input radio 체크유무 검사

<SCRIPT LANGUAGE="JavaScript">
< !--
 function chk_val(f){
  if(f.a[0].checked==false&&f.a[1].checked==false){
   alert("둘중하나는 체크해야함");
  } else {
   alert("체크완료");
  }
 }
 //-->
< /SCRIPT>

< FORM name="form_name" METHOD=POST ACTION="">
 <INPUT TYPE="radio" NAME="a" value="1">
 <INPUT TYPE="radio" NAME="a" value="2">
 <INPUT TYPE="button" VALUE="" ONCLICK="chk_val(this.form)">
< /FORM>