Richard Harrison wrote:
Ian White, G/GM3SEK wrote:
"An alternative possibility is that the Bird 43 does give valid readings
by sampling at the point where it physically is."

Sorry, Richard, apparently my attempt at irony fell flat. Let me put it
another way:

The instrument can only make measurements at the point on the line where
it physically IS. Therefore the Bird 43 cannot be measuring "SWR" by
sampling the maximum and minimum voltages at locations further up and
down the line.

Therefore it follows that the instrument must actually be measuring
something else... namely, what you described in your follow-up:

Why is there power from the reverse direction for a Bird Model 43 to
indicate? There is no second generator sending power in the peverse
direction. The reverse r-f comes from a reflection. The coax enforces a
voltage to current ratio equal to Zo in each direction of flow. Zo is 50
ohms in the Model 43.

Reflection does a peculiar thing. It produces a 180-degree phase
reversal between a wave`s voltage and current. Bird uses the fact that
the current is in-phase with the voltage in one direction of travel and
out-of-phase in the opposite direction of travel to distinguish between
the two directions.

To distinguish, Bird takes a voltage sample and a current sample at the
same point in a 50 ohm line. These two samples are scaled and calibrated
to produce identical deflections of the power indicator.

Out-of-phase samples thus cancel leaving the in-phase samples to produce
double the deflection either would produce alone. This deflection is
carefully calibrated in watts.

Reversing the direction of the wattmeter element,

reverses the polarity of the current sample, while not affecting the
voltage sample...

and reverses the direction in which the samples
of voltage and current cancel.


Yup. It measures the reflection coefficient of whatever impedance is
connected to the port on the opposite side from the transmitter. This
measurement is made at one physical point along the line.

The subsequent conversion to VSWR is a mathematical relationship only.


The Bird has been satisfactory for about a half century.

As I've often said before, you don't need to defend the Bird 43 to me.
I own and use one, and admire the design. It only needs to be defended
from weird notions about how it works.



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