GTO-15 wire's full name is "Gas Tube Sign and Ignition Cable". It's rated
for 15,000 volts and has a stranded copper 14 gauge (4110 circular mils)
core. It is single conductor, not coax. There are 19 very fine strands if
wire, a layer of high voltage insulation, and an outer covering of sunlight
resistant PVC. It's used for the high voltage portion of neon signs and for
oil burner ignition cable. It also happens to work very well for connecting
an antenna tuner to an insulated backstay.

Spark plug wire has a much lighter duty core and is often not sunlight
resistant. Most spark plug wires don't even have a metal wire core. They use
a resistance material to suppress RF noise. The resistance is usually
anywhere from 1000 to 4000 ohms per foot. You wouldn't want to use this for
antenna lead wire.

Ancor Marine Grade Products sells a 25 foot (7.62 meter) spool of GTO-15 as
part number 150102. If your local marine supplier doesn't carry GTO-15 wire
then check with a neon sign supply house.

Rusty O

PS: As to the second part of your question. There have been very long
'discussions' on this group about antennas and grounds. Mostly name calling
and opinions with very few facts. I don't wish to start another one.