I have a question...Polar coodinate..

i'm sorry. i can't english. my question is Antenna pattern. matlab M-file

   clc; clear all;
   filename = 'gb.txt';
   a=fopen(filename, 'r');
   z=fscanf(a, '%g %g', [25 inf]);
   z=z';
   fclose(a);
   z1=z(1,:);
   theta=[0:pi/12:2*pi];
   x=z1.*cos(theta);
   y=z1.*sin(theta);
   R=sqrt(x.^2+y.^2);
   polar(theta,20-R,'r');

this polar plot. dB level is 0~20. but, i hope db level changing. (-35~+10) what i can? help me. please~