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
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?