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 Coupler Type:","dc_coax_6_30.html");

  if (option_no == "coax") {
    f.category.length = 4;	
    f.category.options[1] = new Option("6 to 30 dB Coupling","dc_coax_6_30.html");
    f.category.options[2] = new Option("Bi-Directional, High Power","dc_coax_hi_pwr.html");
    f.category.options[3] = new Option("Directional, High Power","dc_coax_hi.html");

  }
  else if (option_no == "pi") {
    f.category.length = 3;
    f.category.options[1] = new Option("6 to 30 dB Coupling","dc_pic_6_30.html");
    f.category.options[2] = new Option("Bi-Directional, High Power","dc_pic_hi_pwr.html");

  }
  else if (option_no == "sm") {
    f.category.length = 3;
    f.category.options[1] = new Option("6 to 25 dB Coupling","dc_sm_6_20.html");
    f.category.options[2] = new Option("Bi-Directional, High Power","dc_sm_bd_hi_pwr.html");
  }
  else if (option_no == "lt") {
    f.category.length = 2;
    f.category.options[1] = new Option("Bi-Directional, High Power","dc_sm_bd_hi_pwr_ltcc.html");
  }	
}