About ASP.NET WebControl Control Adatpers and PostBack

Further to my earleir question about custom rendering of web controls here: SImple ASP.NET web usercontrol to overwrite default rendered HTML I have tried to achieve the same thing using Control Adapters (linked to Radio buttons in the MyAdapter.Browser file)

Imports System.Web.UI.WebControls.Adapters

Public Class myRadio
   Inherits WebControlAdapter

   Protected Overrides Sub RenderBeginTag(ByVal writer As System.Web.UI.HtmlTextWriter)
      ' MyBase.RenderBeginTag(writer)
   End Sub

   Protected Overrides Sub RenderEndTag(ByVal writer As System.Web.UI.HtmlTextWriter)
      ' MyBase.RenderEndTag(writer)
   End Sub

   Protected Overrides Sub Render(ByVal output As HtmlTextWriter)
      Dim ctrl As RadioButton = CType(Me.Control, RadioButton)
      Dim sb As New StringBuilder
      Try
         sb.Append("<label><input class=""")
         sb.Append(ctrl.CssClass)
         sb.Append(""" type=""radio""")
         sb.Append(" name=""")
         sb.Append(ctrl.GroupName)
         sb.Append("""")
         sb.Append(" id=""")
         sb.Append(ctrl.ID)
         sb.Append("""")
         If ctrl.Checked Then sb.Append(" checked")
         sb.Append("> ")
         sb.Append(ctrl.Text)
         sb.Append("</label>")
      Catch ex As Exception
         sb = New StringBuilder
         sb.Append("Error building Control:<br />")
         sb.Append(ex.Message)
      End Try
      output.Write(sb.ToString)
   End Sub

End Class

This renders the controls just as I want - but again, state is not preserved on PostBack (specifically, teh "checked" state). As this is linked to RadioButtons and is supposed to just change how the HTML is rendered, I don't understand why. But whyever, how can I get the checked state of it on PostBack?