"John Smith" kd5yiatmindspringdotcom wrote in message
...

"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

========================================

Measured values of Rho greater than 1.0 are not necessarily due to
measurement errors. They can be true. The true theoretical maximum value of
Rho is 1 + Sqrt(2) which occurs on transmission lines only at extremely low
frequencies and with a purely inductive terminating impedance. It arises due
to resonance between line impedance and the termination and cannot be of any
consequence at RF.

Values of Rho greater than 1.0 are mathematically legitimate as may be
demonstrated by exact computer programs when presented with exact data. In
any case, the Smith Chart is inherently inexact at the lower frequencies in
addition to the user's difficulties of reading from it.

The reflection coefficient can quite easily lie in any of the four 90-degree
quadrants. If the real component of the vector is negative it may be thought
to correspond somewhere to an imaginary negative resistance. But when Rho is
correctly used in line input and output impedance calculations it always
gives the correct answers. ie., negative resistances cannot exist.
Resistances are always lossy.
----
Reg, G4FGQ