Dave wrote:

"Cecil Moore" wrote:
I don't think you understand what I was saying. It takes a
certain length of feedline to establish a Z0 environment
for the forward and reverse traveling TEM waves, i.e. to
force Vfor/Ifor = Vref/Iref = Z0.

and what the experiment pointed out was that the length of feedline attached
is immaterial, the 50ohm environment is established within the meter itself
and it doesn't care what the length of feedline attached is.

The experiment proved that the 50 ohm environment is NOT established
because the Bird readings were wrong. The Bird reported 100 watts
forward power where the actual forward power was 104.1667 watts. The
Bird reported zero reflected power where the reflected power was
4.1667 watts. Thus the experiment proves my point. The Bird does not
yield accurate results when embedded in an environment other than
50 ohms.

All the Bird does is sample the voltage and current essentially at
a point. If the ratio of net voltage to net current happens to be
50 ohms, the Bird will report a matched condition, no matter what
the Z0 of the transmission line and no matter how much reflected
power actually exists at the measurement point. For instance:

XMTR--balun--1/2WL 450 ohm line--x--1/2WL 450 ohm line--50 ohm load

The SWR on the line is 9:1. The reflected power is 64% of the forward
power. If forward power equals 100 watts, reflected power will be
64 watts at point 'x'. Yet a Bird wattmeter inserted at point 'x'
will read zero reflected power. The environment is clearly 450 ohms.
The reflected energy doesn't tuck tail and run away when the Bird
is inserted. The Bird is simply giving the wrong reading for reflected
power because all it is looking at is voltage and current at a point.
This is such elementary transmission line stuff that I cannot believe
there is any argument about it.

The insertion of a Bird wattmeter in the middle of a non-50 ohm
environment does NOT eliminate the reflected energy that exists
before the insertion.
--
73, Cecil http://www.qsl.net/w5dxp