cst ms microstrip antenna

Hi

I have to simulate a 10 GHz MS Antenna.
ξ = 4.3 substrate=1.6

IE3D refused to define the port for this saying thickness to frequency ratio is large.
Then i tried CST but its issuing warning.

"some higher propagating modes at port 1 have not been cosidered in time domain calculation. This probably leads to a inacurate energy balance. Consider to decrease port size or decrease upper frequency limit"

My question is why i am getting this error ? Though im getting the results hopefully well as atenna resonating at desired frequency. Still i am interested to know why getting this error. As in a example i read 10 times width and 5 times height is good to define a wave port. I used less dimension still i got the above error.

Also i want to know what is wave port and how do i define it ? In CST do i need to take the complete face of the bounding box to define it? I tried google but didn't help much.

Hi crazy_dude,

If I understand correctly, you feed the MS antenna using a microstrip.
Usually, the dimensions you've stated above should define the w.g. port very well.
(I assume you know how to "mesh" the port area) .

BUT:
It depends on the geometry of your structure.
For example:
1. If the microstrip feed line is embedded in a metal plane with a release gap, your port is probably large enough to excite a coplanar line mode (as it overlapps edges of the metal plane).

2. If the PCB contains several conductive and dielectric layers, your port overlapps lower (or higher) layers and excites a parallel plate mode.

Try a smaller waveguide port that overlaps the MS line and ground geometry alone(even if its not as recommended by the manual).

Good luck,
P.

Hi, Crazy_dude:

IE3D should allow you to define the port. If you continue the simulation, IE3D should yield some results to you even thought it may not guarantee the accuracy. When the substrate is too thick compared to wavelength, traditional microwave network theory will not work. That is the reason for the warning from IE3D or other software packages. In some sense, you should avoid using such thick substrate because you may not get good and consistent results from it because its results are dependent upon many things. It can be good and it can be bad depending upon how you connect this antenna to other parts of your system.

Regards.

Hi pushhead and jian
Thanx for help

Yes i am using microstrip line inset feed.
ground metal layer 20x20x.01 mm
substrate (ξ=4.3) 20x20x1.6 mm
antenna and its feed structure with a thin metal. antenna size is (9.1x6.5)

I tried reducing port size but it came up with a new warning that x number of critical cells have been expanded to full size. And old warning about higher mode was also coming along with it and after certain extent it told that port size is too low to simulate.

About IE3d yes it gave warning that results can be inacurate and simulated. Now antenna was designed for 10 GHz and results it gave were of 9 GHz that makes a huge difference and that made me shift to CST.

I feel bad to say but im quite a n00b in microwave so can u tell me the topic of limitation. any pdf or link will be good but even the topic name will be fine so that i can search for some material in the book.
Also i m very much interested to know what are other parameter which affect the performence when using a thick substrate. I really need this thing. All i know that it produces surface waves and causes spurious radiations.

can you upload the geometry files (*.mod, *.par) or mayB a picture of the antenna layout ?

Regards,
P.

Hi

Hmm.. It worked some how. I reduced only the heigth of the port on advice of a friend and it worked. So my final port size was 20x5 mm

The second problem "x number of critical cells have been expanded to full size" i found the prob was -- while defining my port i took height -.01 to 5 mm.
The point to be noticed here is that i took the ground plane(0 to -.01mm) (thickness .01) in account. And it was creating problem. I don't know why but i took from 0 to 5 and this problem was solved
I think i read in manual that CST takes care for ground plane. even if its true whats the harm if i include it in my port ?

Hi, Crazy dude: I tried your antenna on IE3D. It predicts the resonant frequency at 10.1 GHz and it is close to what you designed. I am not sure why you got the resonance at 9 GHz. For those missing dimensions, I use the inlet depth of 1.45 mm and the feed line width = 1 mm. The feed line is beyond the edge by 2.55 mm. Attached are the files.

Seems i have old version of the software. I have 10.0 I will go and try in my lab there they have 11 i hope u used that one. It didn't open in 10.

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