Electronic – RF Antenna Matching circuit and PCB design for 868 MHz SigFox device

Tye following may be useful. Most are 2.4 GHz focused but should provide some useful ideas.

TI 2.4 GHz PCB Antenna - AN 043 - 2.4 GHz but should have significant value.

Compact Integrated antennas - Freescale AN 2731 - significant relevance

A few zillion PCB antennas - all images linked to webpages

Commercial products - but some good idea starters

Inverted F design - 2.4 GHz but significant relevance

Wow Another inverted ed F design paper - looks superb

NO PCB antennas here BUT many432 MHz antennas

Related:

144/432 diplexers

It's ideal when the trace width ends up being the same width as your pad because the pad simply works as an extension of the transmission line. However, in most cases this is not possible, but don't panic because this is not the end of the world. When the pad is much wider than the 50Ω trace it's a good idea to have a void on the ground plane(s) immediately under the pad to reduce the parasitic capacitance represented by the pad and the GND underneath. The module and the antenna manufacturer (I'm assuming you're using a chip antenna) should provide guidance on what to do.

Having said that, simply use a 50Ω microstrip transmission line with the width you calculated based on the dielectric height and dielectric constant you have. I would use a clearance to GND on the top layer of at least two times the trace width. It's very critical that you place GND vias underneath the GND pads next to your module antenna feed pad. The same for the GND pads of your antenna (I'm assuming you're using a chip antenna).

I don't recommend adding a matching network for the purpose of compensating for the presence of a non-50Ω transmission line. The problem is that the matching will be narrowband and you need it to work properly over a wide frequency range. Having said that, it's always a good idea to have provision for antenna impedance matching (instead of transmission line impedance compensation). I would keep the pi network right next to the antenna you already have. I would keep the shunt components a little closer to the series component though. I can't tell what component size you're using - I would use 0201s (so you have a wider range of component values below the self-resonance frequency you can use).