----- Original Message -----
From: "Thomas C. Sefranek"
Newsgroups: rec.radio.amateur.antenna
Sent: Monday, January 19, 2004 7:07 AM
Subject: Advice Needed for Super J-Pole Design: Inductive Loops
Overheating


"Dr. Slick" wrote in message
om...
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?

Knowledgable advice much appreciated.


Slick

Why do you have this choke at the feed point?


A J-Pole is inherently a balanced antenna (half-wave dipole) fed with a
quarter-wave matching section. To feed it with unbalanced line permits
common-mode currents on the outside of the line. Coiling the coax to make a
common-mode choke is a practice recommended in the ARRL literature (and
probably other places) to prevent the currents from traveling on down the
line to the source.



ASS-U-ME the feed is correctly adjusted for a perfect match to the
feedline,
what is to be gained by choking the feed line?
(You already know some of what could be lost!)

Ask yourself what currents are necessary to cause the inner conductor to
heat
to the point of melting the insulation.


If the antenna had an intermittent problem, I suppose the choke could have
taken the brunt of the incident power and melted.

It would be good to know if the melting occurs again without the
intermittent condition. If so, the OP will require help from a more
knowledgeable source than myself.



(Could the melting have been caused by the origional installation
soldering?)

Ask where are these magnitudes of current generated, and why.


--
*
| __O Thomas C. Sefranek
|_-\,_ Amateur Radio Operator: WA1RHP
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

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