I can't see that it makes much difference, but is there any deference which
way the coax is connected to a J pole antenna. Is it better to have the
centre of the coax connected to the main element or the stub?
I notice that most if not all J pole designs on the net have the centre of
the coax connected to the main element, however when I refer to a 1977 ARRL
antenna book it has it connected the other way.

Is there any difference?

Peter

"Peter" wrote in
:

I can't see that it makes much difference, but is there any deference
which way the coax is connected to a J pole antenna. Is it better to
have the centre of the coax connected to the main element or the stub?
I notice that most if not all J pole designs on the net have the
centre of the coax connected to the main element, however when I refer
to a 1977 ARRL antenna book it has it connected the other way.

Is there any difference?

It is doubtful that one is better than the other, though the various
proponents are usually adamant that there is only one 'correct' way.

A better method is to feed the coax inside the tube at the centre /
bottom of the U section, and have it emerge at the feedpoint where you
bond the shield to tube at the emergence, and take the centre conductor
directly to the other side and connect it. Not as adjustable, but this
method incorporates a balun.

You can get nearly the same effect by strapping the coax to the outside
of the tube and bonding the shield at the bottom of the U section (as
well as at the 'feed point' of course)... but no one seems to do it. Not
surprising, as it is a measure to reduce common mode current which most J
pole devotees pretend does not exist.

Owen

On Oct 12, 7:02*pm, Owen Duffy wrote:
"Peter" wrote :

I can't see that it makes much difference, but is there any deference
which way the coax is connected to a J pole antenna. Is it better to
have the centre of the coax connected to the main element or the stub?
I notice that most if not all J pole designs on the net have the
centre of the coax connected to the main element, however when I refer
to a 1977 ARRL antenna book it has it connected the other way.

Is there any difference?

It is doubtful that one is better than the other, though the various
proponents are usually adamant that there is only one 'correct' way.

A better method is to feed the coax inside the tube at the centre /
bottom of the U section, and have it emerge at the feedpoint where you
bond the shield to tube at the emergence, and take the centre conductor
directly to the other side and connect it. Not as adjustable, but this
method incorporates a balun.

You can get nearly the same effect by strapping the coax to the outside
of the tube and bonding the shield at the bottom of the U section (as
well as at the 'feed point' of course)... but no one seems to do it. Not
surprising, as it is a measure to reduce common mode current which most J
pole devotees pretend does not exist.

Owen

Owen
Every set of instructions I found when I was building my collinear J-
pole included a BalUn in the design. Some of the users of these
antennas are not fanatics but simply find it easier to use six turns
of coax as the the BalUn. And as to why it is simply easier to
apply. Since I am getting to the place were I don't like the
inelegance of the coax BalUn I will soon be changing over to a ferrite
bead BalUn for the sake of it's lesser ice collecting properties, it's
improved appearance, and smaller visual profile. Not everyone
believes that the antenna that they are using is magic. Many have
other reasons for not doing it in a particular way. Other than the No
BalUn voodoo are there other reasons that you don't like J-poles?
--
Tom Horne, W3TDH

Tom Horne wrote in
:

....
other reasons for not doing it in a particular way. Other than the No
BalUn voodoo are there other reasons that you don't like J-poles?

It is not that I "don't like J-Poles", but that they have certain
characteristics that should be considered in their design and
implementation.

They are an 'end fed' antenna, and there is necessarily some common mode
current excited on the feed line / supporting mast structure. A balun
helps to reduce that, and I described an integrated balun that is an
elegant construction, though a further ferrite current mode choke would
be of benefit.

The plumber's delight construction which is often hailed as an advantage
is actually a frustration when trying to reduce common mode current.

Though the J-Pole is often promoted as a easy thing to implement, they
figure disproportionately in online forums with questions like "I
adjusted my SWR real good in the shop, and when I raise the thing, the
SWR changes". The common sensitivity of measured SWR to antenna position
and line placement is a sure sign of high common mode current.

The common mode current doesn't necessarily stop them working, but it
does frustrate setup, a trap for the inexperienced.

BTW, J-Poles are not popular commercially in this country, I cannot
recall ever seeing a commercial J-Pole for non-amateur use. Perhaps there
is a message there about convenience and predictability for the
installers.

If a J-Pole suits your requirement, go for it. They warrant an effective
common mode choke, if for no other reason than to make VSWR less
dependent on the layout below the feed point.

Owen

On 10/13/10 12:46 PM, Owen Duffy wrote:

It is not that I "don't like J-Poles", but that they have certain
characteristics that should be considered in their design and
implementation.

They are an 'end fed' antenna, and there is necessarily some common mode
current excited on the feed line / supporting mast structure. A balun
helps to reduce that, and I described an integrated balun that is an
elegant construction, though a further ferrite current mode choke would
be of benefit.

The big question I have always had regarding the balun was are we
reducing performance by eliminating feedline radiation? For a lot of
antennas, that may be a big part of the antenna system.

- Mike -

On Oct 13, 11:53*am, Mike Coslo wrote:
The big question I have always had regarding the balun was are we
reducing performance by eliminating feedline radiation?

For shorter wavelengths, like 2m, feedline radiation is bad news for
the radiation take-off-angle. It essentially makes the antenna a lot
longer than 5/8WL, i.e. end-fire, sending most of the radiation up at
a high take-off-angle.
--
73, Cecil, w5dxp.com

Mike Coslo wrote in :

The big question I have always had regarding the balun was are we
reducing performance by eliminating feedline radiation? For a lot of
antennas, that may be a big part of the antenna system.

Claiming feed line contribution to radiation as a performance factor is
usually a specious claim, and more likely to be made by someone with a
commercial imperative than a genuine interest in radiation performance. You
know, if you can't solve a problem characteristic, call it out as a
feature.

In this case, feed line radiation would usually skew the pattern upwards,
not necessarily a desirable feature, probably not desirable in most
situations.

Owen