Re: RF ground - what if you can't use a ground rod?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

"Above ground radial systems for vertical antennas are usually designed
so
that they are sort of resonant, being 1/4 wavelength long.  This isn't
something you're going to practically be able to do with your TC, since
at
300 kHz, the wavelength is around 1000 meters.

Antennas also strive for efficient radiation, something that TCs don't
worry about. In fact, we'd like to reduce radiated power, since radiated
power can't go into heating up the air for sparks.

What you want for a TC is the "other plate" of the capacitor formed by
the
topload and secondary's self C.  Looking at a drawing of the electric
field
distribution, you can see that the vast majority of the energy stored in
that field is within a fairly small radius of the coil (roughly the
height
of the coil).  That's the area where you want to have good conductivity
to
reduce IR losses.  You also want to have low impedance between the
bottom
of the secondary and that ground plane, so that the RF currents flow
there,
rather than somewhere else."

1. For efficiency at RF ground radials should be as long and as frequent
as space and economics will permit.  As you point out, the point of
diminishing returns is when they a somewhat longer than the antenna (or
coil) height.  It's common in big antenna systems to have the radials in
the form of a "counterpoise" mounted high enough that the electric field
from the antenna doesn't encounter anything like grass, which can
introduce significant losses.

2. TC's are extremely lousy radiators and I doubt that the power loss
due to radiation from any coil is significant compared to other circuit
losses, particularly in the primary gap.  Loss in the streamers really
represents loss in the desired output and, of course, doesn't count.
Look up the charactistics of "Helical Radiators" in a communications
handbook.  If their length is only a hundredth of a wavelength (say 5
feet at 200 kHz) their radiation resistance is negligible (less than 32
milliohms).

3. The analogy between the ground plane and the plate of a capacitor is
a good one, and pretty much tells the whole story.  I maintain that the
ground plane should also be connected to a power ground for safety.

Ed