Tom wrote:
> I don't agree.
>
> If an antenna charges from collecting charges from the
> surrounding air, how does it discharge to something of the
> same potential? Corona occurs only because the clouds above
> and around the antenna have a different potential than
> earth.
>
> It charges towards the potential of the air Jim, and
> discharges to earth! That's why we use drain coils to earth.
> That's one of the reasons why the utility companies ground
> the open lines when doing service work, to prevent linemen
> at ground potential from the shock if they touch a very long
> wire that charges to the V/m field potential caused by the
> height and the natural charge as we move above the earth's
> surface.
It's not charging from the air, it's charging from the particles, which DO
have a potential relative to their surroundings (i.e. the air). The
potential is defined by the work required to move the charge from some
reference point to whereever it is now. You can put a charge on an object,
which will now be at some potential, move it through a field (which will
require or supply work, depending on the relative polarity), and the object
will now be at another potential.
When speaking of isolated objects, the reference used is "ground" but really
means a mathematical fiction of an infinitely large surface infinitely far
away. Since the field drops off as some function of distance (depending on
the shape of the object), at some distance, it really doesn't matter whether
there's actually something there.
The long wire doesn't charge from the earth potential. It charges by being
touched by charged particles being pushed by the wind. The wind slows down
slightly as a result (the energy has to come from somewhere).
Consider this experiment. Put two big plates a few meters apart in a
vacuum. Put a DC voltage of some amount between the plates. (this
represents the earth's DC field) Now string a wire between the plates. Hook
an ammeter up to the wire and to one of the plates. You'll measure no
current (unless the field is strong enough to cause field emission, and, of
course, if there are radioactive particles, they can transfer some energy).
Putting a dielectric in the system won't make any difference. It's a
capacitor, and no DC current will flow.
As far as the breakdown itself goes, if the field exceeds the breakdown
field, the air will ionize. It doesn't need to "go anywhere". Think of RF
breakdown in a waveguide, for instance. There's a field that exceeds the
breakdown field.
When the high field happens to be next to a conducting surface (as in a
charged object at relatively high voltage, compared to the radius of
curvature), the physics tends to make the breakdown form long
fingers/streamers under some situations. From this you get the familiar
dendritic sparks apparently to the air from a Tesla coil. The actual details
of what goes on in the sparks from tesla coil is substantially more complex
and poorly understood. Having little or no commercial value to the
understanding, there's not much research. The research concentrates more on
leader formation with impulses or DC voltages (like lightning, or long
sparks in HV systems)
Jim, W6RMK
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|