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.
## 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
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