Antenna systems and antenna filters are provided, for example for use in a windshield or on a roof of a vehicle. An antenna system comprises a first antenna, a second antenna, and a filter. The first antenna is configured to operate at a first frequency. The second antenna is configured to operate at a second frequency. The filter is coupled to the first antenna. The filter is configured to create an open circuit condition at the second frequency and reduce secondary radiation between the first and second antennas.