[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.
> 
> ##  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 ?

it turns out that this is a bit tricky - voltage relative to what?
I ran some quick NEC4 models with a 20 meter long dipole, driven at 7.15 
MHz (so it's a bit out of tune) at various heights above the earth 
(5,10,50 meters).

NEC can sum the electric field along a line, which is the voltage 
between the ends of the line. I summed 4 lines:
 From just under the end of the dipole to the earth (10m)
from mid point (5m)
from near the center (1m)
And, along a line 1cm away from the dipole from end to end.

Here's the results (all excited with 1 Volt)

jimlux, w6rmk, 27 june 2020				
20m long dipole, 1mm diameter, 1V at feed point at 7.15 MHz, 13/0.005 soil		
5m high			
	Re	Im	Mag
10m	0.70	-2.80	Volts	2.88
5m	0.80	-2.96	Volts	3.07
1m	0.19	-0.65	Volts	0.68
				
along	9.93	35.37	Volts	36.74
				
10m off ground				
	Re	Im		Mag
10m	-0.09	-0.51	Volts	0.52
5m	-0.06	-0.53	Volts	0.53
1m	0.03	-0.12	Volts	0.12
				
along	0.59	2.45	Volts	2.52
				
				
50m off ground
	Re	Im		Mag
10m	3.26	-8.06	Volts	8.70
5m	3.74	-7.86	Volts	8.70
1m	1.52	-1.71	Volts	2.29
				
along	-0.80	2.45	Volts	2.58



For what it's worth, on the 50m high one, the feed point impedance was 
63.3-31.1j
for the 10m high case, the impedance was closer to 78+6j - due to the 
loading effect of the soil under the antenna.

the power was 6.36E-3 watt for the 50m case. - so maybe multiply the 
voltages by 400 (=sqrt(1000/3.36E-3)) to scale to 1 kW.


There are some oddities here..

The voltage along the dipole is about the same for 10m and 50m, but the 
voltage along for the low dipole is a LOT higher - I'm not sure why yet.

And the voltage from "ground" to the dipole is highest for the 50m case, 
and lowest for the 10m case.

I'm thinking it's because 10m is 1/4 wavelength

The odd results for the low dipole are perhaps because of the 
"transmission line" effects

When I get some time, I'll look at the theory and see if I can make 
sense of this.


> 
> ##  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.
> 
> ##  I suspect  dacron rope, when wet, is not much better than nylon rope.
> 
> Jim   VE7RF
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>