Javascript form validation

I'm writing a function to do some form validation. The form contains several radio buttons (named pickbutton) and I want the condition to say.. if the last radio button (valued ownbutton) is picked.. make sure the textbox is filled by more than "http://" in it. I have the following code for starters.. but it doesn't work.

Any help would be very much appreciated!

function button(f)
{
var msgerr="";
valid = true;
if ((document.thisform.pickbutton.value == "ownbutton")
&& (f.value=="http://"))
msgerr += "\n\n Please pick a valid url location for your button image.";
valid = false;
}