K1TTT wrote:
> Not for corona noise, that is from the tips and pointy spots and has nothing
> to do with current flowing to ground... in fact it is current flowing 'from
> ground' so the high impedance to ground won't help. This type of noise
> occurs even on completely dc grounded construction.
>
>
I was thinking that the static dissipation wicks on planes basically
turn periodic big zaps into a constant hiss of small discharges. The
charge accumulates regardless.
There's also two charging mechanisms at work:
1) charge transfer from airborne particles (dust, raindrops, snow, ice
crystals)
2) overall current flow from surface to sky (obviously, this one
doesn't occur in the airplane scenario)
For an antenna, you have one of two situations:
1) the antenna is isolated from ground, so charge accumulates just like
in an airplane, until the voltage gets high enough that breakdown
occurs. The bleeder impedance would fix this one nicely. The static
dissipator might, but it might just turn periodic snaps into low level
hiss. Or, some other part of the antenna system breaks down (after all,
the antenna isn't magically suspended in free space with an optical
fiber interconnect to the rig.. somewhere there's a physical connection
with wires in it.. perhaps galvanically isolated by transformer or
capacitor)
2) The antenna is connected to ground, in which case you have the
"current flowing from ground to the antenna, and thence to the sky"
problem. I think the best answer here is to have antenna elements with
sufficient radius of curvature that you don't exceed the breakdown of
air. The static dissipator things might actually aggravate the problem.
What radius? you'd have to look at the fields.. but let's say the field
is 10kV/meter under the thunderstorm. You have a 20 meter high tower,
so the potential of "the air" at 20 meters, if the tower weren't there,
would be about 200kV. If the tower didn't perturb the field (which it
does, but that just helps, here).. the grounded antenna would be at that
potential relative to the surroundings. The radius of curvature needed
to prevent breakdown at 200kV is on the order of 3" (6" diameter) which
is pretty big.
In the not under a thunderstorm environment, where the field is on the
order of 1kV/meter, a radius of 0.3" would be more appropriate.
On the other hand, the static dissipators aren't going to change the
field all that much, so you still have the same problem: corona from the
ends of elements and such.
Maybe this is one of those situations where you just live with the problem.
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