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



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 07:48 PM 6/28/2005, Tesla list wrote:
Original poster: "Bart B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Yes, as Ed said, running a 50 foot cable to a ground connection is not a good ground. I personally did the unthinkable. I drilled (masonry bit of course) a hole below my coil in the center of my cement garage floor.

Warning... if you have a "post tension slab" this might be more exciting than you expected. The cables in these slabs (which are quite popular for houses built in the last decade or so, because they reduce the cost of the concrete significantly) are under a lot of tension, and if you hit it with the drill, aside from the steel dulling the masonry bit, the sudden change in tension can cause cracking and spalling.





I then pounded an 8 foot copper rod down until I had about 1.5" sticking up. The rod "just fit" the hole, so, a very tight fit (and a lot of sweat!). I added a little epoxy around the rod and cement edge to help seal it. It's been a "super" contribution to my "garage coiling"! Of course, I wouldn't even attempt to ever pull the rod up. Should I ever move, the rod will be pounded down about 1/4" below the cement level and epoxied over. But it is very "cool" ;-) to have this ground rod available where I build my coiling gizmo's.

If I had a ground nearly impossible to pound a rod into, I would simply lay down a flat plate and use it as the ground plane. I actually keep a round plate for "outside" coil measurement projects. There are a lot of flat plate large objects out there which would suit well as a ground plane. As a matter of fact, hardware stores sell a large metal pan for water heaters (I think). Seems they were maybe 30 or so inches in diameter. I bet they are reasonably priced as well.

Take care,
Bart

Tesla list wrote:

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