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.
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