VerilogA Modelling for Skin Effect

Hi all,

I'm trying to model skin effect using VerilogA. And I got the sample from  Ken's website: https://designers-guide.org/modeling/ind.pdf

Here's the model:

`include "constants.vams"
`include "disciplines.vams"

module skin_effect(p, n);
electrical p, n;
parameter real f0=1 from (0:inf);
parameter real f1=10 from (f0:inf);
parameter real r0=1 from (0:inf);
`define lumps 100
real mult, mult2, wp, wz;
real zeros[0:2*`lumps-1], poles[0:2*`lumps-1];
integer i;
    analog begin
        @(initial_step("static")) begin
            mult = pow(f1/f0, 1.0/(4*`lumps)); //May the factor in front of lumps, we can change.
            mult2 = mult*mult;
            wz = 2*`M_PI*mult*f0;
            wp = mult2*wz;
            for (i=0; i<`lumps; i=i+1) begin
                zeros[2*i] = -wz;
                zeros[2*i+1] = 0;
                poles[2*i] = -wp;
                poles[2*i+1] = 0;
                wz = mult2 * wp;
                wp = mult2 * wz;
            end
        end
        V(p,n) <+ r0*laplace_zp(I(p,n), zeros, poles);
    end
endmodule

When I simulated this, I found that both the real and imaginary parts are cut off at around 5MHz. 

I experimented with lumps around, but still got the same cut off, any one can shine some light on this problem is greatly appreciated!

Thank you very much,

Menghan