Advice Needed for Super J-Pole Design: Inductive Loops Overheating
 

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

"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?

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.

(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

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

----- 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

http://hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

(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

How much radiation, in watts, usually occurs from feedlines? Does it make
much difference to anything? Is it worth worrying about?

If it works don't fix it.
----
Reg.

==================================


"Mark Keith" wrote in message
om...
(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

"John Smith" wrote in message ...

Knowledgable advice much appreciated.

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.


Thanks for explaining it to the "expert".





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.


Well this is what i need to know, can i get away without the inductive loops?





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

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



Ask yourself why you answered when i wanted knowledgable advice.


Slick

"Thomas C. Sefranek" wrote in message ...
"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?

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?

SWR and common modes currents are unrelated. You can have a perfect
match, and still have excess current on the coax shield. The gain? No
skewing of the pattern up off the horizon. MK

"Reg Edwards" wrote in message ...
How much radiation, in watts, usually occurs from feedlines?

Dunno in watts. But it's a lot if it's frying coax innerds.
Does it make
much difference to anything?

Absolutely.

Is it worth worrying about?
Absolutely.

If it works don't fix it.

It's not working near it's potential. So it needs fixing. MK