On 10/4/2022 10:30 PM, KD7JYK DM09 wrote:
I may be crazy here, but, am I the only one that remembers using, and
all the drawings in all the books of using, a meter to measure voltage,
or current, along an antenna, starting at the feed-point, if one
desires, and moving to the tips?
Is this no longer a thing?
Hi Kurt,
There may have been illustrations to convey concepts of voltage and
current distributions in textbooks, but actually making those
measurements is another story. There is a fundamental principle of
measurement we learn in engineering classes that it is impossible to
perfectly measure a system without changing it. The disturbance may be
small, but there will be one. We enter a closed room to measure the
temperature; the opening of the door allows air flow, our body heat
raises the temperature. We want to measure the behavior of sound
bouncing around a room; the presence of our body, and the equipment we
use to make the measurement, changes the acoustics of the space.
Measuring voltage and current of an antenna that is in the air is a
great example -- how would you physically do that? The physical presence
of a person next to the antenna holding the current probe changes the
antenna by the coupling of his body to the antenna!
With respect to the voltage at the end, since it is a very high
impedance point, any conductive object next to it, even a high impedance
voltmeter, changes it. And if you're measuring the voltage with respect
to ground, there must be a wire connecting the meter to ground. :)
Many years ago, I saw some good work measuring current distribution
along radials in the radial system for a vertical. Here, the disturbance
could be small enough to provide meaningful data, but I would expect
accuracy to degrade as the current approaches zero at the end.
73, Jim K9YC
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