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

[jimlux]

On 6/27/20 2:30 PM, Dan Maguire wrote:
> Jim, VE7RF, wrote:
>>>> 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.
> 
> A few years ago I did a little EZNEC study on that.  Details here (Reply #16):
> 
> https://forums.qrz.com/index.php?threads/touching-the-antenna-during-transmission.556342/page-2#post-4157410
> 
> 


That's a 5:1 voltage ratio.

However, I'm not sure about the "large resistor to measure voltage" 
technique when the wire gets to be long enough to become part of the 
circuit. In your experiment it was 6 ft long, which is about 1/10th 
wavelength. you split it into 9 segments, so it's like having a 3 foot 
long wire hanging off the end of the dipole, then the voltmeter, then 3 
feet sticking up from the "ground".

Another scheme might be to put a whole series of high resistance 
segments (Load all segments with a megohm or something), and then sum them.

I suspect, in the limit, it's like integrating the e field over the same 
path, using the LE card.


Running my 20m long dipole at 7.15 MHz, and putting it 4 m above the 
ground (lambda/10)

   TAG   SEG.    VOLTAGE (VOLTS)         CURRENT (AMPS) IMPEDANCE (OHMS) 
        ADMITTANCE (MHOS)      POWER
    NO.   NO.    REAL        IMAG.       REAL        IMAG.       REAL 
  IMAG.       REAL        IMAG.     (WATTS)
    901   101 1.00000E+00 0.00000E+00.   -3.66642E-02 4.61848E-02

Z= -1.05439E+01 -1.32818E+01   Y=  -3.66642E-02 4.61848E-02

Pwr     -1.83321E-02


at the ends
LE integrates to 1.33416E+01 9.97624E+00
multiplying by the segment length -> .040303

I get 2.7+2.02j volts at the end, about 3.37 magnitude... that's not far 
off your 5:1




This is a tricky thing, like measuring the voltage at the top of a tesla 
coil.  For tesla coils, what one usually does is set up a voltage probe 
(high Z voltmeter or scope with a sphere or flat plate antenna) at some 
distance (far enough so it won't get hit by sparks).  You then put a 
known voltage into the coil at the top load, measure the voltage at the 
probe, and now you have a "cal factor" for the probe, voltage on top 
load = k*voltage measured at probe.

Then you fire up the coil at full power.


This is basically "sampling the e field at one point and relying on 
linearity" to make the measurement.

It gets harder to do at MHz frequencies - for a Tesla coil at 100kHz, 
the wavelength is 3000 meters and stuff is a tiny fraction of a wavelength.