On 10/18/16 1:00 PM, Steve, W3AHL wrote:
Rob,
Since I needed to retune a portable 75M dipole, I took advantage of
the opportunity to measure the voltage at the end. I also modeled
the dipole in EZNEC, duplicating the as-installed configuration, to
hopefully verify the model’s accuracy. The dipole was installed on
my 60ft mobile tower trailer with the center at 32ft, the far end at
30ft and the test end at 11ft. Testing was done at the new resonant
frequency (3.64 MHz) that resulted from the lower overall height and
one end being at 11ft. My K3 was placed near the test end, connected
to the dipole with 60ft of RG-8X and the TX power set to 10 W.
A Tektronix THS720 portable scope was placed on a 10 ft step ladder
near the test end, with a 100 MHz Tek 10 megohm probe. The probe is
only rated for 300 vPeak at 3.5 MHz, but I exceeded that a little to
get up to 10W TX power. A ground reference wire was run to the top
of the ladder and connected to a ground rod and the K3 chassis with
about 16 ft of wire.
<snip>
Interesting.. There's a fair amount of interaction between the antenna
and the measurement setup (evidenced by sensitivity to movement.. it's
not just the High Z probe)
For instance, the scope chassis is coupled via its capacitance to the
ladder (even if not connected explicitly), and, of course, the ladder
itself is coupled to the antenna.
But "hundreds of volts" is a reasonable measurement and since it is
close to what EZNEC predicted, all the better.
The exotic way to do this kind of thing is to use resistive measurement
leads that have an impedance that matches free space (377 ohms/square)
Another way is to use a long string of resistors as the probe - you want
the resistance high enough that there's no appreciable current induced
in the "probe"
You still have to model the measurement system, if it's bigger than,
say, 5-10% of a wavelength.
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