JB wrote:
The accuracy of a Bird for measuring power goes out the window for an SWR
other than 1:1. Are you saying that if you put an unterminated 3 inch piece
of coax on the Bird you have a 50 ohm system? That sounds like semantics
rather than Math. In fact you would have a 50 ohm system if you left the
antenna port disconnected, Then you could say you had an unterminated 50
ohm system. Real handy huh?

The Bird readings are consistent. In such a case it will read
equal forward power and reflected power (within its accuracy).

This same subject was thrashed out on one of these discussion
groups a few years ago. (I have had a hard disk crash since then
and lost all my notes.) Back then I thought like you are thinking
now. A person a lot smarter than I came up with the equations to
back the proof that a 50 ohm environment is established in only
a few inches of ideal 50 ohm coax. If I remember right, the
necessary length to establish the Z0 of coax was less than ten
radii. It really surprised me.

He proved that a Bird directional wattmeter didn't need any
surrounding coax to establish the 50 ohm environment - that
the internal Thruline establishes a 50 ohm environment all
by itself. Perhaps someone smarter than I who is reading
this exchange can do the math for you.

When light is emerging from glass into free space, how long
does it take space to establish the 377 ohm characteristic
impedance? I suspect that all necessary reflections take place
at the impedance discontinuity and the characteristic impedance
is established immediately.

That is why we can calculate the reflection coefficient at
the junction of two transmission lines with differing Z0s
as (Z02-Z01)/(Z02+Z01).
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein