Sunday, August 4, 2013

An 80 through 10 Meter doublet antenna. Post #218

Sometimes the simple things in life are best.  This reasoning can be applied to amateur radio antennas, where cost and ease of assembly are factors for your "antenna farm".

As much as I like towers and 4-element monoband HF antennas (I've used them during Field Day events), the cost of such structures can really ruin my retirement income.  So, as I've done in the past, I've designed and built simple antennas that "do the job" without depleting my bank account.  In most cases, limited funds, restrictive operating conditions, lack of space, and proximity to neighbors have dictated easily built antennas such as verticals, inverted vees, delta loops, and small flat-top dipoles.

Recently, my xyl and I were clearing some of her property in the Puna District when I saw two Norfolk Pine Trees separated by approximately 120 feet/36.58 meters.  There was a branch on each tree approximately 50-feet/15.24 meters above ground.  These branches would make a suitable support for an 80-10 Meter half-wave doublet or flat-top dipole.  The extra length needed to make the antenna "work" on 80 Meters could be suspend from each branch, with most of the length going from tree to tree.

A basic doublet is a half-wave dipole cut for the lowest frequency of use (in this case 80 meters), fed with 450-ohm ladder line, connected to a 4:1 balun.  A length of 50-ohm coaxial cable would run from the balun to the shack antenna transmatch and then on to the transceiver.  You could also use coaxial cable for monoband operation on the frequency of your choosing.

Since I had the necessary equipment in my "go kit", I could build a simple dipole fed with ladder line and get on the air without much difficulty.

MATERIALS:

100-feet/30.48 meters of 450-ohm ladder line.
One homebrew center connector.
Two ceramic insulators.
150-feet/45.73 meters of #14 AWG housewire for the dipole antenna.  I always measure out more wire than I need.  Actually, you could use any gauge wire for a temporary antenna.  I've found that #12 or #14 AWG wire is tough and will stand up to most weather conditions.


25-feet/7.62 meters of RG-8X with UHF connectors.
One W9INN 4:1 current balun.
One antenna transmatch.  My "tuner" was the classic Drake MN-4.
Two, 60-foot/18.29 meters of monofilament fishing line tied off with sinkers.
One WalMart slingshot.  The slinghshot would be used to launch the dipole into the trees.
One deep cycle marine battery with solar panel charger.
One folding table and chair.
Note paper, pen, calculator, sunshade.
A 7-foot/2.13 meters wooden stake to support the ladder line as it came off the center connector.
ASSEMBLY:

Since I wanted to cover amateur radio bands from 80 to 10 Meters, I cut the dipole for my chosen frequency of 3.500 MHz.  With the ladder line, balun, and antenna transmatch, I could work all bands without worrying about the severe mismatch that would occur on certain frequencies.

Using the general formula 468/f (MHz) = L (feet), I cut the dipole to a length of 133.71 feet/40.76 meters.  The dipole was cut into two equal parts measuring 66.85-feet/20.38 meters.

I connected the 450-ohm ladder line to my homebrew center connector.  I then soldered each segment of the dipole to the ladder line held by the center connector.  Each connection was wrapped in several layers of vinyl electrical tape.

Ceramic insulators were attached to each free end of the dipole segments.

Monofilament fishing line (with a sinker attached) was attached to each ceramic insulator.

A slingshot was used to launch each end of the dipole to a corresponding tree limb on each tree.  Since the distance between trees was not 133-feet/40.54 meters, there was approximately 13-feet/3.96 meters of antenna wire hanging down the tree in addition to the fishing line and sinker.

The monofilament fishing line, sinker, and extended piece of antenna wire were secured to each tree with a little "slack" in the dipole to accommodate the effect of wind on the trees.

One the dipole antenna was stabilized, I ran the 450-ohm ladder line to a 7-foot/2.13 meters stake near my operating table.

A 4:1 balun was attached to the ladder line.  A 25-foot/7.62 meters piece of RG-8X was connected to the 4:1 balun and terminated at the Drake MN-4 antenna transmatch.  Several small coaxial patch cords connected the Drake MN-4 to the dummy load and Yaesu FT-7 QRP transceiver.  Once the deep cycle marine battery was attached, I had power to run this portable station.

INITIAL RESULTS:

Thanks to the trusty Drake MN-4, I was able to get a SWR of 1.1 to 1 across 80, 40, 20, 15, and 10 meters.  Tuning was a little "touchy" on 20 meters.  With a power of only 10 watts, I was able to make several contacts on 80 and 40 meters and a few contacts on 20 and 15 meters, both CW and SSB.  Ten meters was quite noisy on Friday afternoon, so I didn't attempt much on that band.

For a simple antenna, the multiband doublet with ladder line does an excellent job.  Even with a bit of antenna wire hanging vertically down the tree support, performance has been quite good.  Antenna experts recommend that a doublet be erected as high as possible, preferably above 60-feet/18.29 meters.  Although my antenna height was below 60-feet/18.29 meters, it provided me with many contacts and an enjoyable few hours on a late Friday afternoon.

When I was done for the day, I cut off the sinkers from the fishing line, collected the collapsed dipole, rolled up the feed line and coaxial cable, and stored the old Yaesu FT-7 in the "go kit" behind the driver's seat of my minivan.  The Drake MN-4 and the 4:1 balun would be taken home for use in the shack.

Try a doublet with ladder line, balun, and a transmatch.  You'll have lots of fun with an antenna your built yourself!

REFERENCES:

http:www.hamuniverse.com/hfdoublet.html.
http://www.eham.net/Reviews/detail/255.
http://norcalqrp.org/norcaldoublet.htm.
http://pc5e.nl/downloads/wireantennae.pdf.

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Aloha es 73 de Russ (KH6JRM).

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