On Wed, 29 Jun 2005 22:53:24 -0700, Frank Gilliland
wrote:


cannot be "zero length" in the present state of reality). If the
impedance of the signal is different than what is expected by the
bridge then your power measurements will probably be wrong (to what
extent they are wrong may or may not be important). But if that's the
case then any error will be the same by percentage and sign for both
forward =AND= reflected power because the impedance of the signal is
the same for both forward and reflected power. IOW, the ratio is the
same -despite- the impedance.

Lets make an assumption that we are talking about lossless lines. (If
you are not, then the reflectometer does not provide an accurate
indication of forward and reverse power.)

If you use an ideal 50 ohm reflectometer (that means it is a
negligibly short 50 ohm through line and it is nulled to show zero
reflected power when connected to a 50+j0 load) to measure conditions
in a line, the power flow at that point is the indicated Pf-Pr. If you
had placed an ideal 75 ohm instrument in that spot, the readings are
not necessarily in the same ratio (they are unlikely to be so), but
the difference between Pf and Pr will be the same.

The only other inference you can make from one instrument with regard
to the other will be if one of the instruments shows zero reflected
power, then you know the VSWR that the other instrument will indicate.

Real instruments aren't of zero length, but some types of design are
so close to it at low HF frequencies, you will not detect the error
that is introduced.

Owen
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