Comment imbedded:

"Dave" wrote in message
...
ok, bonzo, i'll bite on the troll bait. ...

"Lord Snooty" wrote in message
nk.net...
...RF power amp feeding directly into a VSWR meter,
...into a load consisting of a carbon resistor and a variable
capacitor rigged in series. The meter connects to the load via about a
foot of 50 ohm coax. The frequency is between 1 and 10 MHz.
Model the source impedance as Zs = R + jX, and the load impedance as Zl
=
r +
jx (or use phasors if you prefer .
The following two statements are true:
1) The power dissipated in the load (r) is maximised when x = -X
(so-called
"conjugate matching"), whatever the value of (r).

wrong. you must transform the R+jX along the transmission line to get
back
to the load seen by the source. you stipulate a low frequency and short
line, so you are close anyway.

The "transform along the coax" part is correct, but the "power is
maximised" part can be VERY misleading folks.

P.S. Get this MPT theorm blockage out of your minds... It is a synthetic
restriction.
The "maximum power therom" (ZL=Zs) ONLY applies to ONE special case, NOT
all cases. That case is where the source's output power (or if you like
current) capability is limited ONLY by the two resistances. That is, the
case is when the source can put out all the power needed by these resistors
and no other internal limit dominates. A common circuit can be shown to
give maximum power at other than Zs=ZL (aparently violating the above
referred-to therom). There are things other than these resistances that
limit the output power of a
practical source.... (see how long this thread goes.....



2) The classical VSWR is minimised (zero "reflected power") when x = +X,
whatever the value of (r).

doubly wrong. vswr is on a cable and is independent of the source.

Without plodding through the rest, it appears Dave has a handle on the
error.



--
Steve N, K,9;d, c. i My email has no u's.

it
knows nothing of R+jX only the characteristic impedance of the cable. all
following calculations are wrong for this reason alone.


However, my VSWR meter, whch is a conventional 2-diode bridge and short
transmission line, indicates that minimum indicated VSWR
corresponds to max power dissipated in (r).!! (i.e. at conjugate match,
and
NOT when reflected power is zero).

The equation normally used for VSWR is
VSWR = ABS( (1 + |p|) / (1 - |p|) )
where
p = (Zl - Zs) / (Zl + Zs)

wrong again, the impedance used must be that of the cable not of the
source.
its not worth commenting further until you understand this.

and p is a measure of the amount of power reflected back to the source,
called
the "voltage reflection coefficient"

I plotted something I call "conjugate VSWR" or VSWR*. which is the same
expression as above, but with p defined as
p = (Zl - Zs*) / (Zl + Zs)
where Zs* indicates the complex conjugate of Zs.
and the behaviour of this VSWR* thingie absolutely matches what I see on
my
meter. Aye, there's the rub.

Some points to note
a) Classical VSWR shows NO minimum for all r, when x has the opposite
sign
to
X
b) VSWR* always has a minimum at the same r-value which causes maximum
power
to be dissipated in r, whatever the value of x.

Again, I flat don't understand how my VSWR meter can indicate VSWR* when
I
know it should indicate VSWR.

Here are a couple of links to flesh out the theory.

1. Wade through this at your peril - it's you lot fighting abou this
issue
and
is VERY long


http://www.ibiblio.org/pub/academic/...S/20030831.ant

2. This is much more succint - cut to the chase on p47


http://my.ece.ucsb.edu/yorklab/Usefu...%20AN64-1B.pdf

Best,
Andrew