SpamHog wrote:
I have a T2FD antenna for 5-20 MHz next to a good ground point,
and I'd like to make the lead-in coax DC-insulated
to lessen the risk from a lightning strike.

I would line to put in a wideband RF transformer with
1) primary connected to the antenna coax and the ground
2) secondary connected to the lead-in coax, w/o any DC connection at
all.

This is a low power / RX installation, and a bit of loss should not be
a problem. There are already a few neon lamps in the T2FD balun, and
I'll add a spark gap at the primary.

I am very tempted to use either a toroid or a couple of ferrite rods

A good RF ferrite toroid would out perform the rods, especially at the
low frequency end.

Perhaps something like the FT-240-43 on this page:
http://www.surplussales.com/FerToro/FerToro-1.html


with 1:1 windings made of... spark plug wire?! The stuff takes 30kV
pulses without even blinking, If I use non-resistive type, an
appropriate container, and keep it all dry, breakdown voltage between
incoming and outgoing coax could exceed 50kV. A low-ohm, low-Z
grounding could help ensure that it's not easily exceeded.

Hopefully, very little pulse energy would come my way.

Any thoughts?
Any pointers to plans for high-insulation RF transformers?

The silicone wire made for the high voltage connections to cathode ray
tubes is smaller per voltage rating than ignition wire, I think.
It is generally more flexible, also.

Something like:
http://www.surplussales.com/Wire-Cable/HVWire-1.html

If you have a fair ground at the antenna, I doubt you need more than
5kV rated wire, since the arc would have to pierce two wires, and the
instantaneous breakdown voltage is quite a bit more than the rating of
the wire.