[TowerTalk] Fwd: Inductor Design and Voltage Breakdown

[Jim Lux]

On 7/31/15 5:18 AM, Hans Hammarquist via TowerTalk wrote:
>
> Jim,
>
>
> 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
> <jim at audiosystemsgroup.com> To: towertalk reflector
> <towertalk at contesting.com>; amps <amps at contesting.com> Sent: Thu, Jul
> 30, 2015 8:44 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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