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I put a (C-L-C) Pi Filter in series with my antenna trace on a new design, like so:

enter image description here source

The reason you put one of these circuits in, as I understand it, is just in case, during compliance testing (i.e. FCC / CE) you have an issue with your antenna matching causing a failure, you have a decent chance of fixing it by populating appropriately sized inductor and capacitors.

So what is usually done to initially populate the board in that case, before you take it in for testing / certification?

  • DNP the capacitors and populate a 0-ohm resistor in place of the inductor?
  • DNP the capacitors and populate a 100-ohm resistor in place of the inductor?
  • Guess at the value of and populate the inductor, and DNP the capacitors?
  • Guess at the values of both the inductor and capacitors and populate both?
  • Something else?

Feel free to rank these in preference order, and/or comment on the pros and cons of each. Thanks in advance for any demystification you can offer.

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  • \$\begingroup\$ If you add in a switch or another convenient way of hooking up a VNA, you can characterize and tune your RF line. It's not practical in all situations, but it it's a good way of getting things done. See this app note. ww1.microchip.com/downloads/en/AppNotes/… \$\endgroup\$ – BB ON Jul 3 at 14:59
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DNP the capacitors and populate a 0-ohm resistor in place of the inductor?

This is the easiest and most obvious one to do

DNP the capacitors and populate a 100-ohm resistor in place of the inductor?

This is something only a non-RF engineer could dream up. Would you trail a boat anchor behind your pedal-bike? The resistor will absorb most of your power, both inbound and outbound, rendering your new design almost useless.

Guess at the value of and populate the inductor, and DNP the capacitors?
Guess at the values of both the inductor and capacitors and populate both?

May or may not be reasonable, it depends how you've done your original design. If your antenna nominally matches your amplifier and traces (as it should) then these revert to the top one above. If not, then these are the first cut at your matching components.

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    \$\begingroup\$ Thanks for answering. Yea I'm not pretending to be a proper RF engineer; I'm at best a dabbler in the dark arts. I figured my intuition was in the ball park, and I did design a 50-ohm grounded co-planar wave guide for the 200mil trace between the u.fL connector and the RF module's ANT pin. Glad to hear it's not a crazy idea to go in with a zero-ohm resistor. \$\endgroup\$ – vicatcu Jul 3 at 15:41

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