On 23 Oct 2006 17:41:37 -0700, "Brian Kelly" wrote:

I'm in the process of stringing two base-fed half wave verticals up in
a big beech tree, they're Dale Parfitt's End Fedz dipoles for 20 & 15M.
These antennas are described on this Web page:

http://www.universal-radio.com/catal...wire/4456.html

Hi Brian,

There are any number of curiosities about this link.
1. The antennas are marketed as half wave DIPOLES;
2. They have matching networks to 50 Ohms;
3. Some (if not all) have low power restrictions.

Why?

A half wave dipole already comes close enough to match as to beg the
question: what more is necessary?

Well, the best interpretation is they are not DIPOLES at all. They
are half wave monopoles which definitely demand matching. But with
tinker toy sized components in that small box? Now we see why they
are power restricted (those components would be toast).

The matching networks will be 12" - 18" inches above the the surface
and fed with 8X coax. The tops of eight foot ground rods will be
directly (+/-) below the matching network enclosures at the bases of
the verticals. I'd split open the jacket of the coax and solder
pigtails made from 3/8" flat braid to the coax shields just behind the
PL-259 fittings then tape/goop the joints to weatherproof them. The
other ends of the braid will be attached to the ground rods. There will
also be some "nominal" radials attached to the ground rods.

It sounds like it would be worth a try.

Will this arrangement eliminate the need for the usual types of 1:1
choke baluns by taking any I3 currents on the coax shields straight to
ground?

Skip it. Go straight path to ground, and make sure the coax hugs the
weeds all the way home. This will stabilize the match which is
notoriously wild if you have too much cable trailing around in the
open. Your concern for common mode is well founded.

Now, let's return to the technical howler of calling these antennas
DIPOLES. This is marketing to the stupid, or by the stupid. Buy your
own wire, and invest in quality components in a larger enclosure. This
will easily save half the cost and finding even a modest coil and cap
in the junk box will save you a trip to the post office (and the cost
of postage and handling). The worst you can do is equal the design.
(OK, so some could do much worse.)

Still and all, the price is set at the disposable income level; buy
two so you have a spare when the first one blows.

73's
Richard Clark, KB7QHC