Electronics Forum / CAD / August 2007

The simplest way is to use the Spice "URC" - Uniform distributed RC
transmission line:

3.3.5.  Uniform Distributed RC Model (URC)

     The URC model is derived from a model  proposed  by  L.
Gertzberrg  in 1974.  The model is accomplished by a subcir-
cuit type expansion of the URC line into a network of lumped
RC  segments  with  internally generated nodes.  The RC seg-
ments are in a geometric progression, increasing toward  the
middle  of  the  URC  line, with K as a proportionality con-
stant.  The number of lumped segments used, if not specified
for the URC line device, is determined by the following for-
mula:
                                            2
                    |     R C        |(K-1)|  |
                          _ _      2
                 log|F        2 J L  |-----|  |
                      max
                    |     L L        |  K  |  |
            N =  ------------------------------
                             log K
     The URC line is made up strictly of resistor and  capa-
citor  segments  unless the ISPERL parameter is given a non-
zero value, in which case the capacitors are  replaced  with
reverse  biased diodes with a zero-bias junction capacitance
equivalent to the capacitance replaced, and with  a  satura-
tion  current  of ISPERL amps per meter of transmission line
and an optional series resistance equivalent to RSPERL  ohms
per meter.
     name     parameter                            units   default
example   area
 1   K        Propagation Constant                 -       2.0
1.2       -
 2   FMAX     Maximum Frequency of interest        Hz      1.0G
 6.5Meg    -
 3   RPERL    Resistance per unit length           Z/m     1000
       -
 4   CPERL    Capacitance per unit length          F/m     1.0e-15
     -
 5   ISPERL   Saturation Current per unit length   A/m
     -         -
 6   RSPERL   Diode Resistance per unit length     Z/m
     -         -

and called by in the spice netlist:

3.3.4.  Uniform Distributed RC Lines (Lossy)
General form:
    UXXXXXXX N1 N2 N3 MNAME L=LEN <N=LUMPS>
Examples:
    U1 1 2 0 URCMOD L=50U
    URC2 1 12 2 UMODL l=1MIL N=6
     N1 and N2 are the two element nodes the  RC  line  con-
nects,  while  N3  is the node to which the capacitances are
connected.  MNAME is the model name, LEN is  the  length  of
the  RC  line in meters.  LUMPS, if specified, is the number
of lumped segments to use in modeling the RC line  (see  the
model  description for the action taken if this parameter is
omitted).
e.g..MODEL ResisterInTankURC_XN urc(rperl=1G cperl=100p)Most GUI spice
should allow you do do this all automatically. e.g SuperSpice has an
example, and a GUI setup for that model.