J pole question
 

"Steve N." wrote in message ...


Harbin,
You seem to understand the "J-Antenna". What you really seem to be asking is how far apart is "too far" for a balanced transmission line. That's a question I can't answer and I'm sure others can, but , in the common custom of adding obtuse references which sort of seem to be relevant... what about that old one wire transmission like the "G-Line"? [no joke]

73, Steve, K9DCI


Hi Steve:
I have read about the G-Line, which is very low loss (so claimed). Have
you heard of anybody that has used this line?

J pole question
 

"Steve N." writes:

OOPS! Jon,
IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I
guess it is technically a monopole.

Sort of, sort of. The feed point is actually in the middle of the half
wave. Traditionally, it's called coaxial sleeve antenna. Except
originally, the sleeve was outside the coax braid. In this variation,
due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and
the end of the sleeve is a choke. He's written about it in English in
CQ, 1999, Aug pg 22. The version in Norwegian is on the web at
http://www.la2t.org/teknikk/vertikal2m.html.

I found an article from Microwave Journal about a very similar
concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use
different kinds of choke, since they're at much higher
frequencies. They report performance very similar to a dipole.

73 de LA4RT Jon

J pole question
 

"Jon KÃ¥re Hellan" wrote in message
...
"Steve N." writes:

OOPS! Jon,
IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I
guess it is technically a monopole.

Sort of, sort of. The feed point is actually in the middle of the half
wave. Traditionally, it's called coaxial sleeve antenna.

OK, perhaps I missed the full reference. I read "end fed dipole" and
went from that. The sleeve dipole is still a dipole (I have one for 2M) and
is still center fed. The lower half (lower, hollow, quarter wave with the
t-line passing through it) is viewed as a "choke" by some as well. You can
do some mental games with your models and take the sleve and spread it out
into a cone, then a plane and say that the sleeve dipole is sort of a
configuration of a 1/4 wave ground plane of a different color. Various
analogies are possible depending on your particular way of understanding the
basics. This is what I call "mental models".

That Microwave article is showing different concepts, but along the same
line of the "non-sleve" antenna. you can take a coax, strip off 1/4 wave of
outer to leave 1/4 wave on inner projecting out the end, then do various
things with gaps in the the shield (whthout adding a classical sleeve) and
get radiation and minimize coax radiation ( except were you want it, or
perhaps more correctly, where it helps it to be a better antenna (for
whatever charasteristic you want in the first place).).

Rolf Brevig's choke is something I don't think I've seen before, but I
understand the concept. I can't tell, but I suspect it is common "1/4 wave
stub", no?

73, Steve, K9DCI



Except
originally, the sleeve was outside the coax braid. In this variation,
due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and
the end of the sleeve is a choke. He's written about it in English in
CQ, 1999, Aug pg 22. The version in Norwegian is on the web at
http://www.la2t.org/teknikk/vertikal2m.html.

I found an article from Microwave Journal about a very similar
concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use
different kinds of choke, since they're at much higher
frequencies. They report performance very similar to a dipole.

73 de LA4RT Jon

J pole question
 

"Steve N." wrote in message
...

"Jon KÃ¥re Hellan" wrote in message
...
"Steve N." writes:

OOPS! Jon,
IF it is "end-fed" is isn't a "di-pole". It is just a half wave.
I
guess it is technically a monopole.

Sort of, sort of. The feed point is actually in the middle of the half
wave. Traditionally, it's called coaxial sleeve antenna.

OK, perhaps I missed the full reference. I read "end fed dipole" and
went from that. The sleeve dipole is still a dipole (I have one for 2M)
and
is still center fed. The lower half (lower, hollow, quarter wave with
the
t-line passing through it) is viewed as a "choke" by some as well. You
can
do some mental games with your models and take the sleve and spread it out
into a cone, then a plane and say that the sleeve dipole is sort of a
configuration of a 1/4 wave ground plane of a different color. Various
analogies are possible depending on your particular way of understanding
the
basics. This is what I call "mental models".

That Microwave article is showing different concepts, but along the same
line of the "non-sleve" antenna. you can take a coax, strip off 1/4 wave
of
outer to leave 1/4 wave on inner projecting out the end, then do various
things with gaps in the the shield (whthout adding a classical sleeve) and
get radiation and minimize coax radiation ( except were you want it, or
perhaps more correctly, where it helps it to be a better antenna (for
whatever charasteristic you want in the first place).).

Rolf Brevig's choke is something I don't think I've seen before, but I
understand the concept. I can't tell, but I suspect it is common "1/4
wave
stub", no?

73, Steve, K9DCI

Jon,
I also just realized that the choke is very similar to the Microwave
article's concept of simply opening the shield. I think that was your
point.
73, Steve, K9DCI




Except
originally, the sleeve was outside the coax braid. In this variation,
due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and
the end of the sleeve is a choke. He's written about it in English in
CQ, 1999, Aug pg 22. The version in Norwegian is on the web at
http://www.la2t.org/teknikk/vertikal2m.html.

I found an article from Microwave Journal about a very similar
concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use
different kinds of choke, since they're at much higher
frequencies. They report performance very similar to a dipole.

73 de LA4RT Jon

J pole question
 

"Steve N." writes:

"Steve N." wrote in message
...

Rolf Brevig's choke is something I don't think I've seen before, but I
understand the concept. I can't tell, but I suspect it is common "1/4
wave
stub", no?

A 1/4 wave stub would work better, but be less simple to
fabricate. This design is trivial. It's just a coiled length of coax.

I also just realized that the choke is very similar to the Microwave
article's concept of simply opening the shield. I think that was your
point.

That was my point. But I now see that we're putting it at a high
impedence point, so the isolation is going to be far from perfect.

Anyway, my *real* point was that this was a useful antenna. Carry one
rolled up in your pocket and hang it from a tree to extend the range
of your HT. Cut one for 156.8 MHz and keep on your sailboat in case
you're dismasted. Etc.

73
LA4RT Jon

J pole question
 

Jon Kåre Hellan wrote:
. . .
Anyway, my *real* point was that this was a useful antenna. Carry one
rolled up in your pocket and hang it from a tree to extend the range
of your HT. . .

I did a bit of experimenting along this line and found that any fixed
antenna hung from a tree was a poor way to extend range. As you probably
know, multipath propagation causes dead spots every few cm -- the cause
of "picket fencing" when mobile -- when you're in a marginal area where
a better antenna would help. Any fixed antenna is fairly likely to end
up in one of those dead spots. And additional gain due to height is
likely to be largely canceled by feedline loss unless you carry along
some large diameter coax.

What I've found to be the best VHF antenna for portable use in marginal
conditions is a telescoping half wave antenna like the AEA "Hot Rod" or
various imitators like the MFJ. They radiate as well as a J Pole, but
have the advantage that you can move them around to find a "sweet spot".

Roy Lewallen, W7EL

J pole question
 

Nope. Only read about it and, perhaps, understand the theory.
73, Steve, K9DCI

"Harbin" wrote in message ...
"Steve N." wrote in message ...


Harbin,
You seem to understand the "J-Antenna". What you really seem to be asking is how far apart is "too far" for a balanced transmission line. That's a question I can't answer and I'm sure others can, but , in the common custom of adding obtuse references which sort of seem to be relevant... what about that old one wire transmission like the "G-Line"? [no joke]

73, Steve, K9DCI


Hi Steve:
I have read about the G-Line, which is very low loss (so claimed). Have
you heard of anybody that has used this line?