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

[K9MA]

You might get an idea of the voltage by modeling a center fed full wave wire, tuned for maximum impedance.  It will depend strongly on wire diameter. For a center fed dipole, figure half that impedance and twice the power. I.e., 1/(sqrt 2) times the voltage across the feedpoint of the full wave model.

73, 

Scott K9MA  

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Scott Ellington

 --- via iPad

> On Jun 27, 2020, at 11:49 AM, jimlux <jimlux at earthlink.net> wrote:
> 
> 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.
> 
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