Topband: Summary Omega-match

[gamma.tec at t-online.de]

First, big thanks to all !

I received a lot of ideas and facts, I will try to give a summary:


In my present setup I have problems with corona-effects. Last time I heard about that 
was 25 years ago in school.

Dave, WX7G, sent me great calculations:

"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). 
...
The series capacitor current is 5 amps at 1 kW. The parallel capacitor current is 60 
amps. Both capacitors have about 7 kV RMS across them. The 60 amps through the 
doorknob capacitor must surely be heating it up."

So I can try to use my current hardline coax as cap, but:

Riki, 4X4NJ:

"- letting the insulated inner conductor extend several inches beyond the
shield. - fanning out the outer and bending it back so that a rounded
surface is in the direction of the open end (rather than the frayed ends
of outer conductor mini-wires)"

and Dave, WX7G:

"The series coaxial cap can be made workable by bringing the center insulation out a 
couple of centimeters. A parallel cap can be made like the series cap. Twelve paralleled 
hardlines about 3/4 of a meter should do it. Five amps of RF current per coax should be 
good. Dress the ends like the series cap and the tuner will be good for 1 kW- assuming 
the air variables can take 10 kV peak."


Will working, but the efficiency in this system is not the best, as Dave, WX7G 
calculated:

"I ran the simulation again using
copper rather than perfect wire for the omega wires and the radiation
efficiency fell from 100% to 64%. It looks like moving the omega wires
further from the tower helps. Twice as far doubles the input R and so on.
For now I think you can get good results by fixing the present design.
Perhaps in the Spring you can move the omega connection up the mast or
some other change."

And, like Dave finished, and also Riki wrote, it is a lossy installation here and for better 
performance I have to change something with the match or the attach-point.
For now I will try to get my coax working, or get some vacuum caps, fixed are ok for me 
due to my air variable cap in parallel.

In spring, might be earlier if wx is ok, I will change the main setup.
And there is a very nice idea from Uli, DJ2YA. It is not that difficult to isolate the upper 
pipe construction from the grounded tower. Than I can lengthen the upper part to 
ground-level with e.g. wires, and feed it like an isolated monopol, which makes it much 
easier.


But, I hope to bring that 80m spitfire up again, before end of season, and after repairing 
the 160m match I should install a camera near the box for watching the light show....


Many tnx to all and cu on 160,  André  DF9OX