Why S11 down at 0->0.5 GHz in WLAN antenna simulation

Hi, do you know why this S11 seems wrong at very low frequency, it increases from -15 to 0 in the band from 0 to 0.5 GHz,
it would be still at 0 in this band like this picture ( I refer it in a paper).
I use CST to simulate and the mode at port is Quasi-Tem, I use CPW-fed in my design, E at port is ok,
but impedance is 62 ohm, not 50 ohm like in that paper. Port dimenson is 4W*4h.

Thank you!

I don't see a reasonable purpose to discuss characteristics of an unspecified antenna geometry.
Technically, a low S11 number at low frequency requires a resistor built-in to the antenna, doesn't sound like a realistic design.

Thank FvM,
Here is my design, do you use CST ?
Please help me !