately 6 feet (1.82 meters) of the coax into a "choke balun" and secured this coil together with nylon ties.  I hoisted the mast on its support stake and tied off the end insulators to nearby stakes.  The antenna elements added some support to the mast.  Trade winds haven't affected the mast too much.  I will later install some guy ropes to further stabilize the mast.  All antenna connections have been soldered and covered with several layers of plastic vinyl electrical tape.

I ran the RG-8X halfway down the mast (approximately 16 feet/4.87 meters), secured the coax to the mast with nylon ties, and led the cable to the anti-static unit beneath the shack window.  A short piece of RG-8X connected the shack's equipment (Drake MN-4, if needed, a low pass filter, a dummy load, and the Swan 100 MX transceiver).

PRELIMINARY RESULTS:

So far, results have been good.  SWR on 10 and 15 meters stays below 1.7 to 1 without the Drake MN-4.  With the Drake MN-4 connected to the antenna system, SWR stays below 1.2 to 1 across each band.  The results on 20 meters are less satisfactory, with SWR readings of 2.0 to 1 without the Drake MN-4 and 1.5 to 1 with the Drake unit in the antenna system.  A little trimming will be needed to bring the 20 meter portion of the antenna into a better SWR reading.

Despite this minor problem, all bands covered by this antenna work well.  The antenna is a project in progress.  I could possibly add further segments to cover 40 and 80 meters, but, by that time, I will have run out of room to accommodate a larger antenna.  My backyard is a bit small.

With the old Swan 100-MX running 20 to 30 watts CW, I'm getting reports of 569 to 599. SSB reports vary between 56 to 59, depending on propagation and band conditions.  This antenna has its flaws, but it keeps me on the air with a minimum of cost and maintainance.  I'll work on this "skyhook" for a few weeks before I build another antenna.  Right now, my collection of wire and feed line is running low, so I'll have to visit the nearest hardware store and pick up what I lack in the storage room.

I have about 100 feet (30.48 meters) of 450-ohm ladder line which is waiting for a project.  I think a doublet antenna is in the planning stages.

Have fun building your next antenna!

REFERENCES:

Noll, Edward M.(W3FQJ).  Easy-Up Antennas for Radio Listeners and Hams. Limited Edition, 1991.  MFJ Enterprises, Inc. Mississippi State, MS, 39762.  pp. 111-113.

Barry, Larry A. (NV5I).  Hints and Kinks for the Radio Amateur. 13th Edition.  ARRL, Inc.  Newington, CT, 06111. pp. 7-26 and 7-27.

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Aloha es 73 de Russ, KH6JRM

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