Tracing GPS antenna on multi layer PCB

Question

I have been tracing PCB for the first time and came to a few questions regarding GPS antenna selection and placement on PCB.

Basically, I am trying to make as small as possible PCB with GPS/GSM modem. On the top layer, I placed a GPS antenna, on the bottom - GPS/GSM modem. PCB is 30 x 30 mm size.

1. Can I place GPS/GSM modem directly below GPS antenna or should I move modem to the opposite side of PCB as much as possible? (I worry that there will be GSM antenna traces directly below GPS antenna).
2. Now I selected 25 x 25 mm GPS patch antenna. Should I select smaller antenna (18 x 18 mm) to have more space for better placement of modem (not directly below GPS antenna)? What is better: bigger antenna but modem directly below GPS antenna or smaller antenna but the better placement of modem?
3. As I understood, in order to get the best performance, the entire layer with GPS antenna must be plain ground. But what about inner layers (2nd and 3rd)? Should it be etched below antenna or it is better to make there ground pads too?

Thank you in advance for your time answering questions.

Answer 1

Can I place GPS/GSM modem directly below GPS antenna or should I move modem to the opposite side of PCB as much as possible? (I worry that there will be GSM antenna traces directly below GPS antenna).

In many cases manufacturers recommend removing the ground plane beneath the GPS antenna, if this is the case placing anything else underneath the antenna could change the capacitance. If the GPS antenna requires a ground plane, then it would most likely be ok to place something on the opposite side of the board as the ground plane would shield any electric fields from the antenna.

Now I selected 25 x 25 mm GPS patch antenna. Should I select smaller antenna (18 x 18 mm) to have more space for better placement of modem (not directly below GPS antenna)? What is better: bigger antenna but modem directly below GPS antenna or smaller antenna but better placement of modem?

There is no way to answer this question by only knowing the size. I'd imagine that the antennas have different gains associated with them. Check the gains and size in the datasheet and select the best one for your application.

As I understood, in order to get best performance, entire layer with GPS antenna must be plain ground. But what about inner layers (2nd and 3rd)? Should it be etched below antenna or it is better to make there ground pads too?

In some cases, yes, in some the antenna requires nothing beneath it. Manufacturing guidelines for the antenna should suggest the proper PCB layout. The guidelines may or may not be in the datasheet, sometimes they are on a different area of the manufacturers website or need to be requested directly.

Answer 2

The best answer will be to follow the PCB layout guidelines suggested by the manufacturer of the modem module and your patch antenna. They will definitely have a whitepaper or application note about this.