validate a group of radio button in a form with php

[dentaura]

PLEASE CORRECT THIS JAVA SCRIPT TO VALIDATE THE FORM FIELD

<script>
function ValidateContactForm()
{
    var CategoryOpt = document.ContactForm.CategoryOpt;
    if (CategoryOpt.value == "")
    {
        window.alert("Please enter your Membership Category.");
        CategoryOpt.focus();
        return false;
    } 
    return true;
}
</script>


BELOW IS THE FORM FIELD

<td width="176">
<font face="Verdana">
<input type="radio" value="Radiobutton1" name="CategoryOpt" tabindex="5"><font size="2">
<font color="#000080">Radiobutton1</font><br>
</font>
<input type="radio" value="Radiobutton2" name="CategoryOpt" tabindex="7">
<font size="2" color="#000080">Radiobutton2</font></font></td>
<td width="161">
<font face="Verdana">
<input type="radio" value="Radiobutton3" name="CategoryOpt" tabindex="6">
<font size="2"><font color="#000080">Radiobutton3</font><br>
</font>
<input type="radio" value="Radiobutton4" name="CategoryOpt" tabindex="8">
<font size="2" color="#000080">Radiobutton4</font></font></td>