Topband: Omega-match

[R. Kline]

Been there, done that and it ain't workin....

In a past post I've explained various techniques used in attempting to
reduce the voltage gradient at the open end of the coax, including:
- 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).
Although both of these (and other techniques) will greatly improve the
situation, they are not always sufficient.

The last time that I tried this, the insulation on the inner conductor broke
down about 3/4 inch from where it came out of the outer conductor and more
than an inch from the open end.  This was high quality teflon coax whose
physical size was like that of RG-213. If you want to use coaxial
capacitors, you should:
- determine the voltage at the open end.
- determine the maximum voltage allowable on the coax. This will either be
specified directly by the manufacturer, or it can be calculated using the
power rating of the coax and its characteristic impedance.
- make sure you don't exceed 80% of the cable's voltage capability to ensure
reliable operation.

There is another issue with the Omega-match in that the parallel capacitance
to ground increases the circulating currents (and therefore the losses) in
the circuit. If only because of this, it would be wiser to choose the Gamma
match (which is more difficult to adjust).

73,
riki :-)
K7NJ - 4X4NJ

-----Original Message-----

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).