Not quite. Look closer - http://www.arrowantennas.com/j-pole.html
You assume it should result in common-mode currents.
And you know what happens when you Ass u me.

But....Common mode current on the feedline can't be avoided
no matter how the antenna is fed. I agree that the antenna is
quite usable without decoupling, and I've found that the 1/2 WL
ringo's usual "gamma loop" feed is also fairly good as far as that.
But there is no way to totally avoid all currents, without some form
of decoupling. If you believe you can, try some tests adding decoupling
sections. If you don't see an increase in performance, I would
be *very* surprised. In some cases, it's possible for the currents
to add, and produce a gain at low angles, but it's fairly unlikely.
Most of the time, the pattern is skewed upwards. It takes at least
two decoupling sections to fully decouple an elevated vertical.
To me, you have none, although maybe you could argue that your
feed of feeding qualifies as a first decoupling section...Lets say
you win, and this is the case...You still need one more, if you
want to be fully decoupled... If you believe you are now, I'd be
willing to bet you are deluding yourself.
Thats not to say the antenna doesn't work ok as is...I'm sure it
probably does, except the most retentive of users...I've used
loads of halfwaves with no decoupling...But those were all on 10m,
where it's not so critical. BTW...The results of adding decoupling
sections will vary from user to user, depending on their lines.
But if the antenna is totally decoupled, the amount of current will
be the appx same for any user, no matter feed length.
When I say total, I mean as much as practical. I don't believe you
can totally eliminate all current. There will always be a small
amount.
I doubt I would change the antenna....Would make it cost more...
I'd consider a super deluxe version for a higher price, if they wanted
full 2 section decoupling...:/ And if I went that far, I'd use a dual
5/8 design to get more gain. You'd be reinventing the isopole...:/
MK

On 12 Jun 2005 15:44:18 -0700, "Al" wrote:

Hm, why is that? How does an open stub prevent coupling from the antenna
to the feedline?

Roy Lewallen, W7EL

I was hoping we would not have to go through this AGAIN.

The typical J-Pole (Closed Stub) was designed to feed with open feed
line. Because Hams insist on feeding it with coax, a Band-Aid is
needed to choke off feed line radiation & common mode currents.
The Open Stub J-Pole was designed to feed with coax. So it don't need
the Band Aid.
The difference between the two is discussed in several antenna books.
Just ask anyone the owns one.
(http://www.eham.net/reviews/detail/1613)
Besides your (sic) the antenna expert, tell us why it would.


Careful Roy, you're goring a commercial ox.

Al, you have some really ingenious machinery and your products look to
be well made. But you should probably stick to machining and hire two
more folks: one to do antenna design and another to write copy and
manage your web site.

With just a cursory look I see that you sell "beems" instead of
"beams" and a claim that corner reflectors work better on packet than
do Yagis is pure snake oil.

Simply modeling your super J-pole and adding a third element to
represent the transmission line would show that the line radiates like
crazy and affects the feedpoint Z.

On 12 Jun 2005 23:39:08 -0700, wrote:

It takes at least
two decoupling sections to fully decouple an elevated vertical.

That statement is generally correct, however, if you were to use a
bead balun things change a bit. The reason being that on 2-meters the
balun is about 10" long (about 1/8-wavelength). When model correctly,
you can observe that such a balun is pretty good at choking of common
mode current without resorting to adding a second one.

It works even better on 200 MHz and 440MHz.

73,
Danny, K6MHE

With just a cursory look I see that you sell "beems" instead of
"beams"

Look a little more and you will find more typo's I am sure.

and a claim that corner reflectors work better on packet than
do Yagis is pure snake oil.

I think I said long Yagi's, something to do with phase distortion. I
am just going by what customers have told me that have used both.

Simply modeling your super J-pole and adding a third element to
represent the transmission line would show that the line radiates like
crazy and affects the feedpoint Z.

I never said anything about a "super J-Pole" .
Other than maybe it is super simple.

If you simply modeled the OSJ and it showed the feed line radiates like
crazy, Maybe you didn't do it right. Maybe you need to step out into
the real world.

By the way, it has a 3rd element, makes it a dual J-Pole not just a
dual band.

73 Al Lowe N0IMW

They who discuss "power radiated from the feedline", yet are unable to
measure (in watts) or calculate (in watts) the MAGNITUDE of the
effect, belong to a set of waffling old wives.

How is it possible to decide whether or not a choke or balun is
needed, and where to locate it, unless the magnitude of what one MIGHT
wish to prevent is known.

Do we always, without valid reason, just copy the practical
construction details from ARRL handbooks, on the grounds that if it
once worked OK for somebody else it might work for me.

But of course, you'll never get to know, having copied a very simple
system which contains a choke, unless you remove the choke and observe
what happens to system performance. Or alternatively, add a choke if
the ARRL handbook implies that you don't need one, and then make more
observations.

