Cecil Moore wrote:
Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid.

No, they're not. They are requiring a 50-ohm system reference impedance.

What you call the "impedance environment" consists of physical things
like the source impedance, line impedance and load impedance. You're
confusing those with the system reference impedance, which something
completely different.

System reference impedance is purely a matter of definition. The most
common choice is 50 ohms... and by definition, that means 50 ohms
exactly.

Having made that choice, then you obviously design and calibrate your
instruments to give correct readings in an impedance environment that is
as close to your chosen reference impedance as you can practically make
it.

Your example shows the difference between impedance environment and
reference impedance most clearly.

If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.

You have changed the impedance environment to 450 ohms, and that's
fine... but all of the Bird's readings are perfectly correct if the
system reference impedance remains defined at 50 ohms. The reason why
say they are incorrect is that you also changed your definition of
system reference impedance to 450 ohms, without acknowledging that you
did it.

It's like doing a financial calculation without mentioning that you
switched into another base currency... darn right the results are not
valid.


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

Ian White G/GM3SEK wrote:

You have changed the impedance environment to 450 ohms, and that's
fine... but all of the Bird's readings are perfectly correct if the
system reference impedance remains defined at 50 ohms.

I have changed the system reference impedance to 450 ohms. Assuming
a tube PA with a pi-net output, 50 ohms doesn't exist anywhere anymore.
The system reference impedance is no longer 50 ohms so the Bird wattmeter
is being abused and misused. You can do the same thing by using a DC
voltmeter on an RF voltage or by using a hammer on a screw. If you
want to know the SWR on 450 ohm line, use a 450 ohm SWR meter.
--
73, Cecil http://www.qsl.net/w5dxp