– Almost always 50Ω.
Note that an inset feed can slightly skew the radiation pattern compared to other feeding methods like a coaxial probe. Microstrip Patch Antenna Calculator for RF Designers inset fed microstrip patch antenna calculator
But more accurate models (including fringing) require iterative computation—precisely why a good calculator is vital. – Almost always 50Ω
In an inset feed design, the microstrip transmission line is "cut into" the patch. This allows the designer to control the input impedance by varying the depth of the cut (inset distance), eliminating the need for external impedance matching components. Key Design Parameters inset fed microstrip patch antenna calculator
This demonstrates that a well-calibrated calculator gets you >95% of the way to a working design.