By combining two TV antennas together, you are able to receive TV stations in two different directions without needing the use of an antenna rotator. This kind of installation can be done successfully, but proper spacing must be kept between the two antennas to help prevent the antennas from interfering with each other and causing ghosting on the TV picture.
The formula for determining the minimum vertical spacing between the two antenna booms is a half l (wave) of the lowest channel to be received. To calculate this distance take 467 and divide it by the lowest TV channel frequency that will be received by either of the two antennas. So for example, if the lowest channel received by either of the two antennas is channel 2, you would take 467 and divide it by the lower frequency of channel 2 (54 MHz). This would give you the minimum vertical spacing of 8.64 feet or 103.7 inches that would be needed between the two antenna booms.
If two antennas will be spaced apart horizontally (such as in an attic) it is recommended that they be separated by 1 wavelength instead of a half wavelength as in vertical spacing.
There must also be the exact same length of coaxial cable between the two antennas and the combiner, which is basically a backwards two-way signal splitter. The same exact length of coax cable between each of the two antennas and the combiner is needed also to help prevent ghosting from occuring on the TV's connected to the antenna system.
Even if you properly separate the two antenna together and have equal lengths of coaxial cable you can run into problems. The biggest problem is when one of the two antennas starts to receive the station(s) in the opposite direction (from the rear) that it is pointed. This can create ghosting on all of the TV's connected. To reduce or eliminate this problem in residential antenna systems, bandpass filters and channel traps are used.
An inexpensive bandpass filter/channel trap is the Channel Master JOIN-TENNA coupler. The JOIN-TENNA coupler has two 75 Ohm coax cable inputs and one 75 Ohm coax cable output. One of the two inputs is connected to a bandpass filter that lets only the single specified channel through. The other input is connected to a channel trap to filter out the single channel while passing all other channels. By only letting the single channel through one input and trapping it out of the other input, the amount of interference is reduced.
The JOIN-TENNA coupler model is selected by the single channel that you wish to combine with all of the other channels. The JOIN-TENNA coupler cannot be used to combine adjacent channels, except for channels 4 and 5, and channels 6 and 7. The JOIN-TENNA coupler can also be used as a -20dB single channel trap. Be sure not to trap channels ajacent to the ones that you desire. For more information on how to connect the JOIN-TENNA coupler see the connection diagram below.