[Tech-Assist] Marti STL funny readingsHere's a head scratcher.

[RichardBJohnson at comcast.net]

Once you understand how these "reflectometers" work, you can understand how inaccurate they can be. It is easiest to think of the slug-type that you can rotate to get "forward" and "reflected" "power."  They consist of a loop that couples inductively to the transmission line. The loop is made of wide enough material so it also couples capacitively. When it is rotated to sense reflected power, the voltage induced by the capacitive coupling is equal to and of opposite of phase from the voltage induced from the inductive coupling if the size and shape of the element was calibrated for a 50 ohm load. Therefore, if the line is terminated exactly in 50 ohms, there will be no reflected power indication because these two voltages cancel out. When you turn the slug around the voltage from the capacitive coupling and the voltage from the inductive coupling add in-phase. The meter reads volts, but the dial has square-law calibration so if the voltage doubles, the power indication is four times. In no case does it read "power," although, if the load is perfectly resistive, is 50 ohms, and the RF has low harmonic content, it can be calibrated to read the same as a true power meter. Companies like Bird do this before the device leaves the factory.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Tom Bosscher" bosscher.org>