"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.