On 10/4/22 11:28 PM, Jim Brown wrote:
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.
As it happens, when you do "near field" measurements of antennas
(typically microwave, but also for AM broadcast and shortwave) they DO
actually measure the E and H fields at multiple points and then
mathematically transform that to the far field.
In the case of AM BC antennas, the "probe" is very small (on the order
of a foot), and it's not measuring the voltage to ground, but, rather,
the field in Volts/meter (typically in three axes). Then, later you can
mathematically transform it.
In the case of microwave antennas, the probe is usually an open ended
waveguide, which has theoretically well defined properties. In both
cases, one can calibrate the probe separately.
Finally, there is some literature out there on making near field
measurements with tiny sensors suspended by conductors of space cloth
(which has an impedance of 377 ohms/square, just like free space).
Another scheme (Bolomey, if I recall correctly) used an array of little
diodes turned on and off, (via spacecloth, or resistive wires with
impedance 377/square) and they'd measure the changes in the feedpoint
impedance.
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