On 8/2/15 6:42 AM, Jim Thomson wrote:
Date: Fri, 31 Jul 2015 14:28:11 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Inductor Design and Voltage Breakdown
On Fri,7/31/2015 2:11 PM, Howard Hoyt wrote:
I'd love to see the final product, Jim!
So will I. :) If it works well, we'll write it up. And maybe even if it
doesn't, so others can learn from our mistakes. :)
73, Jim
## As others have pointed out, your V per turn isnt an issue. Another option
is
to use polyimide wire....which is just 15 kv rated magnet wire. Then you could
butt the turns..tight up against each other..and uh would really increase a
bunch,
so less turns used vs air spaced coil. The 15 kv breakdown would mean
30 kv between adjacent turns. Then just wind it on a low loss form..like
fiberglass tube etc. The polyimide in the larger gauges like 14-12-10-8 has
the 15 kv rating.
Plane jane airdux is plenty good enough. You can get it in 10 ga. A local
buddy gave me a bunch of it a while back.... in 8 ga..and aprx 3.5 to 4 inch
diam.
You can also get airdux with different turns spacing. The slightly closer
spacing
stuff has more uh..which is ideal for low band applications.
Even 10-12 ga THNN wire would easily work in your application..with turns
butted up tight.
Dont confuse V breakdown with 160m mobile coil setups.
I wouldn't rely on the voltage rating of the insulation to its limit. If
you have two turns that are adjacent, the field is highly non-uniform,
so the breakdown voltage is lower than you'd expect given the gap length.
Typical breakdown strength for Kapton (which is a polyimide) is about
290 kV/mm (7400 V/mil) (compare air at 3kV/mm)
A heavy build insulation for AWG 10 would be 0.0015 mil. So that would
work out to about 10-12 kV.
There are triple and quadruple builds also available, but the
relationship between breakdown strength and thickness is NOT linear for
thin films (in the wrong way.. twice the thickness has less than twice
the breakdown voltage). This is why they use multiple layers in HV
insulation (that, and because a void or defect in one layer is unlikely
to line up exactly with a void in another layer)
However, there's a bunch of other factors that come into play: partial
discharge and treeing are worse with RF than with DC or line
frequencies, so you need to be more conservative in your ratings.
In any case, the turn/turn voltage in Jim's coil is so low, any
insulation will work and it can have holes in it too, because it's below
the minimum sparking voltage.
Polyimide is expensive (really, really durable, good for high
temperatures, etc.).
http://www.mwswire.com/pdf_files/mws_tech_book/TechBook040114.pdf is a
nice source of information about magnet wire of all sorts.
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