I made a model of an aperture coupled patch antenna in CST. Its linearly polarized with a microstrip feed on the bottom layer and a single bowtie aperture slot. Substrate is FR-4 with er 4.23. Frequency of operation is 10GHz. After simulating the model I am getting realized gain of 8dBi and directivity of 8.7dBi. Its a single patch antenna so I thought the maximum gain I can get is around 6-7 dBi on an FR-4.
Any ideas why my gain is so high? I had the same problem on another model at lower frequencies where the gain was unrealistically high for the substrate.
What is the loss tangent of your FR4 at 10 GHz? It is the loss tangent that will affect the gain, not necessarily the er. I think a directivity of 8.7 dBi is possible. For the gain to be so close the losses have to be very low, like 0.7 dB
Dear @Malcolm White, the loss tangent is 0.017 at 10 GHz. The radiation and total efficiencies are both near -2.7 dB.
The total efficiency should be the difference between the gain and the directivity.
The numbers you have given do not add up. Something is not right in your simulation. CST usually gets these things right. Are you sure the gain wasn't 6 dB rather than 8 dB?
If the numbers don't add up I know that I have to repeat the simulation with a better model or mesh or better convergence.
I am sure that the gain was not 6 dB. Please see the attached screenshots. I have simulated the model with high mesh count and results remained the same.
I have a similar problem in a different model at lower frequency (1.65 GHz) where all of the results look good but the gain and directivity are too high.
Dear Jakub Przepiorowski ,
I would second Malcolm White .
It is clear from the radiation pattern that your antenna is quite directive. Then it remains the radiation efficiency which is Prad, the radiated power divided by the input electrical power. Your antenna seems to have too high radiation efficeincy especially you use FR4 at 10 GHz. You have to review the material parameters.
If you display S11 one can see the losses outside the antenna resonance range.
Best wishes.
I've never used the 2D plot to read off the gain - I always have used the table in the corner of the 3D farfield pattern to give me directivity and efficiencies. Maybe the gain in the plot format you are using is not reliable, it certainly doesn't seem to agree with the other parameters CST gives in this case.
I usually check the efficiencies against the S11 and the power loss in different materials, and it is usually right - and if not I try to do the simulation so it is. I would tend to use with the directivity and efficiency to get gain - and check that the difference between total efficiency and radiated efficiency is the same as the loss due to S11, and that the radiation efficiency corresponds to losses in the antenna.
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