some suggestions for an RF PCB

[kalbun]

I am currently designing my first RF PCB (well, not the first exactly, let say the first serious RF PCB).
I would like to ask a couple of suggestions.

1) The antenna path is 50 Ohm. According to microstrip calculation, the trace should be 3mm width for a standard FR-4 PCB with 1.6mm height, but only 1.45mm is the PCB height is reduced to 0.8mm.
So I plan to use a 0.8mm PCB, this should also reduce VIA parasitic inductance and capacitance.
In your experience, is this a good choice? Are there drawbacks?

2) Looking at other PCBs, I noticed that VIAs are always put very near to the pad, but not inside. For room reasons, I plan to put VIAs inside the pad. Again, is that a good choice? If not, why?
I am talking about VIAs that connect pads to ground plane.

3) I would like to use PCB coils for the filters, but currently I only found formulas valid if there is no ground plane below the coil. Does anyone can point me to more general formulas?

Thanks

 

[Mazz]

kalbun

1) It is a good choiche. To further reduce line width in FR4 you can use coplanar with ground instead of microstrip.
2) Via under pad are commonly avoided because solder goes insed the via and the solder joint can fail. If you are doing some prototype, use them, for production it's better to avoid.
3) Do you mean printed inductors in FR4? Not a good choice, as you'll have a large area (respect to lumped inductors), low Q (for dielectric losses) and not a great accuracy in inductor value (if you use simple formulas, an EM simulator is needed).

I hope it can help.

Mazz

p.s.: if you need some other suggestion, show some PCB image

 

[kalbun]

Thank you for your comments. I noted that PCB inductors tend to be quite big, I designed some coils as big as 3500 um diameter... but I need some weird inductance values for the filters, and I would like to avoid wire wounded inductors - which possibly also have a low Q.
Thank you for the suggestions about coplanars and VIAs. I am indeed designing the first prototype and we plan to build it by hand, so I can keep them under pads.