"Steve N." wrote in message
...

"Jon Kåre 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