On Sun, 31 Dec 2006 22:31:41 -0800, "Sal M. Onella"
wrote:

Either way, do we know the impedance? That is an "easy build" and I want to
try some single-band, no-tuner HF antennas.

Hi OM,

The sleeve dipole makes for a pretty tall HF radiator and puts one end
quite close to ground where a ground plane design, if you matched the
heights of the feed points, would probably show less loss.

As for impedance, it would be close to a conventional dipole - 70 to
75 Ohms. However, the proximity of earth (for HF) would alter that.
Further, the jacket material that covers the outer braid is notorious
for being rather lossy in its own right. When you draw the braid back
over it, it creates a lossy transmission line with that material
between the drawn back braid, and the jacketed braid it is covering.
This does not normally concern most who build this style of antenna as
it is one of those "precious" details that is argued endlessly while
others put it to good use.

One more thing: Assuming a nominal 50 ohm match can usually be made, is
there any drawback to tuning the thing by making the braid a little longer
than calculated, moving the end of the braid up and down to find the best
match and then securing the end?

It'll work just fine.

The decoupling
capacitance would be reduced by both the decrease in length and the increase
in spacing, but the changed value would be, in effect, part of the tuning.

This is the first I've heard of "decoupling capacitance." The lower
arm of the vertical dipole is just that, the lower arm. It's
proximity to the feed line will certainly impact the state of tune,
but there is no claim to decoupling built into this design. You will
have to provide for the usual considerations in that regard (but most
neglect that too to no particular pain).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 31 Dec 2006 22:31:41 -0800, "Sal M. Onella"
wrote:

Either way, do we know the impedance? That is an "easy build" and I want
to
try some single-band, no-tuner HF antennas.

Hi OM,

The sleeve dipole makes for a pretty tall HF radiator and puts one end
quite close to ground where a ground plane design, if you matched the
heights of the feed points, would probably show less loss.

As for impedance, it would be close to a conventional dipole - 70 to
75 Ohms. However, the proximity of earth (for HF) would alter that.
Further, the jacket material that covers the outer braid is notorious
for being rather lossy in its own right. When you draw the braid back
over it, it creates a lossy transmission line with that material
between the drawn back braid, and the jacketed braid it is covering.
This does not normally concern most who build this style of antenna as
it is one of those "precious" details that is argued endlessly while
others put it to good use.

One more thing: Assuming a nominal 50 ohm match can usually be made, is
there any drawback to tuning the thing by making the braid a little longer
than calculated, moving the end of the braid up and down to find the best
match and then securing the end?

It'll work just fine.

The decoupling
capacitance would be reduced by both the decrease in length and the
increase
in spacing, but the changed value would be, in effect, part of the tuning.

This is the first I've heard of "decoupling capacitance." The lower
arm of the vertical dipole is just that, the lower arm. It's
proximity to the feed line will certainly impact the state of tune,
but there is no claim to decoupling built into this design. You will
have to provide for the usual considerations in that regard (but most
neglect that too to no particular pain).

73's
Richard Clark, KB7QHC

I made one of these sleeve antennas for 10M with the folded back shield,
put a KW on it and it arced through the black vinyl.
Not exactly what I would call a "precious detail". Many years ago Shakespear
antennas were made this way and they changed there design because of the
same problem. Now they replace the folded back sheild with 4 or 5 wires
imbeded in the fiberglass shell putting a good bit of air dielectric between
the folded back part of the dipole and the sheild.

Jimmie D wrote:
I made one of these sleeve antennas for 10M with the folded back shield,
put a KW on it and it arced through the black vinyl.

It's a standing wave antenna. The voltage at the end
of the shield is approximately 20 times the voltage
at the feedpoint. For a KW, that's more than 5 kV,
too much for ordinary coax.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
et...
Jimmie D wrote:
I made one of these sleeve antennas for 10M with the folded back shield,
put a KW on it and it arced through the black vinyl.

It's a standing wave antenna. The voltage at the end
of the shield is approximately 20 times the voltage
at the feedpoint. For a KW, that's more than 5 kV,
too much for ordinary coax.
--
73, Cecil http://www.w5dxp.com

No kidding!!!!!

Cecil Moore wrote:
Jimmie D wrote:
I made one of these sleeve antennas for 10M with the folded back
shield, put a KW on it and it arced through the black vinyl.

It's a standing wave antenna. The voltage at the end
of the shield is approximately 20 times the voltage
at the feedpoint. For a KW, that's more than 5 kV,
too much for ordinary coax.

