Simulate and plotting S21 of a LowPass Filter

I’m running KiCad 6 on Windows 10 and like anyone new to this software I have questions I can’t seem to find in the ngspice manual.

I don’t understand what the limitations are to simulating with ngspice (in general) and in the version of ngspice(35?) that comes with KiCad6 or whether I should install ngspice36. Knowing what the limitations are would be helpful so I don’t go making more assumptions. As you know there is a barrier to entry for understanding this stuff…but this community seems to act as good gatekeepers.

Anyway I’m trying to create a tunable LPF simulation but before that I’m having problems with how to use .plot equations / directives properly.

How do I / Is it possible to
1- Plot things like S21 or S11?
2-Change the X axis spacing so that it isn’t log spacing?

I read section 15.6.6 of the ngspice manual but can’t figure out how to plot S21. Would it be V(out)/Vin? My attempt didn’t work and threw an error.

I would have posted a screencap of that section for your convenience but being a new user I can only post one image.

Like most things… It’s probably easy once you know how, but I’m embarrassed to say how long I’ve tried and failed.

I’m trying things like the following without success.
.ac dec 100 100k 4G
.plot S21

I’m not familiar with spice but as a circuit you probably need some terminating resistor. Else L5 will have no effect driving an infinite resistance.

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