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

[David Gilbert <ab7echo@gmail.com>]

I just tried modeling something like that.  I used 100 foot 100 feet 
high wire fed at the 25% point at a frequency that would represent a 
fullwave, but checking the current at the mid point and dividing that 
into the power fed at the 25% point.  Using as many segments as EZNEC 
would let me do, I only got about 970 volts at the midpoint.
So I fed the same dipole at the middle, changed the frequency to 
represent a half wave dipole, and divided the current at the endpoint 
segment into the power being fed at the midpoint.  That gave me a 
voltage of 18,767.123287671232876712328767123 volts, which looks a lot 
more accurate to me.
Just kidding, of course.  I'm sure the correct answer is somewhere in 
between those two values.  Glad to help narrow it down for everyone.
73,
Dave   AB7E



On 6/27/2020 11:28 AM, K9MA wrote:
> 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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