I want to use one shared antenna for the Lora/FSK transceiver sx1272 and LTE/GSM SIM700E module, with BGS12SN6 as a switch.

I've matched the antenna with two conductor and validated the result with a VNA after the switch (sx1272/SIM7000E->Switch->TestPoint->Matching->Antenna) with a reasonable performance (-28.9db 50.5+j0.17 @868MHz).

I strictly copied the switchless reference design from Semtech, but for some reason I get really a bad performance, even if I use an external 30db perfectly matched 50ohm antenna. I've already used the antenna for an other board with a Si4463 and get way better results (testing with wireless M-Bus devices, the Si4463 gets about 800 devices in the areas, the Sx1272 only about 150)

So I got curious and tried to rebuild the reference design with qucs (see image below (red line antenna without filter, blue line with reference design filter), but i don't think the result can be right.

ReferenceDesign with matching

The PCB schematic looks like the image below, the VNA was connected to J5, with C9 placed and C30 left open. PCBSchematic

Is there anything I missed or did a big mistake? I'm grateful for any hint or idea.

Just in case I'm adding the application flow: AppFlow

  • 1
    \$\begingroup\$ the reference design is really bad and I rebuild the design with qucs and was a little shocked are meaningless statements. They impart no technical value at all to help you with this: Did I do something wrong?. Then you say this: I can't change the matching network because otherwise the GSM module would perform bad but I'm unclear where it fits into things schematically. \$\endgroup\$
    – Andy aka
    Mar 8, 2022 at 12:46
  • \$\begingroup\$ Sorry for the misunderstanding. I wanted to know if i did something wrong in qucs and get wrong results. I can't change the matching network because otherwise the GSM module would perform bad but i could change the values in the reference design, but i don't know what the sideeffects this would have. \$\endgroup\$
    – Wowa
    Mar 8, 2022 at 13:06
  • \$\begingroup\$ I've updated the question to make it clearer and added some more info. \$\endgroup\$
    – Wowa
    Mar 8, 2022 at 18:59

1 Answer 1


Well, I'm now using the reference design with the antenna switch and now get good results.

After some searching and reading all kind of documentations I finally found out, the optimum match for the sx1272/1262/1232 is 28.5+j41.7 @ 915MHz (source: AN1200.16 of the SX1232). This explains my "wrong" qucs results. Unfortunately I couldn't find the optimum match at 868MHz.

enter image description here


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