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
----------
Scott Ellington
--- via iPad
> On Jun 27, 2020, at 11:49 AM, jimlux <jimlux@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@audiosystemsgroup.com>
>> To: towertalk@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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