Topband: Omega-match

[DAVID CUTHBERT]

Andre,

Looking at the picture of the burned coax I think I see the problem. It may not be a capacitor current issue but a capacitor voltage issue. It appears that the ends arced. This will take very roughly 10 kV RMS for the way you have the ends dressed. To fix this build new hard line capacitors and this time run the inner insulation 2 cm beyond the shield (at both ends). 

While running the rig someone can observe the tuner to see if there is an corona discharge happening at the ends of the coaxial caps. If there is you need to reduce the field stress some more. This can be done by extending the inner conductor 1/2 cm beyond the inner insulation. Dressing the cut ends of the inner conductors with a file to remove burrs. Then tin them to further smooth the edges. 

If more electric field reduction is needed you can fashion corona balls using solder. To do this you apply the soldering iron the the wire. Then, while still feeding solder (it is a ball by now about 1/2 to 1 cm diameter) remove the iron first and then the solder wire. With some practice you can make some nice round corona balls. What you are trying to do is to reduce the field stress at any point to below 35 kV/cm.

Some might tell you to apply RTV. This will work temporarily but due to partial discharges within the RTV (inside gas bubbles) the RTV will fail after awhile. It might hold up in intermittent amateur use and then it might not. 



    Dave   WX7G