Warning when choosing transient analysis (CST port problem)

I meet the following warning when i choose tranient analysis . (I use waveguid port for a microstrip fed patch antenna ).
some higher propagating modes at port 1 are not considered in time domain calculation, this possibly lead to an inaccurate engergy balance, it will be better to reduce the port size or decrease the upper frequency limit.
Looking for your help, thank you.

Time domain calculation can excite the structure only with one mode.
In your case, 2D mode solver calculated except right QTEM mode some higher one.
Check if you have excited really QTEM and if yes, all should be OK (from my experience).
To be sure, do what the message recommends you, isn't your port plane too big?
rgz
eirp

It might have to do with how close you are the open boundary and how close your waveguide is to the antenna.
Try changing the port size and the surrounding boundaries.
What may help is to add air at each open-boundary and extending the microstrip port a little away from the antenna.

Thanks a lot for your help. I will try to your suggestions. and Also I would like to give other addtional information below:
After I increase port size, warning disappears, it is an opposite choose related to CST suggestion warning.