u/Awkward-Week-4989

I simulated an inverted-F antenna in HFSS. I first optimized the antenna parameters with a feeding line length of 5 mm and obtained a good result, with S11 below -10 dB from 2.3 GHz to 2.6 GHz.

However, I then found that the antenna S11 varies drastically with the feeding line length, as shown in Figure 2. The feeding line is a 50 Ω grounded coplanar waveguide (GCPW).

To verify the feeding structure itself, I simulated the GCPW separately. As shown in Figures 3 and 4, the S11 of the feeding line remains below -10 dB for feed lengths ranging from 5 mm to 25 mm.

I am trying to understand why changing the length of a supposedly 50 Ω feed line still has such a large impact on the antenna matching.