Atmospheric breakdown

Updated January 7, 2007

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New for January 2007! This is just the start of a discussion on atmospheric breakdown, starting with a contribution from Duncan. Thanks! When you design or operate an airborne transmitter with perhaps 1000 watts peak outpout power, it's time to think about atmospheric breakdown.

Power tubes are microwave circuits that employ high voltage, often it is the power supply that arcs, not the tube! To some extent breakdown is a function of frequency, as you go to higher frequencies breakdown voltage will decrease.

Things you can do to prevent arcing at high altitude:

• Turn off the high-voltage equipment (this is not always an option!) Don't forget to use bleeder circuits to discharge high voltage capacitors.
• Eliminate sharp points, maximize spacing between high voltage conductors.
• Seal high-voltage components in pressurized containers.
• Pot high-voltage components. Typical potting materials are dielectrically-loaded silicone goop. Potting a component will reduce your ability to troubleshoot it, it isn't easy to remove potting material!

Critical field at breakdown

When the "critical field" is exceeded, arcing occurs. Recall from that freshmen physics class you slipt thorugh what "field" means, its units are volts/meter, not volts, and it is "E" in an equation (for electric field), not V. At STP (standard temperature and pressure) for spherical electrodes, this happens at 3,300,000 volts/meter. As you go up in altitude and air pressure drops, the critical field decrease. By the time you are at 50,000 feet, it might be 400,000 volts per meter.

Because power handling goes as voltage^2, your high-voltage design that works on the bench might be in a world of hurt when you fly it.

Paschen's Law

The classic voltage breakdown dating back to experiments in the nineteenth century is that 30,000 volts/cm will cause breakdown in dry air at one atmosphere. But it ain't that simple! This topic deserves it's own page, we'll get to that later...

More to come!