Call me dense, but after many years of using LTspice, during which many times I inserted return-loss bridges into my circuits to measure S11, someone pointed out to me that LTspice has a directive for that.
Just insert ".net V1 v(out)" somewhere into the schematic, and an S11 trace will become available.
I'm still missing some of the finer points however: The resulting S11 only makes sense with the directive above. The docs seem to say that it should be ".net v(out) V1", but then the resulting S11 makes no sense that I can see. With the latter form, I also get the three missing S- parameters, as well as H, Z and Y parameters. S21 makes no sense either. I don't know yet about the others. If I insert ".net I(R1) V1", with R1 the output load, S11 is OK again. I'm puzzled. Maybe someone can shed some light on this?
For the curious, below is the circuit I've been playing with. It's a constant impedance O(6) Bessel low-pass filter. It's more an exercise in filter synthesis than a practical design...
Jeroen Belleman
Version 4 SHEET 1 1468 680 WIRE -96 -80 -144 -80 WIRE 16 -80 -32 -80 WIRE 64 -80 16 -80 WIRE 176 -80 144 -80 WIRE 288 -80 256 -80 WIRE 400 -80 288 -80 WIRE 464 -80 400 -80 WIRE 608 -80 544 -80 WIRE 736 -80 688 -80 WIRE 848 -80 736 -80 WIRE 896 -80 848 -80 WIRE 1104 -80 976 -80 WIRE 16 -32 16 -80 WIRE 288 -32 288 -80 WIRE 400 -32 400 -80 WIRE 1104 -32 1104 -80 WIRE 736 -16 736 -80 WIRE 848 -16 848 -80 WIRE 16 96 16 48 WIRE 288 96 288 32 WIRE 400 96 400 48 WIRE 736 96 736 48 WIRE 848 96 848 64 WIRE 1104 96 1104 48 WIRE -144 176 -144 -80 WIRE -144 176 -496 176 WIRE -16 176 -144 176 WIRE 160 176 64 176 WIRE 256 176 160 176 WIRE 544 176 336 176 WIRE 592 176 544 176 WIRE 896 176 672 176 WIRE 1008 176 896 176 WIRE 1056 176 1008 176 WIRE 1056 192 1056 176 WIRE -496 208 -496 176 WIRE 160 208 160 176 WIRE 544 208 544 176 WIRE 896 208 896 176 WIRE 160 288 160 272 WIRE 544 288 544 272 WIRE 896 288 896 272 WIRE 1056 288 1056 272 WIRE -496 304 -496 288 FLAG 1056 288 0 FLAG 896 288 0 FLAG 544 288 0 FLAG 160 288 0 FLAG -496 304 0 FLAG 1008 176 out FLAG 1104 96 0 FLAG 848 96 0 FLAG 736 96 0 FLAG 400 96 0 FLAG 288 96 0 FLAG 16 96 0 SYMBOL ind -32 192 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 1.5124 SYMATTR SpiceLine Rser=0 SYMBOL ind 240 192 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value .8125 SYMATTR SpiceLine Rser=0 SYMBOL ind 576 192 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value .3785 SYMATTR SpiceLine Rser=0 SYMBOL cap 144 208 R0 SYMATTR InstName C1 SYMATTR Value 1.0329 SYMBOL cap 528 208 R0 SYMATTR InstName C2 SYMATTR Value .6072 SYMBOL cap 880 208 R0 SYMATTR InstName C3 SYMATTR Value .1287 SYMBOL res 1040 176 R0 SYMATTR InstName R1 SYMATTR Value 1 SYMBOL voltage -496 192 R0 WINDOW 3 -185 164 Left 2 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value PULSE(0 1 1 1m 1m 1m) SYMATTR Value2 ac 1 SYMATTR InstName V1 SYMBOL cap -32 -96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMATTR Value {1/1.06998} SYMBOL res 0 -48 R0 SYMATTR InstName R2 SYMATTR Value 1 SYMBOL ind 48 -64 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 2.44663 SYMATTR SpiceLine Rser=0 SYMBOL res 272 -96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1.61635 SYMBOL cap 272 -32 R0 SYMATTR InstName C5 SYMATTR Value .501225 SYMBOL res 384 -48 R0 SYMATTR InstName R4 SYMATTR Value {1/.23388} SYMBOL res 704 -96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1.0087 SYMBOL ind 448 -64 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L5 SYMATTR Value 1.67931 SYMATTR SpiceLine Rser=0 SYMBOL ind 880 -64 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L6 SYMATTR Value .944529 SYMATTR SpiceLine Rser=0 SYMBOL res 1120 64 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R6 SYMATTR Value 3.55786 SYMBOL cap 720 -16 R0 SYMATTR InstName C6 SYMATTR Value .330091 SYMBOL res 832 -32 R0 SYMATTR InstName R7 SYMATTR Value {1/.397832} TEXT 240 352 Left 2 !.ac dec 100 10m 10 TEXT 240 392 Left 2 !;tran 10 TEXT 248 304 Left 2 !.net v1 v(out)