On Tue, 21 Aug 2007 17:34:23 -0700, Jim Lux
wrote:

Cecil Moore wrote:
Jim Lux wrote:

I doubt there's anything special about the 22 ft.


I've been told that 22 ft. is special, i.e.
virtually everyone can get his vertical
section 22 feet off the ground. (The original
Windom had a vertical radiating section.)

As opposed to 20 ft or 21 ft or 25 ft?

I find it hard to believe that there's an electrical reason for 22ft.
Mechanical or convenience I can believe.

Maybe it fits well with the length of their isolators, etc. and matches
a convenient pole height (e.g. isolator plus sag plus 22ft plus whatever
exactly matches 26 ft 3" or whatever the pole is)

Looking through the Radio Works catalog, the vertical radiators are
22' for 80m & 160m versions, 10' for a 40m version, 18' for an
enhanced 40m version of the Carolina Windom.

So they're figuring something, what, not sure of...

bob
k5qwg

Bob Spooner wrote:
"Jim Lux" wrote in message

...
I doubt there's anything special about the 22 ft. As many have pointed
out, it's a deliberately unbalanced antenna which radiates from both
vertical and horizontal parts (and influenced by the towers, trees, and
buildings nearby), with a nice choke at some point to keep RF from coming
back in the shack on the outside of the coax.
...

If the choke is effective in removing the common mode current, then the
feedline will not radiate very much.
choke at the shack, was my thinking.. I believe that the original intent
of the design was that the feedline could radiate. It's a compromise
antenna of sorts, so one can't be too picky about patterns,
polarization, etc.


I also recall seeing a diagram with choke on the ground, and feedline to
shack laying on ground (which would, of course, tend to reduce any
coupling to it)..


I suppose the real question is whether there is a statistically
significant difference in performance between this and any other random
dipole. If 100 hams put up a Carolina Windom, and 100 hams put up an
OCF dipole of some other sort, and 100 hams put up a multiband dipole,
will the variations within the groups be larger than the variations
between the groups. I suspect they will. For low gain antennas in
random installations, there's so many other factors that influence
pattern and efficiency that I doubt you could accurately measure the
difference.

Probably more important just to get some sort of wire in the air and
somehow get a match so the transmitter doesn't fold back, and not worry
too much.

On Wed, 22 Aug 2007 11:13:11 -0500, Bob Miller wrote:


Looking through the Radio Works catalog, the vertical radiators are 22'
for 80m & 160m versions, 10' for a 40m version, 18' for an enhanced 40m
version of the Carolina Windom.

So they're figuring something, what, not sure of...

bob
k5qwg

My guess is that the length of the vertical segment is such that it acts
as a matching segment to keep the overall impedance of the antenna at a
reasonably matchable level.

That was my primary reason for choosing the New Carolina Windom, over the
venerable G5RV. The impedance mismatch onthe NCW is almost never over
3:1. Which is all that my auto-tuner is rated to handle, while the G5RV
can easily exceeded 5:1 on some bands. (I saw an article that did a
great job of comapring impedances between these two antennas, but I can
not seem to find it now)

I built my own NCW based on the measurements in this QRP Expressions
article. http://www.w5fc.org/files/QRP%20Expressions.pdf

I liked it so much, I built the larger 80m version, with measurments from
the RadioWorks website. I did not have a 22' of coax handy at the time,
so I used a 14' piece that was left over from another project.

It seems to work just fine on everything but 17m. The autotuner tried
and tried but could not cope. Finally I replaced it with the correct 22'
lenth, and 17m once again tunable. So there is -something- to it.

--Teh

I doubt there's anything special about the 22 ft. As many have pointed
out, it's a deliberately unbalanced antenna which radiates from both
vertical and horizontal parts (and influenced by the towers, trees, and
buildings nearby), with a nice choke at some point to keep RF from coming
back in the shack on the outside of the coax.
...

If the choke is effective in removing the common mode current, then the
feedline will not radiate very much.

I believe that's not necessarily true.

If the distance between the main feedpoint, and the common-mode choke
is close to 1/4 wavelength at the frequency in question, then the
impedance "looking down" the outside feedline from the main feedpoint
will be relatively low, and a fair bit of RF current will flow on the
outside of the feedline. I think it's also likely that the choke
will have difficulty being very effective in this location, as it will
have to have a very high choking reactance.

If the distance from feedpoint to choke is close to a half-wavelength,
not much current will flow.

If the distance is somewhere in the middle, the outside-of-the-
feedline impedance will be rather reactive. This reactivity may
interact with the reactivity of the radiator... e.g. if the feedline
impedance is inductive, and the radiator's impedance is capacitive,
the two might tend to cancel out to some extent. This could have two
effects - a significant amount of RF current flow on the outside of
the feedline, and (perhaps) a better impedance match at the
feedpoint/balun and a lower SWR as seen by the rig.

It's possible that the 22' vertical-section length for the Carolina
Windom was selected, after experimentation, to provide the best
matching of the antenna/balun/choke system to a 50-ohm transmitter on
certain bands on which it would otherwise be difficult to match.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!