40m 1/2 wave coaxial sleeve antenna
 

Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I have
a large tree with a branch at 70' and i thought i'd pull the antenna up with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

Alex,

Solder a quarter wave length of wire to the center conductor of the coax
with a loop at the end of the wire to attach your line to raise it. A
quarter of a wavelength from the wire end of the coax, coil enough of
the coax to form an RF choke. (I think the ARRL Antenna Handbook has the
information you need about how many turns and what diameter to make the
coil.) Seal the end of the coax to keep out moisture. Then hoist the
antenna up and you have a coaxial sleeve antenna.

73,
Bob AD3K

Alex wrote:
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I have
a large tree with a branch at 70' and i thought i'd pull the antenna up with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.



--
Robert L. Spooner
Registered Professional Engineer
Associate Research Engineer
Intelligent Control Systems Department

Applied Research Laboratory Phone: (814) 863-4120
The Pennsylvania State University FAX: (814) 863-7841
P. O. Box 30
State College, PA 16804-0030

Just a thought - build a 40M 1/2 vertical out of wire - center fed with coax
running 90 degrees from antenna as far out as you can.
Works well here as I have another shorter tree to secure the coax to.

[ TREE 70 feet
[
[
[
[________________ coax 32 feet TREE
[
[
[
[
[ Antenna 64 feet

-- --------- ground


Incognito By Necessity (:-(

If you can't convince them, confuse them.
- - -Harry S Truman




"Alex" wrote in message
le.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I
have
a large tree with a branch at 70' and i thought i'd pull the antenna up
with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna
would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

I never built this, but had considered an antenna like that. I was going to
patch in a 1/4 wave section of triaxial coax. The two shields would be
connected at the top, and the outermost shield would be left unconnected at
the bottom end. BTW, I hung a 40m 1/4 wave ground plane vertical from a tree
branch once. Feed point was at about 30 - 35 feet. There were 4 radials
sloping down at 45 degrees. It worked reasonably, but unfortunately came
down before I had a chance to do much with it. The sloping radials will give
you very close to a 50 Ohm impedance.

Tam/WB2TT
"Alex" wrote in message
le.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I
have
a large tree with a branch at 70' and i thought i'd pull the antenna up
with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna
would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

"Alex" wrote in message
le.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I
have
a large tree with a branch at 70' and i thought i'd pull the antenna up
with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna
would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.


There was an article in, I believe QEX just a few months ago. The
author made the "sleeve" out of several wires, hanging down, around the
coax. They formed sort of a skelton sleeve. It was for an HF band too.


--
Steve N, K,9;d, c. i My email has no u's.

"Alex" wrote in message ble.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I have
a large tree with a branch at 70' and i thought i'd pull the antenna up with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

Myself, I would probably feed it from the base. One problem often seen
with sleeve dipoles like that is decoupling the feedline from the
antenna. if you do center feed it, pay close attention to feedline
decoupling. MK

On 7 Mar 2004 03:12:05 -0800, (Mark Keith) wrote:

"Alex" wrote in message ble.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I have
a large tree with a branch at 70' and i thought i'd pull the antenna up with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

Myself, I would probably feed it from the base. One problem often seen
with sleeve dipoles like that is decoupling the feedline from the
antenna. if you do center feed it, pay close attention to feedline
decoupling. MK

Hi Mark,

That is the point of a sleeve dipole (and the design of the Isopole)
in that the quarterwave section below is the decoupling.

I've plaid a variant upon this by attaching a ground wire to a point
on the shield, down and away from the feed point. It requires that I
strip away a portion of the coax outer dielectric to do this, and I
use a wire connected to a BNC female jack as the radiator portion (the
line is a conventionally terminated BNC-BNC coax). This way I can
rummage through combinations of lengths and ground or non-ground
situations to suit the purpose (usually field day variations). I
prefer off-center fed designs.

