Two big advantages of the center top feed as opposed to bottom end feed is
center top requires no radials. Bottom L will need a minimum of 8 100 foot
radials in your application and some would argue you need many more than 8.
Have fun burying 800+ feet of wire. Second, top feed gets the high current
radiating part of the antenna up higher. Check out
http://www.cebik.com/ltv.html and his discussion of ladder line at
http://www.cebik.com/gup/gup32.html. He concludes a 1:1 balun is best.

If you have 37 feet of height you can feed at the top and run the wire
straight down, back up and then down again for a total of 3x37= 111 feet of
wire on the vertical side. On the horizontal just bend back at the end or
let the end drop down vertically. Sure it will have narrower bandwidth than
a straight dipole but you have a tuner and you can't have a straight dipole.
--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64


"Jack Painter" wrote in message
news:n890d.31528$Ka6.3203@okepread03...

"Graybyrd" wrote
"Craig Buck" wrote:

With linear loading, you can get up a lot of wire in a small space.
Take a
look at
http://hamgate.sunyerie.edu/~larc/su...inverted_v.htm
for
an example of a 160 meter inverted vee in a 40 foot yard!

If you want multi band operation, put up as much wire as you can and
feed it
with ladder line to a 1:1 balun to a short piece of coax to a tuner.

Feed it at the top so the maximum current is up high.

If an inverted L works for your lot, try feeding it at the top middle
instead of the bottom end and using the linear loading technique to
make
each leg longer. According to Cebik if you can get 3/8s wavelength,
you
are
very close to the full performance of a half wave. Feeding at the top
middle does not require a radial field to work.
--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64
[snip]

Some nice suggestions, there, especially the point about keeping the
maximum current area of the antenna as high as possible.

The antenna can be end-fed (high impedance), off-center fed (moderate
impedance) and center-fed (low impedance). With a good antenna tuner, I
don't s'pose it much matters about impedance, as long as the varying
amounts fall within the tuner's range.

It was a revelation to me to realize that a half-wave length of wire
doesn't really care where it is fed: end, off-center, or mid-point. The
high-current portion will be in the middle and the high-voltage points
will be at the ends, regardless of where the feed point is located, as
long as the wire is resonant at the operating frequency.

Linear loading is effective, but as more wire is tightly folded back
from the ends, the more narrow the 'bandwidth' becomes, limiting the
tuneable range. This would also pretty much kill the multi-band utility
of the antenna.

I use a 130-foot ladderline fed dipole, center-fed. In a location like
yours, I'd keep as much of the center portion of the antenna as high and
level as possible, and then at each far end I'd fold them down and to
the side, and then back along the fence toward to the center as far as
needed, clipping the wire to stand-off insulators and keeping them up
out of reach of curious fingers. The radiation from the end portions is
less, and is a practical compromise for 80 meters and up.

For 160 meter operation, I use "clip on" extensions that run along my
perimeter fence on both sides, and they make a sort of "Z" in respect to
the main antenna. This works very well, letting the center part of the
dipole radiate up high, while providing enough physical wavelength to
allow tuning.

All of this is a compromise, of course, but it's practical and effective.

I suspect that the performance of the "all band" antenna is somewhat
less effective on 20 meters up .. but this is a much more approachable
situation. I've acquired an end-fed trapped half-wave vertical for 20
thru 10 meters that is only 17 feet in length. I plan to mount it on a
mast alongside the house. I suspect that the radiation pattern on 20
meters and up from this "vertical halfwave" will be much more evenly
distributed and predictable than the multi-node variations of the
tuneable dipole. Between the two antennas, one should be pretty well
covered, given the space restrictions.

Gray K7VGW


Many thanks. I do not have the space for this 4th HF antenna to get 1/2
wavelength of 2.182 mhz (214'). I can manage 1/4~ in the fashion of an
inv-L
antenna. A big compromise to begin with, but how would that utilize it's
desired 1/4~ electrical length if I fed it anyplace but from the end? I
can
add radials and will have to do so if the end-fed inv-L turns out to be
the
best by consensus. Earth is sandy with a little clay - not very good. An
MFJ-962D manual or MFJ-994 ATU, whichever performs better, and a 1:1
current
balun at the feedpoint (if helpful) will be used. Estimated height of the
vertical radiator from shield-grounded feedpoint to the "L" would be 37'
and
the remaining 70' of radiator would be horizontal. Comments on the
expected
efficiency of this arrangement for best performance on 2182-4125 khz are
greatly appreciated. Anyone who is capable of modeling this design, that
would be seriously helpful too ;-)

73,

Jack Painter
Virginia Beach VA