[TowerTalk] Static Discharge Wicks

[jimlux]

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.