My guess is that in many cases hardly anything will happen or be
noticed. Simply because the MAGNITUDE of the effects due to line
radiation is too small to be of consequence or detected even.

If you can't measure or calculate the MAGNITUDE of the effects, stop
worrying about it.

The frequency at which "radiation from the line" is mentioned in this
newsgroup is out of all proportion to its importance. There's a
tendency to drag it into the discussion because it is the last of the
few remaining technical topics available to argue about. You have at
least heard about the subject in the magazines.

The very last, of course, will be SWR. Because there is a meter which
supposedly measures it but doesn't. And it is difficult to argue
against meter users, such as Bird, suffering from delusions of
accuracy, who are invariably convinced they are right.
----
Reg, G4FGQ

Al:

In college courses I have pointed out the typos in handouts/papers my
professors have provided me.

Strange, but from the reactions I received, you would almost think the
professors would have rather not had it brought to their attention (I am
quite sure I am mistaken on this point)--I did quickly discover it had a
direct effect on my course grades, at this point I stopped being so
"helpful."

I see others went ahead and developed this "science" of "being helpful"
into an art... grin

Warmest regards,
John

"Al" wrote in message
oups.com...
With just a cursory look I see that you sell "beems" instead of
"beams"

Look a little more and you will find more typo's I am sure.

and a claim that corner reflectors work better on packet than
do Yagis is pure snake oil.

I think I said long Yagi's, something to do with phase distortion. I
am just going by what customers have told me that have used both.

Simply modeling your super J-pole and adding a third element to
represent the transmission line would show that the line radiates like
crazy and affects the feedpoint Z.

I never said anything about a "super J-Pole" .
Other than maybe it is super simple.

If you simply modeled the OSJ and it showed the feed line radiates
like
crazy, Maybe you didn't do it right. Maybe you need to step out into
the real world.

By the way, it has a 3rd element, makes it a dual J-Pole not just a
dual band.

73 Al Lowe N0IMW

Al wrote:
If you simply modeled the OSJ and it showed the feed line radiates like
crazy, Maybe you didn't do it right. Maybe you need to step out into
the real world.

Al, you could get a relative measure of common-mode
current by slipping a #68 toroidal pickup coil over
the coax.
--
73, Cecil http://www.qsl.net/w5dxp

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Reg Edwards wrote:
If you can't measure or calculate the MAGNITUDE of the effects, stop
worrying about it.

Does RF burns on my lip count as a measurement? :-)
--
73, Cecil http://www.qsl.net/w5dxp

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Sniped
I don't believe you can totally eliminate all current. There will always b=
e a small
amount.

Agreed, but it is small enough I don't think most people could see a
difference in performance if it had a choke or not.
All I am saying, to say the antenna won't work with out a choke is just
wrong.

I doubt I would change the antenna....Would make it cost more...
I'd consider a super deluxe version for a higher price, if they wanted
full 2 section decoupling...:/ =A0And if I went that far, I'd use a dual
5/8 design to get more gain. =A0You'd be reinventing the isopole...:/
MK

I have an "Isopole" here in the shop, it works good, maybe one of the
best
5/8 wave antennas I have ever tested. I think it could be built a
little better.
I have seen a lot of them that were broke.
But that is comparing apples to oranges
The OSJ is a simple 1/2 wave antenna that has 0 dBd gain.
No magic.
Again, I just think it is wrong to tell someone the antenna won't work
with out a choke.

73 Al Lowe N0IMW

Reg Edwards wrote:

They who discuss "power radiated from the feedline", yet are unable to
measure (in watts) or calculate (in watts) the MAGNITUDE of the
effect, belong to a set of waffling old wives.


Those who take no precautions to prevent their feedline from becoming
part of the antenna, belong to a set of people who don't even know what
their antenna IS.

There's a part you call "the antenna", and another part you call "the
feedline". Wishful thinking will not stop RF current from flowing
directly from one to the other.

How is it possible to decide whether or not a choke or balun is
needed, and where to locate it, unless the magnitude of what one MIGHT
wish to prevent is known.

Try a clamp-on RF current meter, a little modeling... or even a little
common sense.

There's a place called "the feedpoint" where the antenna and the
feedline are connected directly together. Might that be a good location
for a choke to keep them separate? Yes, it almost certainly would.

Chokes may also be needed at other locations, but it's hard to justify
anywhere else as your *first* choice. (The exception is the Carolina
Windom and similar antennas where part of the feedline is intended to
radiate. But even there, they put a good choke at the point where they
want RF currents on the feedline to stop.)

Queen Elizabeth 1 of England had the good sense to take a bath every six
months "whether I need it or not". If you don't know whether your
feedline smells of RF, then follow her excellent advice and use a
choke.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek