I'm trying to understand an antenna impedance matching / balun network that I came across in the receiving signal path in the schematics for the STEVAL-FKI868V2 S2-LP Evaluation Board, shown in the picture above.
The anennta's characteristic impedance is assumed to be 50 Ohms. Now obviously, C14, C17 and C16 are just DC blocks and large enough to be neglected. C13, C18, L3 and L5 form an LC balun. C15 is not fitted.
But what exactly are L4 and C38 (marked red) for? How did the designers come up with their values? Is there a name for this kind of circuit?
Especially C38 looks odd to me, it makes the whole balanced line asymmetric. Any hints are greatly appreciated!