I have a problem with FDTD
I used method FDTD to simulate Microstrip antenna by using matlab
The propagation of the Gaussian pulse is very will
But by using FFT , I noticed that :
1- The resistances at some frequencies are negative
2- The time domain results ( like current) reached to zero at some time and then oscillated very high
my curve for impedance (resistance and inductance) is very will at required
frequency 1800 MHz but at frequencies far from 1800MHz the rael part of impedance is negative
i used FDTD to obtain time domain values ( current , voltage and fields ), and FFT to obtain
frequency domain