I am having some problem with s-parameter measurement procedure

Please refer to my question at the bottom of this post.

The following plots are obtained by running s-param.cir

As far as I can see, the S-parameters look good, maximum transmission/reflection at 0dB makes sense for low pass filter.

__s-param.cir__ Retrieved from

http://ngspice.sourceforge.net/docs/ngspice-27-manual.pdf#page=364 **Code:**Test for Scattering Parameters
** Two ports
** Examples: Bipolar, Tschebyschef, RC
.param Rbase=50 Vbias_in=0 Vbias_out=0
*** The two-port circuit:
** port 1: in 0
** port 2: out 0
** Bias on both ports through resistor Rbase (to obtain operating point)
** Example RF Bipolar mrf5711
** VCE 1 V, IE = 5mA
** QXXXXXXX nc nb ne
** model obtained from
** http://141.69.160.32/~krausg/Spice_Model_CD/Vendor%20List/Motorola/Spice/RFBJT/
*.include MRF5711.lib
*XMRF5711 out in e MRF5711
*Ie e 0 5m
*Ce e 0 1
** Example Tschebyschef Low Pass filter
C1 in 0 33.2p
L1 in 2 99.2n
C2 2 0 57.2p
L2 2 out 99.2n
C3 out 0 33.2p
** Example RC
** see
** http://www.allenhollister.com/allen/files/scatteringparameters.pdf
*R2 in out 10
*C1 out int5 30p
*R1 int5 0 10
*** End of circuit
** The following subcircuit to be changed only by an experienced user!
*** Driver and readout
X1 in out S22 S12 S_PARAM
.SUBCKT S_PARAM 22 66 5 7
* Resistors emulate switches with Ron=0.001 and Roff=1e12
* to switch driver to input and readout to output (and vice versa, see below)
RS1 22 2 0.001
RS2 66 6 0.001
RS3 22 6 1e12
RS4 66 2 1e12
*Driver
Vacdc 1 0 DC 'Vbias_in' AC 1 $ ac voltage and dc bias at input (applied through load resistor)
R1 1 2 'Rbase'
E1 3 0 2 0 2 $ amplify in port ac voltage by 2
Vac 3 4 DC 0 AC 1 $ subtract driving ac voltage
R_loop 4 5 0.001
R3 5 0 1 $ ground return for measure node 5
*Readout
E2 7 0 6 0 2 $ amplify out port ac voltage by 2
R4 6 8 'Rbase' $ load resistor at output (ac)
Vdc 8 0 DC 'Vbias_out' AC 0 $ dc bias at output (applied through load resistor)
.ends
** Check the two ac lines below for being equal!
.control
set noaskquit
set filetype=ascii
*** measurement for s11 and s21
op
** save bias voltages to vector
let Vdcnew=V(X1.1) $ former Vacdc
let Vacdcnew=v(X1.8) $ former Vdc
** first ac measurement (change this line only together with following ac line)
*ac lin 20 0.1G 2G $ use for bip transistor
ac lin 100 2.5MEG 250MEG $ use for Tschebyschef
*ac lin 101 1k 10G $ use for RC
**
** switch input and output
alter R.X1.RS1=1e12
alter R.X1.RS2=1e12
alter R.X1.RS3=0.001
alter R.X1.RS4=0.001
** switch bias voltages between in and out
alter V.X1.Vacdc DC=op1.Vacdcnew
alter V.X1.Vdc DC=op1.Vdcnew
*** measurement for s12 and s22
op
** second ac measurement (change this line only together with ac line above)
*ac lin 20 0.1G 2G $ use for bip transistor
ac lin 100 2.5MEG 250MEG $ use for Tschebyschef
*ac lin 101 1 10G $ use for RC
**
let s11=ac1.s22
let s21=ac1.s12
settype s-param S11 S21 S22 S12
let S11db = db(s11)
let S12db = db(s12)
let S21db = db(s21)
let S22db = db(s22)
settype decibel S11db S21db S22db S12db
let P11=180*ph(s11)/pi
let P21=180*ph(s21)/pi
let P22=180*ph(S22)/pi
let P12=180*ph(S12)/pi
settype phase P11 P21 P22 P12
let Rbase=@R.X1.R4[Resistance]
settype impedance Rbase
*plot s11db s21db S22db S12db ylimit -50 0 xlog $ used with RC
plot s11db s21db S22db S12db ylimit -0.5 0 $ used with Tschebyschef
plot P11 P21 P22 P12
plot smithgrid S11 S12
*wrdata s3046 mag(S11) P11 mag(S21) P21 mag(S22) P22 mag(S12) P12 $ write simple table
wrs2p s3046.s2p $ write touchstone vers. 1 file s3046.s2p
.endc
.end

**Quote:**R1 1 2 'Rbase'

this 50 ohm Rbase is connected to the netlabel 2 in the screenshot below.

Or did I misread the s-param.cir netlist ?