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@0 Meter Vertical Collinear
On 26 feb, 02:09, Roy Lewallen wrote:
Jim Lux wrote: breakdown? *Is there some distinction to "full?") One often makes a distinction between a corona discharge which exists as a steady state sort of thing and the streamers which precede a "spark". *Both are air breakdown phenomena, but qualitatively different, and both are different from a low pressure discharge like that found in a fluorescent lamp or neon bulb, or from phenomena like St Elmo's Fire. * . . . Most interesting. I've always thought that St. Elmo's fire was a corona discharge, and a quick web search indicates that it's apparently a very widely held misconception. What's the difference? Roy Lewallen, W7EL Hello Richard, A "full air" breakdown is the situation where there is a full conducting path between the two conductors through air. So the current that goes into the conductors is mainly supported by real electron flow through air and not dielectric displacement current. The effect is strongly non-linear and shows hysteresis. Imagine you have an antenna and you increase the input power gradually. At a certain point somewhere in the construction air will break down. When you have a full breakdown in a uniform field, you will notice a stepwise change in SWR. To stop the breakdown, you have to reduce the input power significantly (hysteresis effect). As most amateurs have an in line SWR indicator, you will notice full air breakdown and will check the installation. In an RF "corona discharge", air only breaks down in the high field strength area. As you will probably know, field strength is highest at air/conductor boundary with small curvature (edges, needle tips). Outside that area air will not break down and current in that region is supported by displacement current. This effect may show only minor hysteresis and you may not notice this during normal operation. The voltage required to establish a corona discharge may be far below the level to get a full air breakdown. At the tips of a HW radiator, you will have highest field strength. However when you bring a conductor close to that tip (and retune if required), field strength will increase. This is also the case with HW end-fed radiators. At least the ground of your feeder, or the ground of the matching network will be relative close to the end of the HW resonator. For some formulas see the document referenced before. I hope this clarifies the "full air breakdown" and "corona discharge" issue as used in my postings. Best regards, Wim PA3DJS www.tetech.nl the address is still valid, but don't forget to remove abc. |
@0 Meter Vertical Collinear
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@0 Meter Vertical Collinear
Roy Lewallen wrote:
Jim Lux wrote: breakdown? Is there some distinction to "full?") One often makes a distinction between a corona discharge which exists as a steady state sort of thing and the streamers which precede a "spark". Both are air breakdown phenomena, but qualitatively different, and both are different from a low pressure discharge like that found in a fluorescent lamp or neon bulb, or from phenomena like St Elmo's Fire. . . . Most interesting. I've always thought that St. Elmo's fire was a corona discharge, and a quick web search indicates that it's apparently a very widely held misconception. What's the difference? Roy Lewallen, W7EL Indeed.. but St. Elmos fire has many characteristics not shared with corona.. (the extent of the glow, for one.. corona tends to be a very small sheath or longer leaders/fingers) Most likely, it's glow discharge off small water droplets shed from the surface from which is charged. Basically the process is like an electrostatic sprayer.. small bumps in the liquid surface form from whatever cause, and the surface charge tends to make the droplets come off. They're charged to the maximum charge for the diameter, and as the droplet evaporates, it gets smaller, causing the glow discharge to shed charge. Or, a big droplet splits into smaller droplets because of electrostatic forces. You can set up a nice demo in a dark lab with something like a cork wet with sal****er, a HV power supply, and some cookie sheets for electrodes. |
@0 Meter Vertical Collinear
Jim Lux wrote:
Most likely, it's glow discharge off small water droplets shed from the surface from which is charged. Basically the process is like an electrostatic sprayer.. One very foggy night on Hwy 1 between Carmel and Santa Cruz, I came up behind a mobile radio in operation. The glow off the end of the antenna was bright orange. An oncoming CHP officer stopped the vehicle for having a "red light" visible from the front of the vehicle. When I left the scene, the two were arguing whether it was really a "red light" and whether the operator deserved a ticket or not. I don't know what radio service was involved. -- 73, Cecil http://www.w5dxp.com "Government 'help' to business is just as disastrous as government persecution..." Ayn Rand |
20 Meter Vertical Collinear
Cecil Moore wrote:
One very foggy night on Hwy 1 between Carmel and Santa Cruz, I came up behind a mobile radio in operation. The glow off the end of the antenna was bright orange. An oncoming CHP officer stopped the vehicle for having a "red light" visible from the front of the vehicle. When I left the scene, the two were arguing whether it was really a "red light" and whether the operator deserved a ticket or not. I don't know what radio service was involved. I've read that W6AM used to drive the route between Long Beach and San Francisco. His mobile station is legendary around here. The operator was no doubt explaining to the officer that a glow discharge in air is more of a salmon than a red. Did the officer look like Broderick Crawford by any chance? :-) I couldn't help but fix the subject line. ac6xg |
@0 Meter Vertical Collinear
On Thu, 05 Mar 2009 08:11:33 -0800, Jim Kelley wrote:
Cecil Moore wrote: One very foggy night on Hwy 1 between Carmel and Santa Cruz, I came up behind a mobile radio in operation. The glow off the end of the antenna was bright orange. An oncoming CHP officer stopped the vehicle for having a "red light" visible from the front of the vehicle. When I left the scene, the two were arguing whether it was really a "red light" and whether the operator deserved a ticket or not. I don't know what radio service was involved. I've read that W6AM used to drive the route between Long Beach and San Francisco. His mobile station is legendary around here. The operator was no doubt explaining to the officer that a glow discharge in air is more of a salmon than a red. Did the officer look like Broderick Crawford by any chance? :-) I couldn't help but fix the subject line. ac6xg Rumor has it that W6AM had a 1 KW Swan amp in his trunk. Plus some really big Rhombics. -- 73's Ken Slimmer, WA0SBU |
@0 Meter Vertical Collinear
Ken Slimmer wrote:
Rumor has it that W6AM had a 1 KW Swan amp in his trunk. Plus some really big Rhombics. I wholeheartedly recommend reading his biography "Don C. Wallace: Amateur Radio's Pioneer" by Jan David Perkins. 73, ac6xg |
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