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[TowerTalk] Force 12 Sigma 80

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Subject: [TowerTalk] Force 12 Sigma 80
From: (Guy Olinger, K2AV)
Date: Wed, 23 Jan 2002 01:00:14 -0500

----- Original Message -----
From: "Tom Rauch" <>

>>I'm not speaking of any antenna brand in particular, but only of
system in general. there is no magic bullet.... Making a dipole out of
a Marconi-feed structure of given dimensions quadruples the amount of
loading reactance required, all things equal. That quadruples loss in
a loading system if that system has equal Q! ...snip... <<


It does not take much of an ear to pick out signals on 160, or 80 for
that matter, whose stations have managed to escape antenna system
compromises. What's really hard is how to best approach that
loss-unfettered performance in situations that cannot support the
full-size Marconi fed, etc., and MUST take some loss, some reduced
bandwidth. The more restricted the situation, the more difficult
managing the compromises.

The hard thinking will be done by the fella who wants to put something
that really works on 80-10 when his backyard is a concrete 30' x 30'
pad with raised flower beds next to the 8' board fence around. The
fence is the highest thing in back, and he'll never get away with
hanging something off the house.

If he knows about ground screens before he pours his patio, he'll
insist on tied heavy mesh through the entire pad (for strength
purposes of course :>)). If he's really smart he will also bury radial
copper wire in the pour, coming to a junction box in the center so he
can tie in, and put a flush cover on the box when not in use. A ZR-3
will work very well in the middle of that, and a regular vertical can
be loaded against the buried copper.

But pulling off something like that and all the other possible
cleverness requires a triage among all the competing loss situations.
What to worry about eliminating first...

>From the most beneficial to the least, on my personal calculator of
such things... Others might order these differently or add items.
Eliminate or modify:

1) Eliminate ground loss in ground used as 1/2 the circuit.
2) Eliminate hot spots.
3) Eliminate ground loss due to field penetration and excitation, and
lossy signal reflection from ground.
4) Reduce normal conductor and connection loss in shortened antennas
using larger conductors and careful treatment of connections. Get to
85-90% efficiency.
5) Eliminate feed methods that result in low (less than 25 ohms) and
reactive feedpoint impedances
6) Eliminate folded conductors in the high current parts of the
7) Minimize coupling of miscellaneous conductors in the area.
27.3) Increase your SB200 output by getting it to run 700 watts out
instead of 600.

>>"Melting down" is a poor example of loss, unless you confine the
heat to a small area where you can measure it. ...  Distributed
loading systems (such as linear loading) CAN be very inefficient and
not show the slightest sign of heating. Loss is distributed over a
large air-cooled area with good thermal conductivity.  ...  The only
way we know if efficiency is actually high is to measure the FS in a
A-B test. We won't know that from reading an advertisement, guessing,
or watching for melt-downs. <<

Yes, except when really bad joint design, condition of the antenna, or
misconstruction have created hot spots that turn color or you've
watched a trap turn into hot goo, their elimination really does
improve the signal. Hot spots are a problem category all of their own,
related, but not quite the same as reasonable design efficiency
issues. Joe Ham can usually fix hot spots, but improving design
efficiency is deep stuff, hard to measure, as you say.

73, Guy.

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