[TowerTalk] Peak Voltage at the tips of ants ?

[Jim Brown]

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