function setModelType(f,option_no) {
  var i=0;
  
  // clear selection options	 
  while (i < f.category.length) {
    f.category.options[i] = null;
    i++;
  }

  f.category.options[0] = new Option("Choose Filter Type:","filters_coax_bandpass.html");

  if (option_no == "coax") {
    f.category.length = 7;
    f.category.options[1] = new Option("Band Pass","filters_coax_bandpass.html");
    f.category.options[2] = new Option("Band Stop","filters_coax_bandstop.html");
    f.category.options[3] = new Option("Low Pass","filters_coax_low.html");
    f.category.options[4] = new Option("Low Pass, Flat Time Delay","filters_coax_flattime.html");
    f.category.options[5] = new Option("Low Pass, High Rejection","filters_high_rejection.html");
    f.category.options[6] = new Option("High Pass","filters_coax_high.html");
  }
  else if (option_no == "pi") {
    f.category.length = 5;
    f.category.options[1] = new Option("Band Pass","filters_pic_bandpass.html");
    f.category.options[2] = new Option("Flat Time Delay","filters_pic_flattime.html");
    f.category.options[3] = new Option("Low Pass","filters_pic_low.html");
    f.category.options[4] = new Option("High Pass","filters_pic_high.html");

  }
  else if (option_no == "sm") {
    f.category.length = 7;
    f.category.options[1] = new Option("Band Pass","filters_sm_bandpass.html");
    f.category.options[2] = new Option("Band Stop","filters_sm_bandstop.html");
    f.category.options[3] = new Option("Diplexers","filters_sm_diplexer.html");
    f.category.options[4] = new Option("High Pass","filters_sm_high.html");
    f.category.options[5] = new Option("Low Pass","filters_sm_low.html");
    f.category.options[6] = new Option("Low Pass, Dual","filters_sm_low_dual.html");

  }
}