[Towertalk] Explain this!

[Tom Rauch]

> I spotted something "glowing" about  the spot where my SW wire
> vertical for the 80M 4SQ is.  At first I thought it was an animal, and
> I was just seeing its eyes, but it wasn't moving, and turning on the
> lights on in the back of the house didn't scare it away.

All you need is a little air moving across a very well-insulated 
conductor, and the normal charge gradient in air above earth can 
charge a conductor to many thousands of volts.

I have this happen all the time with my 160 dipole, even on clear 
blue-sky days. If I have the feedline hanging loose and grab the 
feedline, I get knocked right back on my rear! That's true on sunny 
clear days, as long as there is some breeze blowing.

Of course the problem is aggravated in inclement weather, when low 
clouds can store charges and increase the voltage gradient or distant 
lightning discharges can induce voltage in antenna.

I've been back by my tower and seen bluish corona on dangling 
feedlines. This would especially be a problem if you have an non-
vented box that stores or holds ionized air. We often don't realize 
it, but sealed enclosures that hold ionized air can reduce the 
voltage breakdown of components inside. This lowers the voltage 
breakdown and aggravates insulation problems. This is one reason why 
switching relays or switches that are switched "hot" either need to 
have a non-ionizing pressurized gas, a vacuum, or be adequately 
ventilated.

If you are using a series-capacitor or opening a relay contact inside 
you box, I hope you have a bleeder resistor (even one with several 
meg-ohms) from the antenna base to earth!

  

73, Tom W8JI
W8JI@contesting.com