Owen Duffy wrote:
On Wed, 12 Oct 2005 22:09:51 GMT, Owen Duffy wrote:
The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

Can I offer the suggestion that the key to understanding why this is
so, it to understand the sampler.

Again Owen, your own experiment using 75 ohm coax on each side of
the Bird proved why the above is not a myth. The Bird didn't read
the correct forward power on the 75 ohm coax. The Bird didn't read
the correct reflected power on the 75 ohm coax. The SWR calculated
using the Bird's readings does not represent the SWR on the 75 ohm
coax.

The proportions calibrate the instrument for a specific V/I ratio.

Yes, that ratio is 50 ohms for the Bird. Only a piece of 50 ohm
coax will guarantee that Vfor/Ifor=Vref/Iref=50 ohms. You proved
that a piece of 75 ohm coax will not do it.

Did I need to mention environments?

No, but you should have. The Bird gives the correct forward and
reflected power readings on the attached coax only in a 50 ohm
environment. Your experiment proved that to be true.
--
73, Cecil http://www.qsl.net/w5dxp