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Schematic, suggested by the datasheet:

enter image description here

PCB is round with 33mm, FR4 1.6mm

Bottom side

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Top side

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Bottom+top

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3d view

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The antenna drawings is based on AN043 of Texas, HERE.

How about the question on the last picture? It is better to keep the solder mask open or close it? If open, which PCB surface finish is better? Tin, Nickel, Gold ENIG?

I wonder if my ground plane is ok, do you see something that could change on it in order to obtain good results with the antenna? I followed the spacing between antenna and ground according to AN043 drawings.

And also, you can see on schematic that there is an inductor in series (3.9nH) an a capacitor (0.8pF) to ground on the RF line. RF line is 8 mils width, with 8mils isolation to ground. I wonder if these values are the best to use as starting point. Can calculate them correctly? I'm not from RF area.

As said, PCB trace of RF line (4mm lenght only) is 8 mils width, 8 mils clearance to ground (coplanar waveguide). If an internal ground plane layer on the PCB is used, and between these 2 layers we have 0.18mm clearance, the RF line will have 50 ohms, I know this because I already used this before for other boards. So I wonder if I can make the PCB with only 2 layers and dont take care about the impedance that it will have in practice, because the trace lenght is only 4mm.

enter image description here

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  • \$\begingroup\$ What's going on with your ground plane? Close up that silly arc cutout, pour ground copper on top and stitch it to the ground plane with vias. Is that a mounting hole next to the antenna? If so what goes in it, a plastic stud or a metal screw? \$\endgroup\$
    – Chris Stratton
    Jul 27, 2020 at 0:39
  • \$\begingroup\$ Also it rather looks like the ground via shared (!) between your power bypass and crystal cap is going into some contact and not into the ground plane. \$\endgroup\$
    – Chris Stratton
    Jul 27, 2020 at 0:43
  • \$\begingroup\$ The layout is ok, its a plastic enclosure \$\endgroup\$
    – abomin3v3l
    Jul 27, 2020 at 1:06
  • \$\begingroup\$ No, the layout is not very good. It may work but if you have any idea what you are doing, you know that getting it to work is the easy part. \$\endgroup\$
    – Chris Stratton
    Jul 27, 2020 at 1:32
  • \$\begingroup\$ What do you think needs to change? I did not understood well what you said before. \$\endgroup\$
    – abomin3v3l
    Jul 27, 2020 at 1:33

1 Answer 1

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Put on the solder mask. Metal being directly exposed only brings heartburn.

Soldermask (unless made of some weird material) generally has very little effect on 2.4GHz RF. You're likely to have much worse loss from that far-too-simple matching network.

Note that the (old?) Nordic RF beacons (also round) put soldermask over their antennas (which were just a single trace--aka quarter wave whip).

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  • \$\begingroup\$ Thanks. Do you think I really need to make exact 50 ohms impedance RF trace? I want this board to be only double layer. RF trace is 4mm only. \$\endgroup\$
    – abomin3v3l
    Jul 27, 2020 at 0:34
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    \$\begingroup\$ Nah, don't waste your time. It should still work just fine even if you lose a bit of signal (only range would be affected). Detailed RF matching should be put off until you have your enclosure and battery and everything else locked down. Then you can take it to a local lab and have them match it for you. \$\endgroup\$
    – Andrew Lentvorski
    Jul 27, 2020 at 1:54

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