"TOM" wrote in message
...
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


Yes, I see that now, Tom. Thanks.

John, KD5YI