On 14 Aug 2003 18:46:56 -0700, (Dr. Slick) wrote:

PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy
load
1 2 3
The "+" are connector points. You folks are saying that as you
change the PA (source) impedance, that the SWR you read will remain
the same, even if the incident power changes due to the change in
reflected power at point 1.

Reading a SWR at this point 1 (or 2 or 3), as specified above, would
simply be zero deflection confirming the remainder of the circuit has
no return.

This i can agree with, as no matter how much incident power makes
it past the impedance discontinuity at point 1, the system after this
point will theoretically always reflect the same ratio of reflected
power to incident.

It will reflect nothing, by specification of the circuit above.

But, if you place an SWR meter of the SAME IMPEDANCE AS THE OUTPUT

But? What has departed will not turn around after the fact of
changing something as immaterial as this presumed meter Z (which is no
more than the minutia of the scale unless circuit gross dimensions
exceed wavelength considerations). If this were significant, the
meter circuit itself would be the point of discontinuity and thus
reflective, but this would be wholly unobserved because
1. The meter pick-up (as it were) follows the discontinuity
2. The power flowing from the meter encounters no other discontinuity
as specified by the circuit above.

Given 2., it follows that zero deflection will still resolve to zero
deflection irrespective of the meter's guts.

To force a reading would require reflection from the output of the
meter's bridge/coupler which simply casts the issue back to a lousy
meter design (but one with apparent high sensitivity - in other words,
it is sucking out a huge amount of power to do the same chore others
do better with less). One can certainly conspire to screw up by
simply ignoring circuit dimension and wavelength - or taking a chance
with selecting an anonymous instrument from the bottom of the drawer
(which is the same thing). Any reading other than zero deflection
from the circuit above would indeed indicate a system failure (the
instrument being part of the system). Take care that failure here is
measured by degree - any reading forces other tests and in and of it
self is not conclusive of anything.

of the PA at point 1, you will definitely see a change in SWR at point
1 as you change the PA impedance, as you are changing the reference
impedance (center of Smith re-normalized). This is what i thought you
meant when you said "change the source impedance", but you meant to
say "change the source, but keep the reference impedance the same".

Understood, assuming this is what you guys mean.


Slick

Serious SWR measurement at high frequency is far more simply achieved
with greater precision, accuracy, and less impact on the system
employing the rather more creaky methods of a slotted line. With this
approach you can at least relate to the serious considerations of
characteristic Z and wavelength specific issues (and often answered
with simple plumbing components hand crafted once to last decades).
Calibration is as simple as replacing the ruler if it falls off (if
you first confirm that the connections are not reflective, but then we
return to the discussion above).

Once again, it is a chicken and egg problem. You need a better method
of SWR determination to validate a poorer one. There is no such thing
as self validating this particular instrument above HF. A poor Bird
Wattmeter is especially prone to reflective connections (notorious in
fact) and given those connections do this before the reading, many
users are quite satisfied in their state of ignorance. But again,
this hardly amounts to much, unless you wish to achieve an accurate
Power determination. Often the system reflections will eclipse those
of the connectors and still be perfectly useful in an industrial
setting. The user of the Bird in that community doesn't really care
about absolute Power, only relative Power with mediocre absolute
accuracy. Quite often they are simply tasked to confirm levels within
2 or 3dB; the instrument's slop of 1dB is often accounted for within
the methods of measuring those specified levels. The universality of
the Bird is due only to robustness.

Your circuit above is simply the first step in
selecting/calibrating/verifying an instrument. Throw a mismatch into
the game and lights will begin to flash.

73's
Richard Clark, KB7QHC