[Antennas] J-Poles

[J C White]

I have built J-Poles for about 30 years, using everything
from wire inside PVC to copper tubing to TV twin-lead.

Three of my antennas are multiband, one is 6 & 2, one
is 6 & 2 & 0.7 meters, and one is four banc, 6, 2, 1/12,
3/4, all on single main section.  Only the stubs are fed for
each band, and vertical spacing causes no significant
problems.  The 6 & 2 is from an ARRL antenna book,
and I just took off from there.  A friend has built about
50 J-Poles within the last 5 years, all copper.

We both have discovered that only on 6M is there much
advantage to running the coax all the way up inside the
main section.  However, there is nothing to prevent you
from running it inside, IF you make certain you do NOT
weaken the tubing where the coax exits.

As for ground planes, neither of us have found much value
in the form of gain from them.  Usually all it takes to keep
RF off the shield of the coax is a couple of turns of coax 
taped in a small loop...remember we are talking VHF/UHF
here so not much inductance is needed.  I HAVE used the
ground planes for isolation between the stubs, with some
increase in gain, but this is only useful on multiband J-Poles
we generally conclude.  

My 4 band J has the 6M on the bottom and each following
band as the next stub up.  When I do run the coax inside the
main section for more than 2 bands, I go to larger tubine than
the 1/2 and 3/4 used in the ARRL book.  This allows me to
place my RF blocking coils just below the feedpoint for each
of the stubs.

Be careful about the multiple shields of the coax feedlines
all connected to the main section, on multi-band Js..  Though
ground loops are far less troublesome at these frequencies, it
is something to check on to see if you are getting any ground
loop current, even in microamps, measured between shields 
at the transceiver end of the line.

Hope this helps.  I think I have the design of the 6 & 2 M
dual band J as gif file, and the 6 & 3/4 as well.  May even
have the 6, 2, 3/4 M J design as a gif.

Remember if you go multiband with feedline INSIDE main
section, you're using larger diameter tubing is going to change
the length of the main sections and the stubs.  Most issues of
the ARRL Handbook give the correction factor for changes
in antenna material diameter (often called "k" factor).

Have fun, and check out the "copper cactus" J website.  The
very simplest J is just a piece of TV twinlead, with the stub 
part cut short.  You can extend the twinlead well below the
feed point, if you want to use it as a universal matching stub
to nit pick your way to absolute 1:1 SWR (adjust the length
of the shorting section).

Remember to NOT use steel screws if you are fastening the
feedline to the copper with screws, due to electrogalvanic
action.  Brass hardware is fine.  Or, if you must mismatch
metals, say for strength of the main section, get a tube of the
anti-galvanic paste electricians use when joining aluminum and
copper wire or copper and steel wire.

WB6BLV@inreach.com