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

  if (option_no == "coax") {
    f.category.options[0] = new Option("Choose Attenuator Type:","attenuators_coax_fixed.html");
    f.category.options[1] = new Option("Adaptenuators","adaptenuators.html");
    f.category.options[2] = new Option("Fixed","attenuators_coax_fixed.html");
    f.category.options[3] = new Option("Fixed, Precision & Data","attenuators_coax_precision.html");
    f.category.options[4] = new Option("Fixed, DC Passing", "attenuators_coax_passing.html");
    f.category.options[5] = new Option("Digital Step","attenuators_coax_digital.html");
    f.category.options[6] = new Option("Voltage Variable","attenuators_coax_variable.html");
  }
  else if (option_no == "pi") {
    f.category.options[0] = new Option("Choose Attenuator Type:","attenuators_pi_fixed.html");
    f.category.options[1] = new Option("Digital Step","attenuators_pic_digital.html");
  }
  else if (option_no == "sm") {
    f.category.options[0] = new Option("Choose Attenuator Type:","attenuators_sm_fixed.html");
    f.category.options[1] = new Option("Fixed","attenuators_sm_fixed.html");
    f.category.options[2] = new Option("Digital Step","attenuators_sm_digital.html");
    f.category.options[3] = new Option("Voltage Variable","attenuators_sm_variable.html");	
  }
  else if (option_no == "ad") {

    location.href="adaptenuators.html";

  }

}