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![]() Updated: 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 distortionless 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 -- |