Length of microstrip line

[GIGER]

Hello guys!

I have a problem. I'm designing a patch antenna array. I'm feeding the antenna with a microstrip line with a characteristic impedance of 50ohm.
My problem is that when i increase the length of the microstrip line i have serious oscillations in s11(RL).
Serious meaning that s11 is between -30dB and -55dB.

Is microstrip line acting like this or is it something that i' m doing wrong?

Here's a picture of my design. Im using CST MWS.

 

[Hawaslsh]

Hello,

I am rather new at this myself so take this with a grain of salt. Looking from the Patch to the port, the length of microstrip line is acting as a loaded t-line, with the port impedance as the termination. As the length changes, so does the impedance it presents to the patch. At some lengths the loaded line will present a somewhat better match to the patch impedance causing the return loss to drop.

Hope that helps, maybe someone more experienced can chime in.
-Sami

 

[PlanarMetamaterials]

Hi GIGER,

This seems like just a transmission-line (theory) problem. If the impedance changes with the length of the line, then the load is not matched to the line (although -30dB to -55 dB is pretty negligible, in my opinion, therefore you are pretty well matched as is). Does CST calculate the port impedance for the line?

 

[FvM]

For antennas, |S11| values below 0.1 (-20 db) will be usually considered as good matching. In so far I doubt that you see "serious oscillations".

 

[vfone]

If S11 of the antenna is between -30dB and -55dB (or whatever value between) don't touch anything an let as it is.
Probably with "oscillations", actually you want to say "variation of S11 values".

 

[GIGER]

If S11 of the antenna is between -30dB and -55dB (or whatever value between) don't touch anything an let as it is.
Probably with "oscillations", actually you want to say "variation of S11 values".

Yeah, thats what i meant.
Thanks guys for all your answers. Greatly appreciated it!