writes:
If it's what I think it is the design is a coaxial feed section with
a 1/2wl radiator. The difference being a coax fed rather than a
balanced line feed. In either case it relies on an unterminated 1/4wl
section reflecting a high impedence to match the end impedence of
a 1/2wl section. That being something in the range of 3000-5000
ohms (at resonance).

Here's a very simple end fed vertical dipole:

http://www.la2t.org/otfgallery/album...plukk/1002.jpg

http://f3wm.free.fr/radio/pocket.html

It's simply a piece of coax. The braid is stripped off the last 1/4
wavelength. Another 1/4 wavelength from the end, there is a choke to isolate
a 1/4 wavelength of braid from the rest of the cable. Presto, dipole!

Support it with a fishing rod, hang it from a tree, whatever.

73 de LA4RT Jon

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.
73, Steve, K9DCI

"Jon KÃ¥re Hellan" wrote in message
...

Here's a very simple end fed vertical dipole:

http://www.la2t.org/otfgallery/album...plukk/1002.jpg

http://f3wm.free.fr/radio/pocket.html

It's simply a piece of coax. The braid is stripped off the last 1/4
wavelength. Another 1/4 wavelength from the end, there is a choke to
isolate
a 1/4 wavelength of braid from the rest of the cable. Presto, dipole!

Support it with a fishing rod, hang it from a tree, whatever.

73 de LA4RT Jon

"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

"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

"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

"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