microstrip instead of coplanar waveguide for antenna related traces

[gepet]

Hello there!

The nrf52832 qfaa reference circuit uses a coplanar waveguide for the antenna section, leaving a keep out area on the inner layers beneath the matching network and the waveguide to, as per the reference manual, reduce stray capacitance and using the bottom layer as a solid ground reference to attain the desired 50 ohm characteristic impedance. In this regard, I have various questions.

Q1) Is it possible to design a microstrip transmission line with a solid ground inner layer (no keep out area)? I fear that this increased inter layer capacitance could alter the matching network impedance and dampen the performance, specially the L network capacitor since the manual says its layout is critical for filtering high frequency harmonics

Q2) If above question answer is a yes, should I stick to the reference network C and L values or take a more general approach and select new values considering the antenna pin of the nrf52832 qfaa package has an impedance of 53+66j ohms?

Q3) If a coplanar waveguide is imperative, would it be a problem if I change a little bit just the trace itself, to better adjust it to my pcb specs?

Q4) What is the rationale behind using an L network instead of a pi network?

Thank you,

Gabriel

[ketiljo]

Q1) Is it possible to design a microstrip transmission line with a solid ground inner layer (no keep out area)? I fear that this increased inter layer capacitance could alter the matching network impedance and dampen the performance, specially the L network capacitor since the manual says its layout is critical for filtering high frequency harmonics

Yes, no problem. A microstip line just takes more room on the PCB, else it's relativity equal to a co-planar waveguide. Just make sure you design it according to the PCB stack up. A correctly designed transmission line will just act as a 50 ohm load on the matching network.

Q2) If above question answer is a yes, should I stick to the reference network C and L values or take a more general approach and select new values considering the antenna pin of the nrf52832 qfaa package has an impedance of 53+66j ohms?

Use the matching network topology and component values for the device you are using. The matching network does more than juts impedance transformation, it's also attenuates the harmonics. There are also the bonding wire inside the chip that has some inductance. Together with the external LC, it forms a T-network with two series inductors. 

Q3) If a coplanar waveguide is imperative, would it be a problem if I change a little bit just the trace itself, to better adjust it to my pcb specs?

Doesn't really matter how you do it as long as you adapt it to your PCB. 

Q4) What is the rationale behind using an L network instead of a pi network?

See Q2.

You can design the transmission line with many tools, for instance Appcad from Avago http://www.hp.woodshot.com/ Remember to set Er for the PCB you are using. 

[gepet]

Thank you for your comprehensive answer ketiljo , I will take your advices. My main concern was about having a solid inner ground layer without a keep out area under the matching components like the reference designs, so it is good to hear it is not a problem. I think a microstrip line may be less affected by manufacturer tolerances, and that's why I'm more inclined to this approach.