73's
Richard Clark, KB7QHC

I made one of these out of RG58 for portable operation by striping off the
jacket of the coax much longer than 1/4 wavelength and then working the
shield down over the jacket below it. That way the feedline is inside the
lower half of the dipole and is in a low field area. I put a current balun,
(ferrite beads) at the point the coax came out of the shield. It was a bit
of a struggle to do and you need more shield than you would think (start
very long) but its easy to do in the end. I didn't water proof the braid
over the jacket but one could cover it with tape or a plastic tube. Anyway
after the braid is folded back over the jacket its then tuned just like any
other dipole.
Also if your not going to run high power, the PAR makes an end feed half
wave antenna that has a matching unit at the feed point.


--
John Passaneau, W3JXP
Penn State University


"Robert Spooner" wrote in message
...
Alex,

Solder a quarter wave length of wire to the center conductor of the coax
with a loop at the end of the wire to attach your line to raise it. A
quarter of a wavelength from the wire end of the coax, coil enough of
the coax to form an RF choke. (I think the ARRL Antenna Handbook has the
information you need about how many turns and what diameter to make the
coil.) Seal the end of the coax to keep out moisture. Then hoist the
antenna up and you have a coaxial sleeve antenna.

73,
Bob AD3K

Alex wrote:
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I
have
a large tree with a branch at 70' and i thought i'd pull the antenna up
with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave
vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna
would
give me a great feed impedance. Any thoughts? The coax are 100' of
RG8.



--
Robert L. Spooner
Registered Professional Engineer
Associate Research Engineer
Intelligent Control Systems Department

Applied Research Laboratory Phone: (814) 863-4120
The Pennsylvania State University FAX: (814) 863-7841
P. O. Box 30
State College, PA 16804-0030

The idea of just pulling the braid back over the outer jacket of the
coax to make a 1/4 wave section turns out to not be a great one. The
reason is that you want 1/4 wave considering the propagation velocity
of the coaxial arrangement consisting of the feedline outer conductor
and the folded-back braid, with the (generally vinyl) coax outer
jacket as dielectric. The result is a "line" with not very well
controlled VF around 0.5 (so you should use only about 1/8 freespace
wavelength of it, resulting in a very off-center-fed dipole). If you
make it a quarter wave in freespace and it does have VF around 0.5, it
will be close to an electrical half-wave and reflect back...a short,
where you wanted an open! Also, the higher the impedance of that line
is, the higher an impedance it will reflect to the lower end where the
feedline comes out, and the close spacing and high-dielectric-constant
jacket conspire to make a low impedance line, which is not good. So
the idea of using a cage of wires forming a narrow cone should be a
very good one. That's a cage version of an Isopole, I guess. Wire's
pretty cheap; a cage of 8 conductors should do a good job (and be
lighter than a sleeve made of copper pipe!). I'd try to make the
bottom end of the cage at least a foot in diameter, with the coax
nominally centered in it. If you do use a pipe, try to use an ID as
large as practical and keep the coax more-or-less centered in it.

Cheers,
Tom

"Alex" wrote in message ble.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I have
a large tree with a branch at 70' and i thought i'd pull the antenna up with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.

Tom,

I don't think that's correct. The velocity factor applies inside the
coax, but not outside. What you have outside is a fat wire in free space.

But you still don't need to fold the shield back if you put a choke a
quarter wavelength from the end of the coax to decouple a quarter wave
of the _outside_ part of the shield. Then it becomes just as if you had
folded back some of the shield. The currents in the center conductor and
the inside of the shield are unaffected by the choke.

73,
Bob AD3K

Tom Bruhns wrote:
The idea of just pulling the braid back over the outer jacket of the
coax to make a 1/4 wave section turns out to not be a great one...
--
Robert L. Spooner
Registered Professional Engineer
Associate Research Engineer
Intelligent Control Systems Department

Applied Research Laboratory Phone: (814) 863-4120
The Pennsylvania State University FAX: (814) 863-7841
P. O. Box 30
State College, PA 16804-0030