By open stub, you mean there is no loop at the bottom of the "J"?

bob k5qwg

No, Open Stub means the 1/4 wave element and the 3/4 wave element are
not connected.

In the typical (closed stub) J-Pole I have seen the center of the coax
connected to ether side of the antenna, seems to work equally poor
ether way.

In the case of the Open Stub J-Pole I have found that a wider bandwidth
can be obtained connecting the center to the 3/4 wave element. (about
12 MHz.)
Connecting to the 1/4 wave element, bandwidth is only about 6 MHz.
The 2 meter band is only 4 MHz. so it really don't matter too much.
Try that with the typical copper pipe J-Pole.
73 Al Lowe N0IMW

Dan:

Some of these arguments carry on into the realm where I lose interest...
but right on when you say, "...put a choke on that sucker!"

This may be due to the fact that I am not an "antenna guru" and can't be
certain when one is best, and when not... so what? Make fun of me
then!!!
tongue-sticking-out-grin

Warmest regards,
John

"Dan Richardson arrl net" k6mheatdot wrote in message
...


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 the antenna expert, tell us why it would.
73 Al Lowe N0IMW

For exactly the same reason that you need a choke on a ground plane or
any other antenna feed with coax. Depending upon the length of the
feed line common mode current can be substantial. This, in turn,
increases higher angle radiation. Now if you are most interested in
talking to airplanes that would be a good thing, but if you prefer
maximum signal towards the horizon then put a choke on that sucker!

Danny, K6MHE

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 the antenna expert, tell us why it would.
73 Al Lowe N0IMW

Sorry, I didn't realize we'd been through it before. If you'll tell me
the date, I'll look up the postings and review them.

There are two things which can cause current on the feedline. One is by
conduction. That mechanism is illustrated in Fig 21-1 at
http://www.w2du/r2ch21.pdf, and elaborated on in
http://eznec.com/Amateur/Articles/Baluns.pdf. Some types of feed systems
can minimize this, and it looks like both the open and shorted stub feed
systems are in this category.

But it's the second mechanism I was talking about -- mutual coupling, or
induction. Even if you have a perfect current balun (common mode choke)
at the feedpoint or any sort of feed system you want, current can still
be coupled to the feedline. In a center fed dipole, it won't occur if
you first make sure there's no conducted current, and place the feedline
symmetrically relative to the dipole to make the coupling from the two
dipole halves cancel. But you can't do this with a ground plane, J-Pole,
or similar antenna -- there will always be coupling from the antenna to
the feedline. Fiddling with the feedpoint won't prevent it, although it
might change the current distribution on the feedline. Changing the
feedline length and/or orientation and/or using a couple of current
baluns spaced about a quarter wavelength apart are two ways it can be
minimized.

I'd be interested in seeing any book which explains how a feedpoint
modification can prevent current being coupled to the feedline (as
opposed to being conducted).

Roy Lewallen, W7EL

Al wrote:
One think you may have missed, the original post was about a Arrow
Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub
type like the copper pipe ones. The OSJ does not need a choke, it does
not have a problem with feedline radiation or a problem with Common
Mode Currents.

Al, have you modeled it with EZNEC and a path to ground?
--
73, Cecil http://www.qsl.net/w5dxp

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For exactly the same reason that you need a choke on a ground plane or
any other antenna feed with coax. sniped

Danny, K6MHE

I agree with the choke needed on a ground plane. But all antennas ?
Sure you can put a choke on a OSJ won't hurt, won't help ether.
Been there, done that.

73 Al Lowe N0IMW

Bob Miller wrote:
By open stub, you mean there is no loop at the bottom of the "J"?

He means the bottom of the stub is the feedpoint, like this:

+------------------------------------------------------------
|
FP
|
+-----------------

And that looks like an unbalanced feedpoint so it should result
in common-mode currents, sorta like an off center-fed dipole.
--
73, Cecil http://www.qsl.net/w5dxp

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Sorry, I didn't realize we'd been through it before. If you'll tell me
the date, I'll look up the postings and review them.

There are two things which can cause current on the feedline. sniped

I don't remember if you were involved or not Roy. It's been about 10
years ago.
One I remember is Tom Raunch (hope I spelled that right).
We argued for weeks about the OSJ until I sent him one.
Some tried modeling it, some even claimed it would not work at all.
The OSJ has been around for a long time, I did not invent it, I just
build it better.
I guess all antennas have feed line radiation and/or common mode
currents to some degree.
I suppose with the right equipment you can measure anything.
But I think most would find a big difference between an OSJ & a CSJ
Just because two antennas look similar, don't mean they work the same.

Ok, that was fun, But I have wasted enough time, I got to get back to
building Antennas.
73 Al Lowe N0IMW

He means the bottom of the stub is the feedpoint, like this:

+----------------------------- ------------------------------ -
|
FP
|
+-----------------

And that looks like an unbalanced feedpoint so it should result
in common-mode currents, sorta like an off center-fed dipole.

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.
73 Al Lowe N0IMW

On 12 Jun 2005 16:18:35 -0700, "Al" wrote:

For exactly the same reason that you need a choke on a ground plane or
any other antenna feed with coax. sniped

Danny, K6MHE

I agree with the choke needed on a ground plane. But all antennas ?
Sure you can put a choke on a OSJ won't hurt, won't help ether.
Been there, done that.

73 Al Lowe N0IMW

Allow me to repeat: *ANY* antenna feed with coax should employee a
choke. It is just good engineering practice.

If your beloved open base j-pole is feed with coax that is any
multiple of a 1/2-wavelength you ARE GOING TO HAVE HIGH COMMON MODE
CURRENT!

So just what makes you beleive that your open base j-pole has some
magical properties that is self-decouples itself from the feed line?

73
Danny, K6MHE

Al wrote:

He means the bottom of the stub is the feedpoint, like this:

+-----------------x-------------------------------------------
|
FP
|
+-----------------y

And that looks like an unbalanced feedpoint so it should result
in common-mode currents, sorta like an off center-fed dipole.

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.

Would you agree this is a 2m Zepp? Zepps are known to have
common mode currents. We know the current at point 'x' is
not the same as the current at point 'y'.
--
73, Cecil http://www.qsl.net/w5dxp


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