Lots of good ideas here, but I'll add the following since there has been an
occasional :



A "dipole" is called a dipole because it has two "halves" or "poles" driven
in the center. If we want to get a little more descriptive what we are
talking about is a "center fed half wave antenna". This is the basic
antenna. (the gran-daddy of all antennas, but I won't entertain arguments
here about others including 1/4 wave ground planes) Just saying "dipole"
could also (much less common) mean something other than the half wave
length. When hams say "dipole" you can bet they're talkin' the half wave
version.

Turn it any way you want, it's the same thing, just keep the ends as far
from things as possible. Half wave antennas like this need no ground plane,
also called radials, when so constructed.



If you "run the coax feed-line out through the center of one of the halves"
you get the sleeve dipole. Since the coax runs right past the one end,
there's a lot of capacitance and you have to adjust the length of that half
a bit (shorter) for best SWR. In this case it may not be a true half wave
long in total, but that doesn't matter. Anything close to a half wave, when
it is getting power into it, will radiate like the full half wave. This can
take the form of the coax braid pulled back or a tube or pipe.

Going just a bit further, if you imagine spreading the very ends of the
braid out to form a big a cone, so it gets wider at the end away from the
center of the half wave - and keep going until it is all 90 degrees from the
coax (and the other half (1/4 wave)) then you have a 1/4 wave ground plane
antenna.





Then there's the "J antenna" now-a-days always called the "J-Pole" (except
by me)



P.S. The original "J-Pole" Was, and I believe still is, a commercial antenna
by, I believe Cushman, consisting of either 2 or, I believe 4, J's spaced
vertically on a supporting "POLE" for increased omni directional gain. Thus
the name "J-POLE".



Getting descriptive again, this is an "end fed half wave antenna". This is
the same radiating element, a half wave, just fed differently. To get the
high impedance feed required at the end of a half-wave, what looks a lot
like a shorted quarter wave stub matching section is used. This beast can
also be placed vertically or horizontally.

And by the way, there is no reason that the "1/4 wave" matching section has
to be in-line with the radiating half wave. It can be at 90 degrees if
necessary. Just keep in mind that the feed-point end is still at a high
impedance and should be kept away from other things to minimize the
capacitance and resulting de-tuning -- just like any end of any half wave
antenna. (I won't entertain arguments of whether the matching section is
really a true 1/4 wave or not)



Now the thing which triggered my comments, that of Richard , KB7QHC



You can follow my former instructions and simply make the upper
portion 5/8ths wavelength (if you have the room). If this makes it
too long (too close to ground) you will need radials.



I believe he was originally describing a sleeve dipole. The upper part
could indeed be 5/8 wave, HOWEVER that radiator has significant reactance,
capacitive. The 5/8 antenna is usually done as a ground plane and you will
see a "loading coil" at the bottom. It turns out that the 5/8 radiator has
a close to 50 ohm real radiation resistance, but with capacitance. In the
series representation it is 5 - Jx. I don't know the value (I have wondered
what it is for years-anybody model it & let me know??), but it can be tuned
out with a series inductor and nothing more. A good design, however, will
use a coil to ground with the coax feed tapped up the coil. This is to get
the radiating element grounded, but the net match is the same.



If I'm not mistaken, the original "Extended Zepp" or was it just called the
"Zep" or Zepplen",was a "di-pole" made up of two 5/8 elements on either
side of center. Should have had a rather thin radiating pattern broadside
and, yes some gain, like the 5/8 vertical does to obtain its "gain".
Should have perhaps had two smaller lobes something like 50 degrees of the
centerline (also like the 5/8)



Zat help any?



Whew!
Steve K;9;D:C:I