John, negative resistance is indicated when the magnitude of the reflection
coefficient is greater than one.
The solution of the impedance equation for rho 1 yields negative values of
resistance. This lends a lot
of insight into possible causes:

The calibration for open or short is slightly lossy. If a measured load
has a little more reflection than the
calibration loads, it would indicate as negative resistance.

Numerical errors creep into the results. The Smith chart is extremely
sensitive around the periphery.
A few tenths of a dB. result in large movements at the edge of the chart.
Thus, small calibration or
computational errors can result in crossing over the rho=1 circle.

And, as mentioned, detection of external RF fields increasing the output
of the reflection detector
past the calibration value can result in a measured value for rho 1.

-- Tom, N5EG






"The other John Smith" wrote in message
ink.net...

The HP paper discusses negative resistance but does not mention where one
would encounter it. I assume it would normally be associated with active
device measurements. Is this true? If I measure a (passive) load and find
that the resistance is negative, shouldn't I doubt my setup and/or
procedure? Is there any condition where a passive, real-world, network
could
show a negative resistance?



John KD5YI