What is the window mullion made of? Plastic, aluminum, steel? That type of antenna needs a ground plane under it. A metal can, metal cookie can lid, a steel pie plate works also. If the mullion is plastic or aluminum, then any scrap piece of sheet metal (preferred) would work. Those antennas are notoriously bad to start with, so shop around for something better. A homemade “spider” antenna works well also.
