"Dave" wrote in message
. ..

"Cecil Moore" wrote in message
. com...
Tam/WB2TT wrote:

"Cecil Moore" wrote:
The actual SWR on a lossless line doesn't change. Yet, in another
posting, I showed that moving the Bird 1/4WL closer to the load
caused a reported SWR change by the Bird from 1:1 to 2.25:1. How
could both results possibly be right?

The Bird does not know squat about transmission lines, foreward, or
reflected. It only cares about impedance. If you connect a 50 Ohm load
to it through 1/4 wave of 75 Ohm coax, the impedance the Bird sees will
be transformed to 112.5 Ohms; hence the 2.25 SWR. (Actually, a 2.25:1
impedance ratio)

Yes, that's exactly what I said in the other posting. But some
people seem to believe that inserting a Bird into a transmission
line with a Z0 other than 50 ohms magically changes it to a 50
ohm environment. The 40mm of transmission line inside the Bird
is supposed to accomplish that miracle.

yep, that is true, and that is what the experiment shows. the 50 ohm
load, even though it is caused by a 75 ohm line, is far enough away from
the sensor that it sees it as 50 ohms. so the 'miracle' length is
obviously less than 40mm.

Specifically, the "miracle length"is 0. My SWCad simulation assumes 0 length
interconnects, 0 spurious capacity, and 0 spurious inductance. Also, I might
add, near infinite bandwidth. In fact, it should work at DC. Haven't tried
that, but it works the same at 100 Hz as 100 MHz.. It gives generally
accepted answers.

Tam