Richard Harrison wrote:
Richard Fry wrote:
"Read Terman`s RADIO ENGINEERS` HANDBOOK, 1943 edition, pp 30-31 for
more on this (or many other sources)."
Amen. Terman doesn`t say different things in different places. He is
consistent. In Terman`s 1955 edition of "Electronic and Radio
Engineering" he writes on page 21: "It is to be noted that some of this
(magnetic) flux exists within the conductor and therefore links with,
i.e., encircles, current near the center of the conductor while not
linking current flowing near the surface. The result is that inductance
of the central part of the conductor is greater than the part of the
conductor nesr the surface; this is because of the greater number of
flux linkages existing in the central region.
What Terman says is true, for the particular example that he chooses.
But it may leave an incorrect impression that the conductor needs to be
completely encircled by flux linkages.
In fact the skin effect will develop on the surface of any conducting
material of any shape, wherever there is RF current flowing.
Here is a link to a detailed mathematical proof, from 'Transmission
Lines for Communications' by C W Davidson (Macmillan Press, 1978, ISBN 0
333 32738 1):
http://www.ifwtech.co.uk/g3sek/misc/skin.htm
Davidson's analysis starts with the most general assumption possible:
that RF current is flowing over any small patch of a conductor's
surface. No assumption is required about the reason for the RF current
to be present, only that it is. Likewise no assumption is required about
the cross-section of the conductor, only that it has an exposed surface
(and by implication, that there are no constraints due to a small radius
or insufficient depth). Davidson then derives all the usual equations
for the skin effect. The only drawback of this derivation is that it is
highly mathematical, and difficult to put into words; but it's still
physically correct.
To repeat, I am not saying that Terman's explanation is incorrect; only
that the skin effect is a far more general phenomenon than his
particular examples imply.
This is important because, by taking the existence of the skin effect as
a guaranteed starting-point, the explanations for the behaviour of
coaxial cables, 'bazooka' baluns, 'shielded' loops and many other
devices will all fall neatly into place.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek