Why is the resistive component of the input impedance negative?
The first part of the answer was taken from NEC-List (See http://www.nittany-scientific.com/nec-list.htm for further information on NEC-List.) NEC-List: Author is unknown. “We have observed this many times when working with short antennas whose input impedances are high. The angle of the impedance vector is close to zero and NEC apparently rounds the resistive component to a small negative number. We never did come up with a good solution.” This part was submitted by L.B. Cebik. The above section on negative input impedances is only correct for single source models that give this result. There are instances of using multiple sources where one or more may correctly show a negative resistance. These values occur in cases where parasitically induced currents are greater than source currents. In those instances where multiple sources are used in advance of tying sources together via transmission lines or networks, inclusion of those features and specification of a single source will normal