Yeah, but tape a couple of ne-2 bulbs there and you can get that kool
old cb look when they light up on key down!!! silly grin

Regards,
JS

"Cecil Moore" wrote in message
et...
Jimmie D wrote:
I made one of these sleeve antennas for 10M with the folded back
shield,
put a KW on it and it arced through the black vinyl.

It's a standing wave antenna. The voltage at the end
of the shield is approximately 20 times the voltage
at the feedpoint. For a KW, that's more than 5 kV,
too much for ordinary coax.

I have 160 watts available. ... about 90 volts at the feedpoint, so a
whopping 1800 at the end of the shield. (Might want to keep the mike gain
throttled back.)

OR ... use a larger braid over a spacer of some kind and solder it to the
coax braid at the feedpoint. The spacer could be a piece of plastic pipe
slipped over the coax at the outset. Actually, the braid doesn't really have
to be braid. It could be a piece of copper pipe. The tuning operation
would need to be slightly different; I'll need to keep a few inches of the
original coax near the feedpoint. I'll tune it by sliding the pipes and
bunching up the coax at the feedpoint. Or tune it by adjusting the length of
the center conductor element.

I wonder ... if the pipe were sufficiently large, it would tend to broadband
the antenna. I know of this effect at UHF. How about 10m???

I'm looking forward to this. g

..

I wonder ... if the pipe were sufficiently large, it would tend to
broadband
the antenna. I know of this effect at UHF. How about 10m???

I'm looking forward to this. g



Yes it does, solves some of the problem you have with the all coax type of
sleeve antenna. stuff some pieces of styrofoam inside the pipe to center the
coax. Wind a choke in you coax and it is a pretty decent antenna. Consider
this, make the sleve out of 2 inch conduit, at the middle of the antenna put
a cap on the pipe with a hole drilled to accept a mount for a whip, RS and
truck stops sell these. Bottom end screws into a 2inch PVC pullbox, other
side of the pullbox screws on another piece of pipe used as a mast. There
should be enough room in the pullbox for the choke.

Jimmie D wrote:
.
I wonder ... if the pipe were sufficiently large, it would tend to
broadband
the antenna. I know of this effect at UHF. How about 10m???

I'm looking forward to this. g



Yes it does, solves some of the problem you have with the all coax type of
sleeve antenna. stuff some pieces of styrofoam inside the pipe to center the
coax. Wind a choke in you coax and it is a pretty decent antenna. Consider
this, make the sleve out of 2 inch conduit, at the middle of the antenna put
a cap on the pipe with a hole drilled to accept a mount for a whip, RS and
truck stops sell these. Bottom end screws into a 2inch PVC pullbox, other
side of the pullbox screws on another piece of pipe used as a mast. There
should be enough room in the pullbox for the choke.



Or, just lengthen it out to a full wave dipole, provide a matching
circuit of your choice, add a 1:1 current balun and end up with a MUCH
more desirable antenna ...

Oh yeah, you still can use the ne-2 bulbs at either end (or both) of
this dipole and end up with that kewl "cb look!" satisfied look

moving on
JS

"Jimmie D" wrote in message
news:Bdymh.17679$_X.15840@bigfe9...

.

I wonder ... if the pipe were sufficiently large, it would tend to
broadband
the antenna. I know of this effect at UHF. How about 10m???

I'm looking forward to this. g



Yes it does, solves some of the problem you have with the all coax type of
sleeve antenna. stuff some pieces of styrofoam inside the pipe to center
the
coax. Wind a choke in you coax and it is a pretty decent antenna.
Consider
this, make the sleve out of 2 inch conduit, at the middle of the antenna
put
a cap on the pipe with a hole drilled to accept a mount for a whip, RS and
truck stops sell these. Bottom end screws into a 2inch PVC pullbox, other
side of the pullbox screws on another piece of pipe used as a mast. There
should be enough room in the pullbox for the choke.



That's a clever idea. I am recalling four turns as the spec for the
feedline choke at 2m. However, at 28 MHz, there's a lot less inductive
reactance in those same four turns. How many turns are we looking at for
the choke?

"Sal M. Onella" wrote in message
news:BFGmh.53728

I am recalling four turns as the spec for the
feedline choke at 2m. However, at 28 MHz, there's a lot less inductive
reactance in those same four turns. How many turns are we looking at for
the choke?


snip

.... answering my own question: 4 feet of RG-58, coiled into 6 - 8 turns.
[ARRL Antenna Handbook]

"Sal"