antenna effect of microstrip

I measured a microstrip structure on PCB, at certain frequency the s11 gets lower sharply, but s21 does not increase at that frequency, so I guess whether the power is radiated at that frequency and the microstrip structure acts as an antenna, could some experienced person give me some guide?

Thanks!

Question.

How did you measure the S21 ?

I used network analyzer to measure the s parameter, the two termial of the microstrip structure are SMA connector. Our lab only have the caliaration kit for 2.4mm connector, so a converter of SMA and 2.4mm connector is used .In the calibration, the reference plane is at the terminal of 2.4mm connector, so the measurement result include the converter. it is assumed the error caused by the converter could be neglected. The frequency range is 0.1GHz to 10 GHz.

- high substrate loss or nonhomogenous material
- proximity coupling to other lines
- stray modes of propagation

Do you mean you antenna have got 2 port or are you using 2 similiar antenna and measuring their coupling ?

Or are you just measuring the radiation characterist c of a microstrip transmission line?