I'm designing a board with a GSM modem, and the plan is to hook this up to an SMA connector and thence to an external antenna which will live outside the metal enclosure. I'm aware of the need to observe impedance rules for the CPW track that does this, but it's my first time doing this and I'm outside my comfort zone. I'd also like to minimise the radiated emissions from the CPW as there's some delicate ADCs on board. So any observations or advice would be appreciated.
To get the required dielectric thickness, I had to skip the L2 copper and go to L3 for a ground plane. This is not ideal, as L2 is in fact my ground plane and L3 is signal, but I can put plenty of ground on L3 under this area. The bottom layer L4 is also ground.
So firstly, have I got this right?
Secondly, if I rotate the GSM module, I could have a much shorter CPW. Would this be advantageous in reducing the emissions from it?
All advice much appreciated.