Jim,
I would think that 23 turns of #10 could be self supporting (no coil form)
and thus no insulation to breakdown. If #10 won't self support then go
bigger with turns and length adjusted to end up with same uH. There are
probably formulas out there somewhere but I'm an ole cut and try guy when I
can't find the formulas. At least one of the commercial all band verticals
uses big self supporting coils. If you are going to be a rover you will
need a coil with good stability as well as no flash over. Let us know how
you make out. 73, Ron W8RJL
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "towertalk reflector" <towertalk@contesting.com>; <amps@contesting.com>
Sent: Thursday, July 30, 2015 8:43 PM
Subject: [TowerTalk] Inductor Design and Voltage Breakdown
I'm designing a special 80M vertical for CQP (California QSO Party) county
expeditions that can be self supporting for places where there are no
trees. For it, I need a 20 uH loading coil. NEC tells me that running
600W, it will see about 1650V and about 3.5A. I'm concerned about voltage
breakdown, so I've scoured the ARRL Handbook and the internet, and can't
find any guidance. W8JI's webpage discusses the issues, but has no
specific advice except to say that it can be a problem.
N6BV pointed me to a couple of ancient DOS programs that are good at
predicting Q, inductance, stray C, and dissipation, but don't address
voltage breakdown. And I had to find an old computer to run them on.
Surely there must be good stuff around.
I'm tentatively thinking about 23 turns of #10 bare or enameled copper,
3-in diameter, about 5.3-in long. That works out to about 4.5 turns/in.
Suggestions?
73, Jim K9YC
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