ai8o wrote in news:1bcefed7-a087-4fda-976d-
:

I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe.

Most plans that I can find on the web call for a 58 inch main leg and
a 20 inch support below, making the antenna
78 inches or so in height.

I want to install it in my attic where I have 66 inches of clear
space.

Can I cut the lower support leg down to 8 inches without a major
impact on the performance of the antenna?

The support leg doesn't carry substantial RF currents, so by itself it
doesn't play a big role in the antenna.

I believe that its length is intended to help keep the
current-carrying portions of the antenna (the two-arm roughly-
quarter-wavelength matching section, and the half-wavelength upper
radiator) up clear of the support mast, so that the mast and roof
structure and etc. don't de-tune the antenna. If you shorten the
support section, and bring the current-carrying parts of the antenna
down into closer proximity with the attic floor, you *may* see some
change in resonant frequency and SWR. However, the same would be true
(perhaps even more so) if the upper part of the radiating arm comes
too close to the roof.

There are several things that you can try:

[1] Try it just as you propose.

[2] Try a "Z-folded J-pole", where the support arm is vertical, the
half-meter matching section is bent horizontally, and the
half-wavelength radiator turns back up into a vertical direction.
This sort of 90-degree bend between matching section and radiator
was used on classic wire "zepp" antennas, and I've seen it used
with center-fed vertical dipoles.

[3] Try an "open sleeve" J-pole. These are a bit shorter than classic
J-poles - a bit tricker to make, though, since the two arms in the
matching section are *not* shorted together at the bottom, and one
must have an insulated mounting of some sort. Arrow makes a nice
dual-band open-sleeve J-pole, and (last time I looked) they had
complete construction plans somewhere on their site so you could
fabricated one yourself if you wish.

In any of these variations, you may need to tune the antenna
somewhat to adjust for the antenna's shape and for its proximity to
the building structure. One way to do this is to cut the pipes a bit
long, and trim to fit. Placing a copper end-cap over the pipe ends is
a way to extend them a bit if necessary. If you drill a hole in the
end cap, braze a brass nut or two onto the hole, and insert a brass
bolt, you can adjust the effective length of the arm by screwing the
bolt into or out of the nut-and-cap portion.

"Fat" J-poles (with larger-diameter pipe) are relatively broad-band,
so you ought to be able to tune one down to an effectively-low SWR
across the 2-meter band.

[4] Build a J-pole out of 450-ohm ladder line, rather than copper
pipe - suspend it beneath the attic's upper support beam using
6" of nylon fishing line, and tension the bottom down vertically
using more fishing line. No need for a lower support arm at
all... just short the bottom two sides of the ladder line together
with a couple of inches of copper wire.

J-poles can be built in very non-traditional ways, and made to work
effectively, although the dimensions may be rather different than the
traditional plans would indicate. I made one several years ago by
taking a bicycle flag (about 5' of thin fiberglass rod with a flag on
top) and fastening ordinary hookup wire to it with black electrical
tape. It took me a while (and a lot of cutting-and-trying) to figure
out the right lengths for the two arms, and the correct attachment
point for the RG-58 coax feed near the bottom, but I eventually got it
down to an SWR of around 1.2 across the repeater portion of the
2-meter band. Fastened to the back of my recumbent bicycle, it gives
much better coverage than a rubber duck when I'm riding bicycle-mobile.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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