>From my own exhaustive research and many lab experiments with HV corona
discharge, it seems to me that at least some of this could be eliminated by
either:
1. Eliminating all the sharp points
2. Properly covering all the sharp points with suitable insulation such as
heat shrink or vinyl caps.
I'm not saying that a high Yagi has to be dipped in a heavy coating of
liquid plastic to eliminate corona (but that sure ought to accomplish
that!). Just cover the ends and joints and the other points where a charge
can build up a strong enough field to initiate a discharge to the air, etc.
It takes an enormous voltage to get a corona discharge from, say, the side
of a length of smooth, bare metal tubing (with insulated ends) such as even
very small Yagis are made from. For example, my own research shows that
when ~5 to 7 kV is applied to a .005 diameter SS wire, discharge into the
air ONLY occurs at the end of the wire, NEVER from any other point along
the length of the wire (unless a grounded point is placed very near it).
Only by reducing the wire diameter to .004 and changing the material to one
with a lower work function does a corona discharge occur throughout the
length of the wire at that voltage.
Surely, with the much larger diameters of materials used in antennas,
something can be done about corona discharge noise?
Having said that, I have seen the outside of an insulated wire take on a
charge. One time, a quality silicone rubber insulated wire rated for 20 or
30 kV was next to a wall, carrying about 6 kV. Over time, the wall where
the wire was touching accumulated dust.
73, Mike
www.w0btu.com
On Thu, Nov 15, 2012 at 2:25 PM, Tom W8JI <w8ji@w8ji.com> wrote:
>
> Clearly the noise was all from corona at sharp points. ... The highest
> and most protruding antenna has the first and worse noise. Grounded
> elements, fiberglass housings, and other tricks make no difference at all.
> The only thing that matters is streamers from the exact point of corona
> leakage.
_______________________________________________
Topband reflector - topband@contesting.com
|