You may want to multiple TV antennas if some digital television channels are broadcast on the UHF frequency and some on the VHF frequency in your area. Combining multiple antennas might also be useful to strengthen the "gain," capture weak TV signals and reduce or eliminate interference or "drift" in the digital signal. Combining multiple antennas is a straightforward task that does not require special expertise in most cases. It can work with both indoor and outdoor antenna units.
Combine two indoor antennas. Use a coaxial splitter that has one coaxial "antenna out" port on one side and two ports on the other. Use an RF coaxial cable to connect the splitter's single port to the "antenna in" port on the television or digital converter box. Connect one antenna to one of the splitter's two remaining ports, and the second antenna to the second port. This combination can work with one VHF antenna and one UHF antenna, two "rabbit ears" antennas or virtually any other combination of two antennas. If you have trouble with signal drifting, move the antennas around until you lock in the signal for the channel you're watching.
Combine an outdoor VHF antenna with a UHF antenna. Start by reading the instructions that came with your outdoor antenna. Most will provide detailed instructions for adding a second antenna. Also, follow all safety precautions for installing an outdoor antenna. Combining outdoor antennas typically involves bolting the UHF antenna to the top of the mast of the VHF unit, then attaching the two antennas with 300-ohm flat twin-lead cable.
Combine two outdoor VHF or two outdoor UHF antennas. This solution can eliminate the need for an extra antenna rotator to help bring in TV stations from multiple directions. Combining two outdoor antennas in this way usually requires professional installation since there must be a minimum vertical spacing between the two antennas. Otherwise, the signals will interfere with each other and cause poor TV picture quality. Antenna accessories -- bandpass filters and channel traps -- can be added to the combined antennas to reduce the interference.
Read the instructions that come with the antenna, especially outdoor antennas. Follow the safety precautions to the letter to avoid the risk of electrocution and other injury.
Don't try to install an outdoor antenna by yourself. Ask a friend to help, or even better, hire a professional installation company.