Sigfox RF module TD1208R: design a 50 ohm microstrip line on a PCB (Proteus ARES)

Question

I'm designing a PCB (I have very little previous experience). Among other components, I am using the microcontroller MSP430G2553 and the RF module Sigfox TD1208R (868 MHz). I have doubts to design the trace from the RF pin of the TD1208 to an U.FL connector. As I understand it, this trace has to be a 50 ohm microstrip-line (same impedance as the RF pin of the TD1208) but, how do I do it with Proteus ARES?

The PCB properties can be seen in this image:

Using this data, I have calculated the trace width in this Microstrip Impedance Online Calculator and the trace width should be 100 mils:

These are the doubts I have:

1. Have I made any mistakes in the previous steps?

2. Both the PAD of the RF pin of the TD1208 and the PAD of the U.FL connector are have a width much less than 100 mils. How should I decrease the width of the track to connect it to the PADs? Should I reduce the track width slowly or quickly?

3. What length should the microstrip line have? How much shorter is better?

4. If the U.FL connector is placed as close as possible to the RF PAD, is the trace width less important?

Finally, searching in google, I found in this blog (ioter.io) the files in EAGLE of a TD1208 development board for the RF module TD1208 but the width of the RF track (70 mils) is lower than it should be according to the online calculators of microstrip-line and the properties of the PCB. For this reason I have decided to ask, because I do not know if that design is correct because it does not agree with the theory (or so I think).

Best regards,

Fran Martin

Answer 1

You don't have to care much about impedance if the length of the trace is significantly shorter (eg. by a factor of 10) than the wavelength. 868MHz is about 34,6cm long, so if your trace is shorter than 3cm - any width should work.

Answer 2

As Filo said if the length is so small is not so important the impedance.

But if you want to reduce the 50ohms trace so its width is more similar to the UFL and module pads you could use a coplanar waveguide, so changing the clearance to the ground plane will affect your impedance. Also you could change your stack up, to make the ground layer closer to your RF line.