I am currently designing a coplanar waveguide (CPW) structure for the attached circuit. In the past, I created a CPW structure for a passive antenna using FR4 material for the PCB, which has a dielectric constant of 3.48. The PCB thickness was 0.5 mm, which I used as the distance between layers, with the second layer serving as the reference ground plane. For that design, I calculated a 1 mm RF signal width and a 0.2 mm clearance from the ground using an online calculator. However, I am encountering difficulties applying the same approach for an active antenna due to differences in the design

Attached is the schematic for the circuit I am designing on the PCB





the coplanar waveguide (CPW) structure I attempted doesn’t seem to be designed correctly


I am seeking advice on the optimal component placement for this circuit to ensure the best performance, particularly for components such as the ferrite bead, capacitor, and resistor in the RF signal path. Additionally, I would appreciate guidance on how to properly route traces. I am a beginner in circuit designing and PCB design, so any help or suggestions would be greatly appreciated

 

Depending on the frequency of the signal, there are PCB materials that would be much better for use than FR4. Somebody else probably has a recollection of the names, I don't recall any of them.

 

Is a ferrite bead (FB) the component of choice for a bias tee? If you have enough space on the PCB why not use something like a distributed, higher impedance (e.g. 100 Ohm) lambda/4 transmission line instead of the FB? For constrained spaces a lumped L-C lambda/4 transmission line or a 2nd/3rd order L-C high-pass network would do.

As for the CPW structure are you looking for a generic CPW, a CPW with ground or a grounded CPW? At this frequency (GPS) why not go for a simple 50 Ohm microstrip? Keep it simple.

 

There are examples with defined specifications. Some use Rogers standards.

 

Your figure for Er of FR4 seems a bit low. Is this for a particular make? It does vary with the make and thickness as the ratio of glass to epoxy varies. There are defined Er substrates available, usually PTFE based, but these are expansive. If you can find out which supplier of substrates your PCB maker is using and can get them to stick to that, then that is the cheaper option.
When using CPW over a ground plane you need to add a row of vias along the ground adjacent to the signal track to connect top and bottom grounds. These are referred to as stitching vias.
The placement of the ferrite bead is not critical, anywhere along your 50R track will do. The capacitor needs to be as close as possible to the other end of the ferrite bead. Allow space for ground vias as close as possible to the ground end of the capacitor.
The break in the CPW top ground to accommodate the ferrite bead is not a big issue as long as the stitching vias are in place.

Chris.