[TowerTalk] Do I need an insulator on the ends of my

[jimlux]

On 6/27/20 8:19 AM, Jim Thomson wrote:
> Date: Fri, 26 Jun 2020 21:33:25 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Do I need an insulator on the ends of my
> dipole?
> 
> On 6/26/2020 11:07 AM, Kevin Zembower via TowerTalk wrote:
>> Why do I need an insulator?
> 
> <Because the open end of an antenna is a high voltage point, and when the
> <rope that holds it is wet, the RF can melt it and the antenna will fall
> <down. Ask me how I know. :)  If instead it's connected to an insulated
> <wire, because it's a high voltage point, the insulation can arc over to
> <the wire. The same is true of radials.
> 
> <73, Jim K9YC
> 
> ##  Does anybody know what the  EXACT  peak voltage is....on the ends of a half wave dipole,
> say with  exactly 1000  watts cxr  applied  to the feedpoint ???   Say a 80m dipole up  100  ft... flat swr, fed with coax +  CM choke.

It's difficult to measure or calculate. And it is almost certainly 
limited by corona discharge (3kV/mm of radius on a clean smooth conductor)

In an idealized sense, one can look at it like a 1/4 wave transmission 
line, but since it radiates, it's not.


> 
> ##  what is peak Voltage at various points  along the dipole ??   again with 1 kw cxr applied.  What happens to peak  V  along the  dipole as swr is increased ?
> 

Interesting question. For "small" changes in length, even though the 
feedpoint reactane gets big, I would think that the voltage distribution 
doesn't change.

But for large changes (an electrically short diple)


> ##  I would have thought  rain water  was distilled  water,  high resistance... but have never  tested  rain water.
> Have tested  store bought distilled water..and   then compared distilled water to  tap water....  huge difference.
> 

Rain water is pretty clean although it can have dissolved gases in it 
(e.g. Acid Rain downwind of areas burning high sulfur coal)

The rope, though, is almost certainly not clean - at first.  If you get 
enough rain, then it probably washes off.