Cecil Moore wrote:
Ian White GM3SEK wrote:
Evidently I was in too much of a hurry to leave for the GMDX
convention, because what I mean to write next was:
"Cecil's theory does not work for this test case, "
[ I definitely did type the word "not", but it accidentally
disappeared from the version that was posted. ]

Well there you have it, folks. Gurus don't even make typo mistakes.
Some evil server removed that "not" on purpose from Ian's posting.


In trying to respond to your points as clearly as possible, some parts
of my previous posting went through several versions. I simply made a
mistake in editing that particular sentence. I have already apologised
for any confusion that might have caused.


Cecil's theory does not work for this test case,
because it requires that basic electrical properties like current
and inductance switch into a different kind of behaviour in what he
calls a "standing wave environment".

RF current switches from a different kind of behavior than DC current.

True, but irrelevant. You are asking for RF current to switch its
behaviour while still being RF current.

Phase, capacitance, inductance, and wavelength all have to be taken
into account in the steady-state analysis. That is a technical fact
that I'm sure you appreciate.

Yes...

Why is it such a stretch to recognize that standing wave current behaves
differently from traveling wave current? That standing wave current is
different from traveling wave current is readily apparent from the
equations. In the following equations, 'K' is used for a constant,
'z' is the linear distance up and down the line, and 'w' is omega.

Forward traveling wave current = K1*cos(kz+wt)
Reflected traveling wave current = K2*cos(kz-wt)
Standing wave current =
K1*cos(kz+wt) + K2*cos(kz-wt) = K3*cos(kz)*cos(wt)

If tK1 = K2, then the standing wave doesn't move. Please dust off
your old math books and realize what the above equations imply at
a physical level.

Let's try it a different way. At any point located a distance z along
the antenna, there is the normal cyclical variation in current I with
TIME, so:

I(t, z) = Ipk(z) cos(wt)

where Ipk(z) is the peak value of the current at point z. The cos(wt)
term represents the cyclical time dependence of the back-and-forth
movement of electrons; it has no dependence on z.

Ipk(z) is simply a scaling factor whose value depends only on the
LOCATION of point z within the antenna. It has NO time dependence.

The next issue to describe how Ipk varies with the location z along the
wire. The aim of antenna analysis is to find out what the current
distribution along the wire(s) actually is. All the rest of the
antenna's properties can be calculated from this.

Ipk(z) does not have to be a simple cosine function as you seem to
assume above. A cosine function may be a good approximation for very
simple (or simplified) cases; but when the antenna includes a physical
discontinuity such as a loading coil, Ipk(z) will definitely NOT be a
simple cosine function of distance z. So in general it will not be
correct to bundle the z dependence into the same cosine function as
(wt).

There are several methods of finding the current distribution. If you
choose a method based on forward, reflected and standing waves (which
can be done), the "standing wave" is simply a plot of Ipk as a function
of location z. Ipk(z) is a scalar quantity representing the peak
magnitude of the current, and its only dependence is on LOCATION. It is
not an alternating RF current because it has no time dependence.

"Current" remains what it always was: simply the movement of charge
(electrons). If it's an alternating RF current, the cos(wt) term
describes how the charge moves cyclically forward and back past the
observation point. A loading coil, the RF ammeter or the
current-transformer measuring probe all respond to exactly the same
cyclical back-and-forth movement of charge.

In the standing wave analysis, the current is still the net movement of
charge, ie the instantaneous difference between the forward and
reflected currents. These vary together in time according to cos(wt). It
is not possible to measure the "wrong kind" of current by mistake,
because there is only one kind.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek