FFT and FDTD

Dear friends

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

When u say resistance do u mean impedance? What does your curve look like?

At what frequency range do you anticipate resonance and why do you use FFT to obtain timedomain quantities like current?

Give some more details about your code and structure and maybe i can help

:D

Hi settmar


I means by rsistance real component of impedance

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

thanks

Hi no_no
you can see the last fig. in this attach. which from xfdtd

Hi
Maybe for higher frequency you need mesh refinement and in that frequencies dalta to lambda ratio is less than numerical cutoff frequency.

Regards