Re: Topband: Covered /bare antennn wire

["Tom W8JI" <w8ji@w8ji.com>]

> 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
That's one reason why car antennas had round balls on the tips. Take it off 
and drive 70 MPH in dry air with old rubber tires, and listen to the radio.
> 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.
That definitely helps.

> 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.
This is the only reason a quad antenna is often quieter than a Yagi in bad 
weather. There is no protruding end.
People think it is because it is a closed loop, but the real reason is it 
doesn't have a protruding open end.
Your work with electrostatics gives you good insight. The effect of 
insulated wires having a charge on the insulation that attracts dust even 
occurs in amplifiers. This is why storms charge insulated antennas almost as 
easily as bare wire. Charge migrates right through the insulation.
73 Tom 

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