CST-Feed line structure_port and boundary size

Hi , friends!
I am studying the structure feeding by a microtrip line on a vertical ground plane!
I don't know I should use the large port size (full plane in waveguide port definition) or just use small port size for feeding?
Besides, boundary condition also make me to be in many mind!
I tried using the large port size and large boundary (open add space ) to simulate, I got the good result in RL and radiation pattern, but the antenna efficiency (GAIN) was very low!
Please share some experience!

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The port size is three times as the width of the feed line. The boundary I do not know, I open all the boundary

Hi Khangkhang,
Firstly, i'm not quite clear what "vertical ground plane" means. You'd better offer a picture to show this feedline.
When it comes to the port size of microstrip feedline, some training class suggested the full plane is usually defined as the port, but you may find a rectangular plane of free size which includes the strip, substrate and ground is defined for as the port, if you check the "rectangular patch antenna"(CST example).
So my conclusion is never make the port size too small unless some warning pops out.
I cannot offer any explanation of the low efficiency at present, but maybe we could study your structure together if you'd like to uplowd your .mod file.

Hi Drunkbear!
This is my designing structure !

If we use too large port, the simulation system give a warning to suggest a reducing port size of reducing max fre.

At first, I use the full plane wave port to simulate, and got the good matching , but the Gain and antenna efficiency were very low (-5dBi, 21%).

I tried to reduce the port size , boundary was also reduced as a result, and the Gain and efficiency was raised. However, the time simulation increase significatly, about a double! Besides, the antenna return loss also became mismatch.

Now, I don"t know how big the port size I should use for my structure!

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