On 11 Nov, 14:18, Ian White GM3SEK wrote:
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,
Ian, I have no disagreement to your reply above other than you are
being to king in your response
I personaly would have put more emphasis on what you stated with
respect
to RF traveling along a path that has no external surface .With
emphasising
where many have about RF travel without which one CANNOT understand
coaxial cables or braid The inside of braid on a coax CAN and DOES
carry
RF current but it does NOT radiate, because it does NOT have an
exposed surface
other than a dielectric interface. The outside surface can and DOES
radiate if a
RF current flows on the outside of the braid. I would also add that
copper/braid
itself does not turn into a dielectric or contain a diode thus it
also WILL
also pass a RF current at its centre but of course does NOT radiate.
This very fact was refuted by popular vote on this newsgroup where
poll
standings always overule science. So yes, without true understandings
errors
are sure to congregate and eventually will create a "fact".
Art KB9MZ...xg
'bazooka' baluns, 'shielded' loops and many other
devices will all fall neatly into place.
--
73 fromIanGM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek