help on javascript code url

[MDanz]

i have a 2 frames.  the top frame's id is called link_frame.  the bottom is menu.php

 

in menu.php i have a text field named "url".

 

i want this javascript to put the top frame's URL into the textfield in the bottom frame.

 

i tried below but it comes up blank.  how can i change this so that the url of the top frame gets input into the bottom frame's textfield.

<script language="text/javascript">

function getFrameURL {

   var url2 = document.getElementById(link_frame).location.href;
   document.getElementById('url').value = url2;
}

</script>