Thursday, February 27, 2014

Simple Ham Radio Antennas: An 80-10 meter inverted vee dipole. Post #263

A few days ago I read a fascinating antenna article by Cecil Moore (W5DXP) on the http://www.eham.net website.  Cecil described what he called a "Multi-Band 33-ft dipole" that works on 20, 17, 15, and 10 meters without an antenna "tuner" (i.e. transmatch).  The secret to this outstanding antenna was the use of 450 ohm ladder line in conjunction with a series of switches and relays that selected the proper length of ladder line used to feed the antenna.  Depending on the band of use, Cecil could insert and remove sections of ladder line with his relay/switch system so that swr across his chosen band remained low.  By carefully selecting which section of feed line to add or subtract, he got a decent performing antenna without having to resort to a transmatch or "tuner."

Cecil's idea got me thinking of ways to improve the dipoles I'm using at my new home site.  Although his idea has a lot of merit, I felt the need to build something a bit simpler that would get similar results.  My requirements were simple:

The antenna would require one tall support.  I didn't want to climb trees to erect an antenna.  At my age, a fall really can ruin your day.

The antenna had to cover Hawaii Afternoon Net frequencies, 3.888 MHz and 7.088 MHz.  I would also like to work some DX on 20 through 10 meters.  So, the antenna would have to cover 80 through 10 meters.

Although I had erected a basic doublet in December 2013, it had fallen victim to a winter storm and high winds.  I needed a replacement for this general coverage antenna.

The 40-meter loop had been removed to make way for a family garden in the back yard.  My xyl and I are trying to become more food self-sufficient.

So, I decided to re-invent my first novice class antenna--the 80 to 10 meter inverted vee.  As I remember, the antenna was cheap, only took one tall support, and could cover several amateur radio bands with the use of an antenna transmatch, a 4:1 current balun, some surplus speaker wire, and a length of 450 ohm ladder line.  Most of the materials were stored in the new garage along with ceramic insulators, a "ladder lock" center connector, some wooden support stakes, various lengths of RG-8X coax with UHF connectors, and some spare tools.

The construction would be straight forward, with most of the building done on the ground and the antenna later raised into position on a fiberglass mast.  The inverted vee is a simple antenna, which is nothing more than a drooping 1/2 wave dipole cut for your lowest operating frequency and fed with ladder or open wire line to a balanced "tuner" capable of accepting such a line.  In the absence of a balanced "tuner", you can connect the ladder line to a 4:1 current balun and use a short length of 50 ohm coaxial cable to connect the antenna system to your transmatch or "tuner."

CONSTRUCTION PHASE:

Using the general dipole formula 468/f (MHz)=L (ft) and the lowest design frequency of 3.500 MHz, I came up with a total dipole length of 137.71 ft/40.76 meters.  Each sloping dipole element would measure 68.85 ft (around 68 ft, 10 in.)/20.38 meters.  The only wire available last weekend for the project was about 150 ft/45.73 meters of #18 AWG speaker wire from Radio Shack.  For most of my antennas, I try to use stronger wire, such as #14 AWG household wire.  But in this case, the speaker wire was all I had.

I attached a ceramic insulator to one end of each antenna element.

I had a spare 33 ft/10.06 meters  MFJ telescoping fiberglass mast in the garage that would serve as the support for the inverted vee.  I extended the mast to its full length and proceeded to attach the 450 ohm ladder line to the mast.  I would be using 90 ft/27.43 meters of ladder line, because the antenna would be erected at one end of the garden which was approximately 65 ft/19.81 meters from the radio room in the garage.  I attached a "ladder lock" device to one end of the ladder line, looped a piece of nylon rope through the top support hole, and tied the "ladder lock" to the tip of the mast.

I ran the ladder line down from the top of the 33 ft/10.06 meter mast to a point about 8 ft/2.43 meters above the bottom of the mast.  The ladder line was secured to the mast by nylon ties.

Each antenna element was soldered to a leg of the ladder line.  Connections were wrapped with several layers of vinyl electrical tape.

With construction complete, I drove a 5 ft/1.52 meter wooden support stake into the ground to support the fiberglass mast.

I then hoisted the mast into position, tied off each antenna element to nearby trees with nylon rope, and adjusted the antenna to a uniform shape.  The ends of each element were approximately 10 ft/3.04 meters above ground level.

I then pounded six, five-ft/1.52 wooden stakes into the ground, each 10 ft/3.04 meters apart, to support the ladder line off the ground until the ladder line reached the window of the shack, some 65 ft/19.81 meters from the base of the mast.

I slipped the ladder line through the bottom of the shack window and connected it to a W9INN 4:1 current balun.  A three-ft/0.91 meters length of RG-8X coax with UHF connectors mated the antenna with the Drake MN-4 transmatch ("tuner").  Short pieces of RG-8X connected the Ten-Tec Argosy II with the Drake MN-4, a low pass filter, and a Heathkit Dummy Load.  If I want to use 30 meters, I substitute a MFJ-941E Versa Tuner II for the Drake MN-4.  The Drake "tuner" doesn't cover 30 meters.  As a final step, I attached a 68-ft/20.73 meters "counterpoise" to the ground lug of the antenna transmatch.  The "counterpoise" is in addition to the regular station ground which uses a short length of copper braid clipped  to an 8-ft/2.43 meters copper ground rod.

INITIAL RESULTS:

With the Drake MN-4 or MFJ-941E in the line, I can get a swr of 1.1 to 1 on all bands from 80 to 10 meters.  Unlike other multi-band designs, this antenna requires an antenna transmatch or "tuner" to work properly.  Because of its relatively low height, the inverted vee works very well as a NVIS (near vertical incident skywave) antenna on 80 and 40 meters.  This antenna provides excellent local and state wide coverage throughout the Hawaiian Islands.  DX begins to pick up at 20 meters, where the apex of the antenna is nearly 1/2 wavelength above ground.  When 15 and 10 meters are open, I get good signal reports on both cw and ssb.

The inverted vee gives me satisfactory performance at a cheap price.  It's easy to build, requires no ground radial system, and can be used in portable or emergency situations.  Most of the materials for this simple, effective antenna can be found at the nearest hardware store or home improvement outlet.  Some of the accessories, such as insulators and center supports can be "homebrewed" with plastic, teflon, or even plexiglass.

REFERENCES:

http://www.hamuniverse.com/hfdoublet.html.

http://www.w4rnl.net.46/abd.html.

http://www.designerweb.net/downloads/DoubletAntenna_80-10m.pdf.

http://www.theladderline.com/doublets-ladder-line-and-automatic-remote-tuners.

http://www.eham.net/articles/31603.  This is the outstanding antenna article by Cecil Moore (W5DXP).

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Aloha es 73 de Russ (KH6JRM)

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


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