Richard Harrison wrote:
Ian White, G3SEK wrote:
"Step 1: I am starting from the fundamental AC current behaviour of an
ideal inductor."

Good start, but incomplete. Fundamental a-c does not include a
standing-wave wehich causes a variation of voltage, current, and
impedance along the length of the inductor whether straight or coiled.

I was going to get back to you about that (but Life intervened), to say
that I agree with your entire analysis of standing waves on antennas...
except for that one point.

We need to be very clear about the difference between an ideal inductor
and any practical inductor. An ideal inductor has only one property:
inductance. It does not have length, diameter, self-capacitance,
parasitic capacitance or any of the extra properties that a practical
coil has. Let's ignore those for the moment, and try to understand how
an antenna is loaded by pure inductance alone.

The behaviour of loading inductANCE does not not involve any variation
in current between one terminal of the inductance and the other. It
cannot, because that is not one of the properties of inductance - not
ever, in any circumstances. With a pure loading inductance, the current
profile on the antenna has to fit the constraint that the current
immediately above the inductance must be equal to the current
immediately below it.

In the real-life case this is not necessarily true - but we need to be
absolutely clear that any difference in current between the top and
bottom of a practical loading coil has to be due to those "extra"
properties mentioned above. It is not due to the inductance alone,
because inductance alone doesn't do that.

Unless we understand this fundamental point about how an antenna is
loaded by pure inductance, we have no hope of understanding how
real-life loading coils actually behave.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek