var popupFenster;

function fensterAuf(url, name, w, h) {  
if (popupFenster == null || popupFenster.closed)	{
popupFenster = window.open(url, name, 'scrollbars,width=' + w + ',height=' + h);
}
else	{
popupFenster.location.href = url;
}
popupFenster.focus();
}