Co-channel interference suppression using stacked antennas:

If you want to receive a distant or even overseas TV station that happens to use the same frequency as a strong local transmitter, the received signals will be weak and always suffer from ghost effects, moiré and other interference. In some cases, reception will be totally impossible because of the much greater signal strength of the station around the corner. This article describes an interference reduction method that may be used for UHF TV as well as VHF radio (including amateurs).