On Sat, 08 Oct 2005 14:13:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

I think if you will recheck that posting you will find the assertion

Cecil, it is someone else who has on a number of occasions suggested
the quarter wave thing in email correspondence, and here in postings.

My suggestion is that the sampler inside a Bird 43 coupler section is
sufficiently far inside the 50 ohm coupler line to provide
measurements within the instrument's stated accuracy of what is
happening within the 50 ohm coupler, irrespective of whether, for
instance, a 75 ohm line is attached to the coupler on the load side.

The measurements of what is happening within the Bird 43 coupler could
then be used to model what is happening on the adjacent line, having
regard for any Zo changes, loss, length etc.

Owen
--

Owen Duffy wrote:
Cecil, it is someone else who has on a number of occasions suggested
the quarter wave thing in email correspondence, and here in postings?

Yep, it's not me, it's Reg. I have defended the Bird wattmeter
design. Reg sez one needs at least 1/4WL and preferably 1/2WL
in order to accurately ascertain the "real" SWR.
--
73, Cecil http://www.qsl.net/w5dxp

On Sat, 08 Oct 2005 20:33:14 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Cecil, it is someone else who has on a number of occasions suggested
the quarter wave thing in email correspondence, and here in postings?

Yep, it's not me, it's Reg. I have defended the Bird wattmeter
design. Reg sez one needs at least 1/4WL and preferably 1/2WL
in order to accurately ascertain the "real" SWR.

Not it was not Reg... end of the guessing game.

Having regard to the definition of VSWR (SWR), I can understand Reg's
point that the direct way to measure VSWR requires sampling voltage or
current over a quarter wave of line where you can find / observe the
actual minimum and maximum.

Having said that, there are other measurements that one can make that
allow one to reasonably predict what the VSWR would be.

Owen
--

Owen wrote:
"---I can understand Reg`s point that the direct way to measure VSWR
requires sampling voltage or current over a quarter wave of line where
you can find / observe the actual minimum and maximum."

Yes, and the direct way to measure MPH would be to measure the number of
miles and divide by the number of hours to get an average value. It`s
not often done that way. Maybe about as often as people find maxima and
minima on a transmission line and compute their ratio.

It is more convenient and sufficiently accurate to use indirect methods
for MPH and SWR.

Best regards, Richard Harrison, KB5WZI