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Re: [TowerTalk] Fwd: Inductor Design and Voltage Breakdown

Subject: Re: [TowerTalk] Fwd: Inductor Design and Voltage Breakdown
From: Jim Lux <>
Date: Fri, 31 Jul 2015 06:15:20 -0700
List-post: <">>
On 7/31/15 5:18 AM, Hans Hammarquist via TowerTalk wrote:


I assume you consider a single layer coil. In that case, 23 turns
over 5.3", you get about .23" between each turn. A 10AWG wire, .101"
diameter you have about .128" air gap between each wire, provided the
turns are even. The voltage will be distributed roughly even between
the turns (except at the ends) you will have about 72 V between each
turn. I think that coil will do OK at 600 W. You can put a wire/cord
between the turns when you wind the coil just to get an even distance
between the turn and, maybe needless to say, put some
paint/varnish/glue on the coil when you are finished just to make
sure the wire doesn't move when you use the antenna.

If the turn to turn voltage is less than the "minimum sparking voltage" then it won't flashover, no matter how close the turns are.
for DC/50-60 Hz, that's 327V, so you'll have no problem.

For what it's worth, your turn/turn voltage is so low, you could use almost anything as a form or spacer. I'd go for a "comb" type spacer if you're worried about cooling. At 3.5Amps, I wouldn't worry.

Why not close-wind with enameled wire? The skin effect loss might be a bit more because the turns are closer, but you'll need fewer turns, so it might be a wash. Bare wire in that size and short lengths is MUCH easier to come by, though.

A quick run through Wheeler's equation says that a 3" diameter coil 2" long with 18 turns (basically closewound) will be 21.8 uH
[L (mh) = (R^2 x N^2) / 9R + 10B]

BTW, with 3 amps, AWG16 would be just fine (as would AWG 20, for that matter) from a thermal standpoint. With AWG 16, you could get twice as many turns in the same space. 1.3" diameter, 2" long, 36 turns is 19.8 uH. Parasitic/Self C will be about 1.5 pF for this.

  -----Original Message----- From: Jim Brown
<> To: towertalk reflector
<>; amps <> Sent: Thu, Jul
30, 2015 8:44 pm Subject: [TowerTalk] Inductor Design and Voltage

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


73, Jim K9YC _______________________________________________

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