var popupWindow = null;

function popup(w,h,site) {
  if (popupWindow) {
    popupWindow.close();
  }
  x = screen.availWidth/2-w/2;
  y = screen.availHeight/2-h/2;
  popupWindow = window.open(
    site,'','width='+w+',height='+h+',left='+x+',top='+y+',screenX='+x+',screenY='+y);
}