[AMPS] Arcing in air vs. arcing in a vacuum

[Richard W. Ehrhorn]

Hi Jon...

Sorry you caught me w/my head under the table Saturday and were gone by the 
time I stood up & figured out who'd said "Hi."

If your friend's theory was correct, vacuum capacitors surely wouldn't 
work, would they? There's a big difference between "reduced air pressure" 
and a hard vacuum - sounds to me like he made an incorrect extrapolation 
from one to the other.

As you go from sea level to, say, 10,700 feet altitude where W0ID is, the 
breakdown voltage of a given air gap decreases. IF this led to arcing in an 
air variable tuning cap that worked okay at sea level, one solution would 
be to replace it with a vac cap - which also would be smaller because the 
spacing in typical vac caps is much closer than in their counterpart air 
variables.

I guess maybe the answer is that when the air molecules get far enough 
apart it's hard to find enough of them to ionize an (arc) path. I'm no 
physicist but bet John L. can explain the physics without going back to a 
textbook.

BTW, all ALPHA amps since 1977 have been built and checked out between 5000 
and 6500 feet, and all have been extensively tested at 10,700 feet. It's a 
useful stress test, 'cuz cooling efficiency also is lower in lower-density 
air at high altitude.

73,  Dick

-----Original Message-----
From:	Jon Ogden [SMTP:jono@enteract.com]
Sent:	Monday, May 17, 1999 8:35 AM
To:	amps@contesting.com
Subject:	[AMPS] Arcing in air vs. arcing in a vacuum


Hi all,

I got a question.  I had a discussion with a ham friend of mine on our
way to Dayton this weekend.  We were talking about the *gas* theories in
tubes and tube arcing.  His comment was that from his experience, he has
determined that the voltage breakdown potential of a vacuum is LESS than
that of the voltage breakdown potential in air.  He said that while doing
some EMC testing of a product he has designed for his company, he found
this to be the case.  He had to simulate lightning strikes at high
altitude and thinner air.  The breakdown potential was much less at
higher altitude (thinner air) than at the thicker air of sea level.  The
arcs happened at less voltage at high altitude.  So he has made the
conclusion that if the breakdown potential of thin air is less than
regular air, the breakdown potential of a vacuum is less still.  I don't
know if this jump can be made.

Can someone help explain this?  All my instincts tell me that a vacuum
has the highest breakdown withstanding potential.  I seem to remember
hearing once though that thin air ionizes easier than "thick" air so
perhaps that explains it.

Thanks a lot.

73,

Jon
KE9NA


--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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