get radio value inside iframe

how i doing jquery?

basically structure:

<form id="myform">
  <iframe>
    <!-- normal html headers wanting -->
    <input type=radio name="myradio" value=1>first
    <input type=radio name="myradio" value=2>second
    <input type=radio name="myradio" value=3>third
  </iframe>
  <input type=button value="submit" />
</form>

i attempted several examples net such

$("input[@type=radio][@checked]");

but failed. even jquery form plugin's failed.