According to wikipedia, the impedance of a monopole antenna is 37.5 + j21.25. according to another website the real part represent energy radiated or absorbed. The imaginary part represents energy stored in the near field. Does that mean if the input voltage is 10 volts, the input current is 0.267 amp?

 

First, it is impossible to state that any Antenna design will have an exact Impedance,
there are too many other factors that must be taken into consideration when
ESTIMATING Antenna-Impedance.

Why do You need to know the required Current ?
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First, it is impossible to state that any Antenna design will have an exact Impedance,
there are too many other factors that must be taken into consideration when
ESTIMATING Antenna-Impedance.

Why do You need to know the required Current ?
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.
.
View attachment 335988.
View attachment 335989.

What other factors? Can you list them?

 

That's what the attached pictures are for, and they are not "all-inclusive", just an introduction.
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So impedance of monopole depends on angle of groundplane?

 

And a bucket-load of other variables that most people don't even know exist.

An Antenna is a Transducer which turns one form of Energy into a different form of Energy.
Some methods / materials / configurations of Transducing these different forms of Energy
are more or less, Efficient, and/or, more or less, "Cost-Efficient" than others.

Variables,
Variables,
Variables,
and more Variables,
then keep experimenting until the outcome is deemed acceptable.

Many very sharp people have been doing this for many decades
and have come up with some fairly reliable "rules of thumb",
that usually work "pretty well", "most of the time".

It's just a matter of how deep that You want to go down the proverbial rabbit-hole.
How good is "good-enough" ?

And BTW, there are other subjects in Electronics that follow this same trend, such as Inductors.
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REally, the impedance of a specific antenna can be calculated for a specific set of conditions. That part is straight forward. But in the real physical world the conditions are seldom exactly like in theory. So it is always the details. So is is possible to come close and then make adjustments.