hi, I'm working on the PCB layout of Si4432. I plan to use a generic 1 lead antenna and connect directly to the output of the matching network, like the 1st attached E1.
It's recommended by the layout design guide AN414 that grounded coplanar line should be place between the matching output and the antenna, 2nd attached.
My question is what if I put the antenna pad right next to the output C5 in my diagram, is the grounded coplanar line necessary in this case if I keep the the transmission line very short? I want a
smaller PCB size. Please advise. Thanks.
First of all, Si4432 radio is not recommended for new designs, the corresponding device is Si4463 from the Si446x series. The Si446x family has several improved parameters and new features compared to the Si443x series.
Coplanar lines are recommended, since this way the ground plane around traces, transmission lines helps to avoid unwanted radiation of these lines (e.g. typically high harmonics can be radiated through VDD lines). The radiation can be so critical that it can violate the emission standards. Coplanar lines also help to provide better ground reference for the related components.
The usage of 50 ohm coplanar transmission lines are necessary to minimize the loss of these lines. Between components, when the trace is very short, it is not so critical, since the loss of the short line is negligible. If you place your antenna close to C5 capacitor, it is not so important to have a 50 ohm line between them, but if you use long traces (e.g. Figure 8 in your attached file), using a non-50 ohm line significant losses can occur.
You can use the following online calculator to get the necessary width of the trace and the gap between the transmission line and the ground plane:
The corresponding layout design guide application note for Si446x radios is AN629:
That really helped. Appreciated.