At 09:14 29/01/2002 -0500, you wrote:

Hello,
  I have not experimented in a while with my partially built tesla coil,

but

has anyone used their Tesla coil as an antenna for ELF or other Freqs? If we
try to use it for 137 KHz how do you determine ERP?

Dear Tom, LF group,

Well, occasionally I have inadvertently used my LF antenna as a Tesla coil :-) But seriously, The Tesla coil is essentially a helical antenna (see ON7YD's LF antenna web pages http://www.qsl.net/on7yd/136ant.htm ). So if it is several metres tall, and you can persuade it to resonate on 136kHz, it might make a usable LFantenna. Of course, you would want to avoid the discharges that occur in the normal Tesla coil mode of operation, since these generate QRM and absorb lots of RF power.

A Tesla coil and helical antenna have in common that they have a
'distributed' inductance, ie. the inductance is not located at a single
point as with a traditional short vertical monopole but distributed all
over the antenna.
When trying to model the helical antenna I assumed that it was a tall and
slim structure, where the distributed inductance does not create any
significant magnetic field but only improves the current distribution (and
thus radiation resistance) of the antenna. So even though it look like a
coil it is an 'electrical' antenna.
I assume that a typical tesla coil is a rather short and fat structure. I
am not sure that in this case the magnetic field can be ignored, compared
to the electric field. And since a Tesla coil has rather small dimensions -
compared to other LF transmitting antennas - I'm afraid that it will have a
very low efficiency.
BUT ... since you have the Tesla coil available I would certainly recommend
to do some tests (that's what all the fun is about).

73, Rik  ON7YD