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Gold Medal Software - Volume 3 (Gold Medal) (1994).iso
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1994-02-25
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5KB
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79 lines
The TV-Twin Lead J-Pole Antenna
By Kelly Gray VE3VGP
Like most beginning Hams these days, I've started out on 2 metres.
Also like most beginning Hams, I don't have the cash to run out and buy the
latest and greatest of radios. I didn't manage to get on the air at all
until a month after I got my licence, when I finally managed to get the
use of a handheld transceiver. Once I was on the air, I found the things
worked great here in the city, but not so great once I got away from the
hordes of repeaters. Since I was busy saving money to buy any radio, buying
an amplifier to increase my power output was out of the question. What I
needed was something to make better use of the limited amount of power
available from my HT, preferably something that was also easy to pack up
and carry along on backpacking trips.
Digging through the local Library, I soon discovered that most
published designs had one of two "features." Either they were "portable"
only if you had a car to mount them on, or they were designed for HF only.
It seems that for most people, portable 2M meant a rubber ducky to talk to
the repeater down the block. Digging through the amateur radio message
areas on a couple of computer systems, I came across some ideas for a
J-Pole that seemed to fit my needs perfectly, cheap, easy to build, highly
portable, and reasonably effective.
The J-Pole antenna consists of a short vertical element that is 1/4
wavelength long placed close beside and parallel to a longer vertical
element that is 3/4 of a wavelength long. This is effectively a half wave
dipole fed from one end by way of a 1/4 wave section of transmission line.
Making this from a section of 300 ohm TV twinlead is simple, requiring not
much more in the way of tools than wirestrippers, a soldering iron, and
something to measure with. Materials required are a length of 300 ohm TV
twinlead, the cheap flat stuff, not the foam type twinlead, and a 50 ohm
coax feedline with the appropriate connector to connect to your radio.
Starting with a 54 1/2 inch length of twinlead, strip off 1/2 inch
of the insulation at one end, and short the exposed wires together. This
forms the base of the "J."
At a point 16 3/4 inches from the base of the antenna, cut a 1/4
inch notch in the side of the twinlead. This creates the shorter side of
the J from one side of the twinlead.
At a point 1 1/4 inches above the base, strip away just enough
insulation from the twinlead to expose the conductors in the twinlead. The
centre conductor of the coax feedline is then soldered to one conductor of
the twinlead, and the braid is soldered to the other. In the various J-Pole
plans that I've seen, some have the centre conductor soldered to the longer
side of the J-Pole, and some have it soldered to the shorter side. I really
don't think it makes all that much of a difference though.
Tuning the antenna is a matter of adjusting the solder points of the
feedline up or down to find the best match. Beware though, the TV twinlead
antenna is very sensitive to changes in the material around it. I tuned my
antenna while it was hanging from the ceiling in the middle of a room, and
it was totally unusable taped to the concrete bock wall, or even when taped
to a wooden stick! I would suggest that the tuning be done with the
antenna in a position as close as possible to the position it will be in
when operating, and you either raise the antenna above you, or move at
least a yard or two away from it while testing.
When taking my TV-twinlead J-Pole on a trip, I bring along a few
yards of light nylon cord, the kind used sold for camping clotheslines etc.
I toss that over the branch of a tree, tie the end to a hole punched in the
insulation at the top end of the J-Pole. Then I simply pull on the cord to
raise the antenna as high as permitted by the length of the feedline.
Typically this means that the top of the antenna will be something like 15
feet above the ground. Using only 2.5W from my handheld, I've managed to
reach the VE3MUS repeater from a point some 15 miles away inside Algonquin
park. Not bad for something that can be rolled up and stuffed into a pocket.
For parts, I went down to the local Canadian Tire store where I was
able to obtain a huge roll of twinlead for $10. A stop at an electronics
surplus shop turned up a "test lead" that consisted of a length of RG-58
coax with a BNC connector on one end, and a pair of alligator clips on the
other. A quick snip with the wirecutters, and I had my feedline and a spare
set of alligator clips. Now that I have my antenna, and a handheld, I'm all
set. All I have left to do is figure out what I'm going to do with the
remaining 95 feet or so of twinlead I have left over.