Reg Edwards wrote:
Roy said -

While a
short inductor can be modeled as a lumped load, that doesn't work

well

in cases like this where the current changes significantly from one

end

of the coil to the other due to radiation. The coil has to be

modeled as

a helix.


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The non-uniform current does not arise from radiation but from the
distributed capacitance of the wire turns on the helix to its
surroundings.

You're correct. I apologize for the error.

Distributed inductance and capacitance of the helix behave as a short
transmission line and there is a corresponding phase shift between one
end and the other in addition to the non-uniform current distribution.

The effects can be estimated by approximate calculations with an
adequate degree of accuracy but it requires the right type of computer
program (which probably doesn't exist) to provide exact answers.

Moment method programs such as NEC-2, EZNEC, or even MININEC, do a very
good job.

Roy Lewallen, W7EL