[Ccarc] 20-meter J-Pole

[Tom Murray]

Saw this on eham website
Tom  KB9WSL

The 20-Meter J-Pole
(N2KMF) on August 14, 2006
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The 20-Meter J-Pole

Recently, I decided that I was getting bored with my sloping longwire 
antenna. It worked fine, to be sure, but there were some serious holes in 
the coverage outside of the major lobes. I decided that I wanted to put up a 
vertical antenna, but the thought of burying a ton of radials in my backyard 
left me cold. A vertical dipole seemed to be called for, but I didn't have 
enough supports to run the coax perpendicular to the dipole for any 
appreciable length.

After some thought, I hit upon the idea of making a 20 Meter J-Pole. I've 
been a fan of J-Poles for years, having built them out of copper pipe for 
use on 2 Meters. Obviously, I wasn't going to be able to use copper pipe on 
20. I settled on a design that used insulated 12 gauge stranded wire. After 
modeling it several different ways on EZNEC, I was prepared to mount the 
antenna with the matching section runs parallel to the ground a few feet up, 
whit the radiating 1/2 wave section vertical. That allows for a lower 
support, and only lowers the gain slightly. It also 'de-circularizes' the 
pattern, but only by about 1 dB.

I modeled the antenna with a 1-foot gap for the matching section. When I 
went to build it, I found that the scrap Lexan I planned on using as spacers 
wasn't long enough, so I re-modeled it with a 6-inch gap. The change had 
little effect according to EZNEC. I made the spacers by cutting the lexan 
into 7 inch by 3 1/2 inch rectangles, then drilling four holes, one near 
each of the corners. I made 5 spacers this way. I had planned on making 6, 
but one of them broke a corner while I was drilling it.

I took my measuring tape and staked a 16.5-foot distance on the ground. I 
wrapped the wire around it four times, then cut it about a foot past the 
stake to allow for the 6 inch gap and to allow for some extra at the top of 
the antenna for the insulator. I then staked the antenna down in the final 
shape. I cut and stripped the wire at the bottom, then started sliding the 
spacers up the matching section. The wire is stiff enough that they don't 
move once locked in place (the wire is threaded up through one hole, then 
down through the other on both sides of the spacer). If I ever need to use 
open wire line for a different antenna project, this is how I am going to 
make it. I put a small knot in the wire at the top of the matching section 
to keep it from sliding out of the spacer.

Feeding the antenna is dirt simple: An SO-236 chassis connector soldered on 
the bottom, with the center conductor going to the longer wire and the 
ground going to the matching section. I was concerned that I might need to 
have some kind of elevated feed, like I have done in the past with copper 
J-Poles on 2 Meters, but modeling suggested that it wasn't necessary. That 
was borne out in practice. I now think that the elevated feed seen in 
"Plumber's Delight" J-Poles is just a more convenient way to feed them, 
given their construction.

I first tried to get the antenna up into the tree I was going to use for 
support using a wrench tied to a rope, swinging it around my head. After 
several tries, all of them either failing to get enough height or missing 
the tree entirely, I gave up and gave my friend Jack KE7NL a call. He 
brought his slingshot/fishing reel antenna raisin' contraption, and a good 
deal more experience, with him. After a few tries, we were able to get the 
antenna high enough so that all but the very top of the antenna is vertical. 
Since the length of the antenna is about 50 feet, that is no small 
accomplishment. The bottom of the antenna is about 1 foot off the ground. I 
am indebted to Jack for his assistance and advice.

The antenna seems to work quite well on 20 Meters. The VSWR is between 1.5 
to 2 for the entire 20-Meter band, and I have received good reports in the 
few short days it has been up. Contacts have been made from my Upstate NY 
QTH to FL, SC, WI, and OH on 20 Meters. Using a tuner, it will load up on 
80, 40, 15, and 10 Meters. I have made a couple of contacts on 40 Meters 
with it, however I could tell that the antenna wasn't performing well. Since 
the 1/2 Wave section on 20 meters is 1/4 wavelength on 40 Meters, I am 
thinking about burying some 40 Meter radials underneath the antenna and 
possibly using a clip lead to connect them to the matching section of the 
antenna while I am on 40 Meters to see if that will help. On 15 and 10 
Meters, modeling suggests that most of the radiation will go up, instead of 
towards the horizon. In any case, this is a much better 20 Meter vertical 
than you can buy, and since I already had all of the materials at home, it 
cost me all of zero dollars to put up.

73 de Bill N2KMF