I am trying to make a LoRa board with an RFM95W module.

Using the calculator shown in the image below, I designed a 50 ohm impedance transmission line. I added options for both a monopole copper wire antenna and a proper U.Fl antenna. However, after further research I am unclear whether a wire antenna even needs the 50 ohm. Further, I am worried that if I end up going with the U.Fl, the board trace impedance will add with the connector impedance and create 100 ohms.

enter image description here (There will be copper pour on the red side) enter image description here

So do wire antennas need a 50 ohm transmission line? Will the impedance of a 50 ohm transmission line and a 50 ohm ufl antenna connector add to make 100 ohms in series?


The reason for using a consistent impedance from the RF driver to the antenna is to avoid reflections and so improve standing-wave ratio (SWR).

The impedances don't 'add' together, transmission lines don't work that way. Rather, the 50 ohm line continues on to the 50 ohm antenna as smoothly as possible (that is, without impedance discontinuity), so that maximum power transfer is achieved (that is, lowest SWR.)

If possible, if the chip / module vendor provides a reference layout, try to follow it exactly, using the same geometries and stack-up.

  • \$\begingroup\$ There is no downside to using 50 Ohms. As long as you have the option to use a u.fl jack, I think it makes sense to use a 50 Ohm trace on the PCB. If the wire antenna is not 50 Ohms, then things won't work as well, but the solution for that would be to add a matching network to transform the wire to 50 Ohms. \$\endgroup\$ – mkeith Jan 27 at 19:18
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    \$\begingroup\$ Given the nature of the question, the OP should follow the reference design if there is one available. It's possible there would be a stuff option for a matching network to allow for tuning. \$\endgroup\$ – hacktastical Jan 27 at 19:25
  • \$\begingroup\$ Yes following or copying the reference design would be a very good idea. \$\endgroup\$ – mkeith Jan 27 at 20:45

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