QUESTION

Question

I am controlling for the impedance of the antenna connection exiting my cellular modem using a coplanar waveguide with ground plane, but I am not sure how to calculate the non-grounded portion of the microstrip as it exits the ground plane and enters the antenna (pin 3). I am afraid that if I do not specify the correct antenna track width at that location, there will be an impedance discontinuity at that un-grounded portion (at both pin 3 and pin 4 ground return). The impedance calculators I'm using always assume a ground plane and they do not provide an impedance calculation without the ground plane where it enters the antenna itself. Alternately, I may need to measure (as best I can) the antenna eval board track width, but it would have been best if that information was documented in the data sheet. Can you let me know what that track width should be to optimize the antenna performance?

Answer 1

Hello,

The Lucida has a transition between the coplanar waveguide and the antenna feed pin of around 3mm. For this transition the trace width can remain the same as it is between the radio and the antenna. Anything longer than 3mm would need to start transitioning to a different size trace.

There are also several matching components recommended for tuning the transition between the radio and the antenna. I would recommend designing in these components to improve the mismatch between the radio and the antenna.

Best regards,

Geoff Schulteis