On Tue, 29 Mar 2005 15:09:25 -0500, Buck wrote:

How well would that work for shortening a J-Pole? would it be better
to use a shortened 1/2 wave dipole or would the J-Pole design be
better?

Hi Buck,

You may have noticed the vacuum of response - most have already seen
this movie and skipped out. However, for your benefit:

Like any small antenna, narrow bandwidth. Like any Hi-Z antenna, very
skitterish. Like any small antenna, lower gain.

For a helix standing roughly 4M tall for 40M band (which isn't
particularly short) under an 8 spoke top hat 6M wide, this exhibits a
drive point Z of 40000 Ohms. Put 100W into it and you will see 2KV at
the connector. That should take care of grass growing nearby.

EZNEC boldly announces there is loss, and the best gain is -2.6dBi.

The bandwidth spanned from 7.18MHz to 7.25MHz.

From there it is only a matter of adding the critical matching
section.

OR

Throw away the helix and use a straight support, shorten the 8 spoke
top hat to 4M wide and the drive point is an easier match at 13 Ohms.
No lethal voltages at the drive point, lower loss, more gain (albeit
of 0.65dBi), more bandwidth 7.14MHz to 7.4MHz.

Still reduces to it's hardly worth the effort to change from one to
the other, but all the signs point to a better implementation without
the air cooled wire wound resistor. If you want to keep a coil in the
picture, you can put it in the conventional place, shorten the top hat
spokes yet some more, and find no more gain, and less bandwidth.

OR

Buy an SGC which proudly proclaims it will match a wet string.

OR

Throw away all this folderol, and build a full size antenna (roughly
twice as tall as these gomers). No matching issue, bandwidth up the
kazoo (7-7.5 MHz), no appreciable change in gain though (in other
words, that imperceptible 1dB delta).

All modeling performed using "Real/MiniNEC Ground" (not usually my
choice, but then no one else has stepped up to the bar).

OR

Go watch a movie you haven't seen before, it may have a surprise in
it.

73's
Richard Clark, KB7QHC