On Tue, 11 Oct 2005 21:25:53 GMT, 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. 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.

(I am assuming your 450 ohm line to be an unbalanced line, impractical
as that is, but the issues of balance to unbalanced transition are
just noise to the discussion.)

Is this about whether the Bird readings are correct for the conditions
on the Bird Thruline, or whether the meter readings are extensible to
the adjacent transmission line without further interpretation /
modelling?

The Bird readings should be correct for the conditions on the Bird
Thruline. You can safely extend those measurements literally to the
adjacent line where the adjacent line is the same as the Bird Thruline
and of negligible loss. In other cases, knowing the line parameters,
you may be able to use the measurements to some extent to calculating
some conditions on the other line.

Though the Bird readings in your example for Forward and Reflect Power
cannot be assumed valid for the adjacent line, the net power should be
correct.

I don't think anyone is suggesting that the Bird could be used in a
general sense to estimate the VSWR on your 450 ohm line.

Owen
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