HFSS and antenna resonance:urgent please help me!
I'm using HFSS...
first of all a question.
If I've an antenna fed by a microstrip and I want to use a lumped port inside the model between the strip and ground what's the dimension of the port?
If I use a waveport at the edge of the model I know I've to take dimensions approx. 4h x 5w but if I use a lumped port inside can I take its dimension just h x w?so just large as the width of strip and the heigth as that of substrate?
Changing the subject, I've simulated an F-antenna which resonates near 2.4 GHz and I've put a lumped port between the feed point and ground.
Then I've measured S11 and Z11.
I expected that the frequency corresponding to the minimum of S11 (the resonance point) should be equivalent to the point where the imaginary part of Z11 goes to zero. Instead the frequency of S11 minimum is 2.485 GHz while the frequency where Im(Z11) goes to zero is approx. 2.50 GHz.
What's wrong? Is a problem of the simulator or I'm in error thinking about the equivalence of the two measures?
Moreover I've found this problem some other times with other models...
Thanks in adv to anyone will help me!
Bye!
Actually, the minimum is not when Im(Z)goes to zero, instead is when the
reflection coefficient goes to zero or when VSWR get closer to one.
the reference impedance is 50 Ohm and when you get Im(Z) zero the Re(Z)
is not 50, so the S11 is minimum when combination of Re(Z) and Im(Z) makes
de reflection coefficient goes closer to zero
Using a lumped port with the dimensions of the trace should be fine. You can always do a comparison on the width if you are interested in checking it, making it equal to the trace width and then smaller.
Using a Smith chart will aid in understanding the complex impedance response of your antenna. The minimum S11 will occur when the impedance is closest to the center of the chart, which is the impedance of the feedline. The imaginary part of the impedance goes to zero when the impedance crosses the real axis (horizontal line on the center of the chart). Only if the impedance passes directly through the center of the Smith chart will these two frequencies coincide.