Help on a search form based on radio button selection, please

I am kinda new to javascript and I would like to make a search form with a text input field and two radio buttons, i.e, button A and button B, with A auto checked. If A is checked, a search will redirect to the page: http://www.urlA.com/SEARCH_TEXT , and if B is checked, after submitting the form it redirects to http://www.urlB.com/SEARCH_TEXT. Below is my code but it is not working, I guess there might be a problem on finding the value of the radio button:

<script type="text/javascript">

function get_radio_value()
{
for (var i=0; i < document.form1.R1.length; i++)
   {
   if (document.form1.R1[i].checked)
      {
      return document.form1.R1[i].value;
      }
   }
}


function getURL(val){

radio_value = get_radio_value();

if (radio_value==A)
{base = 'http://www.domainA.com/';}
else
{base = 'http://www.domainB.com/';}
   location = base + val;
   return false;
}
</script>
</head>

<form id="form1" name="form1" method="post" action="" onsubmit="return getURL(this.url.value)">
  <label>
  <input type="text" name="url" size="20" /><input type="radio" name="R1" value="A" checked>A<input type="radio" name="R1" value="B">B
  <input type="submit" name="Submit" value="Submit" />
  </label>
  </form>

Code (markup):

Any help on the codes or suggestion of a new way to do this would be highly appreciated. Thanks in advance!