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I'm designing a PCB including Quectel's BG77 LTE module. In the application note I read:

The antenna ports have an impedance of 50 Ω.

Then this reference design is proposed for the antenna interface:

It says:

It is recommended to reserve a π-type matching circuit for better RF performance, and the π-type matching components (R1/C1/C2) should be placed as close to the antenna as possible. The capacitors are not mounted by default.

I can't understand the aim of this network when the antenna's impedance (50 Ω) is the same of the module's input port and the transmission line that connects the antenna to the module is designed to have the same impedance.

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  • \$\begingroup\$ The point is that you don't have a 50 Ohm antenna. You may measure it with an VNA and report. \$\endgroup\$
    – Marko Buršič
    Jan 3, 2022 at 22:08
  • \$\begingroup\$ Since you may benefit from it, Quectel will work with you to qualify your designs before you go to production. They have particular expertise in this area so it's probably worth it to lean on them for some of that if you're weak in this area of design. \$\endgroup\$
    – J...
    Jan 4, 2022 at 13:20

3 Answers 3

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The reference design doesn't include a matching network, it only reserves a spot for one on the PCB. If you later figure out that your transmission line doesn't actually have exactly the same impedance as your antenna, you can populate the matching network to fix this mismatch without having to throw away all the PCBs you have already manufactured. Including the unpopulated footprints doesn't cost you anything so you might as well do it.

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  • \$\begingroup\$ The recommendation says to place the matching components as close to the antenna, this points to the antenna mismatch and not the transmission line. \$\endgroup\$
    – Marko Buršič
    Jan 3, 2022 at 22:41
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    \$\begingroup\$ Both the transmission line and the antenna will have tolerances. It doesn't matter which one of the components has the wrong impedance, you'll still need the matching network in the same place. \$\endgroup\$
    – Jonathan S.
    Jan 3, 2022 at 22:51
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Sure the antenna will never be exactly 50 ohms resistive ,so it is good to keep options open .Also the matching network can attenuate harmonics giving a cleaner carrier which might be handy when you go for approvals.

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The antenna impedance is not given, but it must be matched to the 50-ohm port on the module. That doesn't mean you can just draw a line and connect the components together. Each leg of the cicuit must match the next. There is a match between the module and a transmission waveguide (coplanar wvgd, microstrip), then a match between the waveguide and the antenna structure, be it an antenna or antenna connector. Just because the legs are both at 50-ohms at a junction point-- that doesn't mean their radiation modes line up at all. Try plugging a W connector into a K connector. The impedence is only part of the story.

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