Dear Farell,

The answer is, "It depends."  If you're trying to stack two antennas to point at broadcast towers in two different directions, and you don't have too many tall buildings or hills which may cause reflections, then you should be OK stacking two TV antennas vertically on the same mast, as long as you leave 5 feet or more between each antenna.  Also, if you're stacking antennas of different types (e.g., UHF and VHF), then you'd also be OK stacking these vertically, with the same minimum distance.  You could then send the outputs of these antennas to a simple splitter/combiner and send the full combined signal over a single coax cable to your display. 

If you're in a city with many tall buildings or a hilly area where broadcast signals could reflect and be received by your antennas from two different directions (slightly out of phase from each other), then you could get multipath distortion.  This usually manifests itself as a doubled or "ghosted" image.   If this occurs, then you may be better off running two separate cables and using an A/B switch at the television, rather than a combiner at the antenna end.  You could also use a rotator instead, which allows you to change the antenna's orientation for each channel, but this is more complicated and it requires you to wait while the antenna rotates itself into position. 

If you're trying to stack a standard television antena with a satellite antenna (a dish), then you'll need something called a diplexer at both ends, instead of a splitter/combiner. 

For more details on TV antenna set-up, there's a pretty good FAQ article by Bill Ranck on doityourself.com. 

Hope that helps.

-Chris

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