Cecil Moore wrote:
Ian White GM3SEK wrote:
Now what happens if the load is not exactly 50 ohms?

If the feedline is 50 ohms, what happens is reflected
energy that is easily visible using a TDR, time domain
reflectometer.

One is that if the meter scale says "power", then there genuinely
are forward and reflected traveling waves of power on the line. In the
"93 - 23 = 70W" example, the belief is that there genuinely is a
power flow of 93W towards the load, only 70W of which is accepted and
23W is returned.

One correction. The Bird wattmeter is installed at a point
on the transmission line and it measures the power at that
point. What is traveling is the energy. Power is the number
of joules per second passing a fixed point. "Power flow" is
somewhat of a misnomer.

Sorry, you're right about "power flow". What I meant was a forward
travelling wave carrying 93W towards the load.


The other school of thought is that that's not true. The meter may
*read* more "forward power" than is actually being delivered to the
load, but that is a false indication because the instrument is not
being used in the situation for which the power scale was calibrated.

It certainly is being used in the situation for which it was
calibrated if the Z0 of the transmission line is 50 ohms.

I'm not sure which "transmission line" you meant here, but I don't think
it matters anyway.

The inserts are individually calibrated with a 50 ohm load impedance
connected to the "Antenna" socket. The internal pot is adjusted to give
the correct power reading (at one point on a meter scale that is
pre-printed), and then the insert is reversed and a tab is bent to
adjust the capacitive coupling to give the lowest possible reading.
There may be some interaction requiring the two adjustments to be
repeated, I don't know.

If you meant the transmission line outside of the instrument, the
calibration load may or may not include a length of matched 50 ohm
transmission line - it doesn't matter. Inside the instrument, the
characteristic impedance of the internal line is 50 ohms in order to
avoid introducing an impedance bump into a system that is already
matched, but even with say a 57 ohm internal line, the Bird insert could
be set up to indicate power correctly into a 50 ohm load. The only
difference is that the performance would become frequency-sensitive.


On the other hand, we have yet to see an explanation in equivalent
physical detail that is based entirely and exclusively on the
viewpoint of travelling waves of power ...

Please give up on your misconception. Those are traveling waves
of *ENERGY*. Power is what is measured when traveling energy passes
a fixed point. Perhaps that is your whole point of confusion.

You're right, they would indeed be travelling waves of energy rather
than power. But otherwise the same challenge is still out the if
forward and reflected travelling waves of energy exist, we would expect
to see a detailed explanation of how the Bird or any similar instrument
interacts with such waves as distinct from the explanations that we
already have for travelling waves of voltage and current.


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