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

On 10/13/10 1:14 PM, Owen Duffy wrote:
Mike 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.

I don't disagree, but certain antennas - the EH antenna comes to mind,
which resemble a tuned circuit on a stick. Get rid of the feed line
radiation, and you almost get rid of the whole antenna. But like you
say, its a "feature".

It's also not good practice.


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

Not too desirable. Thanks for the clarification. Mine was a general sort
of response, not limited to this antenna.

- 73 de Mike N3LI -

On Oct 13, 1:42*pm, Mike Coslo wrote:
On 10/13/10 1:14 PM, Owen Duffy wrote:

Mike *wrote :

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.

I don't disagree, but certain antennas - the EH antenna comes to mind,
which resemble a tuned circuit on a stick. Get rid of the feed line
radiation, and you almost get rid of the whole antenna. But like you
say, its a "feature".

It's also not good practice.

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

Not too desirable. Thanks for the clarification. Mine was a general sort
of response, not limited to this antenna.

- 73 de Mike N3LI -

Years ago I built a J antenna for 10m. I was actually pleasantly
surprised at how easily it tuned up compared to the 2m versions I had
buit. I checked for feedline radiation by checking for variarions in
VSWR while coupling a large mass(me) to the feedline near the feed
point. Little change was noted. Do the larger versions of this antenna
normally perform better(more predictably) or was this just a case of
dumb luck.

JImmie