One good solution to a poor, distant ground is to build a twin TC. This is
made up of two identical coils that act as each other's counterpoise.
Primaries are connected is series, each having half the inductance of a
single coil. The bottoms of the secondaries are connected together and
grounded as best you can - e.g. to metal water pipes. Secondaries are out
of phase, so the spark leaders jump between the two top loads.
This avoids large currents in your ground connection, as long as the leaders
are into the air or between the two secondary toploads. Only if the leader
from one of the coils jumps to a ground (e.g. a lamp fixture) will you have
large currents in your ground connection.
The other advantage of a twin is that for a given input power, you can
produce longer leaders between the two coils than for a single coil.
--Steve Y.