J pole/coax radition
 

I was wondering 2 things:

While on hf i have a current balun, thyre pretty common to help elmin
any bad signals on my coax shield

is their such a thing for uhf/vhf?? if so who specifically sells them



second

I have a arrow 2/440 j pole works great---- it dosn't have any radials
hanging off naturally, so since basically just the center conductor is
connected would you suspect my coax shield is 'part of the antenna'
radiating alot of stuff??

(course the shield does attach to the bottom mounting plate of the
antenna but i wasn't counting that



tnx

ml wrote:
second

I have a arrow 2/440 j pole works great---- it dosn't have any radials
hanging off naturally, so since basically just the center conductor is
connected would you suspect my coax shield is 'part of the antenna'
radiating alot of stuff??

(course the shield does attach to the bottom mounting plate of the
antenna but i wasn't counting that

In the typical J-pole antenna, the shield connects to the long element
and the center to the stub. I don't know your antenna in particular,
but I would suspect this is the case and don't believe it is considered
a radiating element.

Modeing shows that radiating current can occur on a J-Pole feedline due
to mutual coupling from the antenna. The amount depends heavily on the
length and orientation of the feedline. The main effect of the current
on the pattern is to distort the main lobe, reducing the amount of
signal directed horizontally. I suspect that the widely varying reports
on the effectiveness of a J-Pole are due, in part, at least, to
different feedline lengths and orientations and therefore different
amounts of feedline radiation.

If there is significant current on the feedline, it might take a couple
of current (choke) baluns, placed about a quarter wave apart, to reduce
it to a small value.

Most people don't realize that ground plane antennas are subject to the
same phenomenon.

Roy Lewallen, W7EL

Larry wrote:
ml wrote:

second

I have a arrow 2/440 j pole works great---- it dosn't have any
radials hanging off naturally, so since basically just the center
conductor is connected would you suspect my coax shield is 'part of
the antenna' radiating alot of stuff??
(course the shield does attach to the bottom mounting plate of the
antenna but i wasn't counting that


In the typical J-pole antenna, the shield connects to the long element
and the center to the stub. I don't know your antenna in particular,
but I would suspect this is the case and don't believe it is considered
a radiating element.

Roy:

My "bumbling goof-abouts" with antennas has proven to me you are once
again right on the money...

I have gotten to where I don't even worry about the loss of a well
constructed choking--balun/unun in the circuit--better to be safe than
sorry...

John

"Roy Lewallen" wrote in message
...
Modeing shows that radiating current can occur on a J-Pole feedline
due to mutual coupling from the antenna. The amount depends heavily on
the length and orientation of the feedline. The main effect of the
current on the pattern is to distort the main lobe, reducing the
amount of signal directed horizontally. I suspect that the widely
varying reports on the effectiveness of a J-Pole are due, in part, at
least, to different feedline lengths and orientations and therefore
different amounts of feedline radiation.

If there is significant current on the feedline, it might take a
couple of current (choke) baluns, placed about a quarter wave apart,
to reduce it to a small value.

Most people don't realize that ground plane antennas are subject to
the same phenomenon.

Roy Lewallen, W7EL

Larry wrote:
ml wrote:

second

I have a arrow 2/440 j pole works great---- it dosn't have any
radials hanging off naturally, so since basically just the center
conductor is connected would you suspect my coax shield is 'part of
the antenna' radiating alot of stuff?? (course the shield does
attach to the bottom mounting plate of the antenna but i wasn't
counting that


In the typical J-pole antenna, the shield connects to the long
element and the center to the stub. I don't know your antenna in
particular, but I would suspect this is the case and don't believe it
is considered a radiating element.

I have gotten to where I don't even worry about the loss of a well
constructed choking--balun/unun in the circuit--better to be safe than
sorry...

If the decoupling is designed into the antenna, using cones,
bells, sleeves, etc, there is little if any loss. Being as the
feed radiation skews the pattern up off the horizon, adding
decoupling usually ends up with a "gain" at that low angle.
Also, I've heard different types of "J" feed methods, naturally
decouple better than others..This also applies to the ringo's,
etc. The gamma loop matching device on a ringo does a
halfway job of decoupling the line, but it can always be improved
by a 2nd decoupling section. I don't use conventional ferrite
chokes when decoupling VHF/UHF antennas. IE: If I were to
add additional decoupling to a ground plane, I would use a
2nd set of radials, or a sleeve, vs a ferrite choke. MK

Hello, Roy & the Group.

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.

In the typical J-pole antenna, the shield connects to the long element
and the center to the stub. =A0I don't know your antenna in particular,
but I would suspect this is the case and don't believe it is considered
a radiating element.

The OSJ is not a typical J-pole.

73 Al Lowe N0IMW Arrow Antenna

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

Roy Lewallen, W7EL

Al wrote:
Hello, Roy & the Group.

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.


In the typical J-pole antenna, the shield connects to the long element
and the center to the stub. �I don't know your antenna in particular,
but I would suspect this is the case and don't believe it is considered
a radiating element.


The OSJ is not a typical J-pole.

73 Al Lowe N0IMW Arrow Antenna

On 12 Jun 2005 14:15:52 -0700, "Al" wrote:

Hello, Roy & the Group.

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.

In the typical J-pole antenna, the shield connects to the long element
and the center to the stub.

I've read, in this group, it doesn't matter which side the center
connects to. That's not true?

don't know your antenna in particular,
but I would suspect this is the case and don't believe it is considered
a radiating element.

The OSJ is not a typical J-pole.

73 Al Lowe N0IMW Arrow Antenna

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

bob
k5qwg

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

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