(Dr. Slick) wrote in message . com...
Hi,

I built a Super J-Pole like this one:

http://www.nemr.net/~aschmitz/antennas/jpolecalc.html

For 300 watts, VHF frequencies. With i believe 6 turns of 4"
diameter
inductive loops in the coax (RG-213), just at the base of the antenna.

Problem was, the antenna had an intermittant problem, which turned
out
to be the dielectric foam material MELTING right at the inductive
loops in
the coax. I tore off the outer jacket, and the foam was melting and
oozing
past the outer braid, really nasty...so that the inner conductor was
intermittantly touching the outer braid, most likely.

A real hair-puller, because the super-J looked fine when i took it
down.

Can i get away without using the inductive loops? They are
supposed to be there to prevent the current from going down the
outside braid, right? So
that you don't have radiation along the coax?

Or perhaps i can make fewer turns, or larger diameter loops? So
that the
impedance discontinuity is less severe at this point?

One note... The 50 inch coax section on the RR2 is for the appx dual
5/8 elements. It's possible it might be better to use a 1/4 wave coax
section on yours, but I'm not sure. You could try both. Use your
receiver, and find a stable repeater and note the signal level.
Whichever length gives the strongest received signal is the one that
is giving the best decoupling. That will be reciprical, so your signal
to them will improve in the same amount. Many years ago, I tried
taking a RR2, and unhooking the lower decoupling section. On the old
IC-22U I used at that time, the signals on most repeaters would drop
appx 4 S units.
Decoupling is critical on VHF/UHF verticals. More so than gain from
using longer elements or arrays of them. I often use a well decoupled
1/4 ground plane and do about as well as a gain vertical. If the tip
of the 1/4 WL GP is at the same height as the tip of the gain antenna,
the difference is quite small. MK