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Attic, Stacking and Combining TV-FM Antennas.

    Stuff Near Antennas.

  1. The closer something is to an antenna, the more connected it is to the antenna. For an antenna, closeness is measured in wavelengths, so an object move closer as the wave length increases, as it would from channel 12 to channel 2. Keep in mind that a directional antenna is supposed to "see" in front, on the "main lobe". Therefore it needs more clearance to objects in front.
  2. A good rule of thumb is that nothing can be closer than 1/2 wave length from any part of the antenna, although 1 wavelength is better. Clearance in the front of the antenna should be at least 10 wave lengths. See the wave length table in the section on Rabbit Ear antennas.

    Attic Location.

  1. A lot of people locate antennas in the attic. If you do, consider that all attic wiring, wood and roofing material bcome part of the antenna. The antenna is compromised by its being connected to all this stuff lying nearby. Why would you want to do that? OK. Maybe you are lazy and don't want to put it on the roof. That takes more effort. Also, maybe winter winds would rip apart a consumer antenna. Fine, so long as you know a lot of the value of the antenna is lost by being near other stuff.

    Stacking Multiple Different Antennas on One Mast.

  1. The same rule applies to stacking as to clearance. If two antennas are closer than one wave length to each other, consider them connected, probably in unpredictable ways. Even 1/2 wave length is pretty serious. If you are an antenna professional, then you MUST consider using a separate mast for each antenna, unless you are stacking to increase antenna gain (see below). As a home user, avoid this problem by getting an all band antenna instead of two.


    Picture to the Right, Antennas Stacked Way Too Close for Comfort --->

  2. An exception might be placing one extra antenna on the mast for a high band VHF or UHF channel. The wave length for UHF is only .6 meters or two feet or so. High Band VHF, that is channels 7-13 are not so bad at about 1 1/2 meters or 6 feet. Yes, the big antenna on low band channel 2 must be 5.5 meters, or 18 feet from other objects. Well, a little UHF antenna is not resonate at channel 2, and although it decidedly WILL be coupled to the larger antenna, the effect will probably not be so hugely bad. The idea is to use common sense when making location decisions, which of course involves compromise.

    Connecting Multiple Antennas.

  1. Connecting antennas together into one feed line is also a problem. You use a directional antenna to reject interference and ghosts (multipath), but then connect a bunch of antennas together so that all of them contribute some interference and ghosting. How do you avoid this?
  2. The only way to avoid this is with a filter for each antenna that passes only the band of frequencies each antenna is intended to receive. This can be expensive. Or not. I can't find any good filters on the web. If you know of some, please e-mail me.
  3. I have seen an inexpensive filter unit with channels 2,4,5,7,9,11 & 13 so a different antenna can be used on each channel. It didn't have a manufacturers label....so I can't tell you where to find it.

    Connecting Multiple Identical Antennas for Increased gain.

  1. You can connect identical antennas together to increase gain, reduce interference and ghosting. Connecting multiple antenna to increase gain is an extention of idea of a multiple wire antenna gain antenna, where the wires are put together in one antenna so that energy is collected in the preferred direction. Combinations of complex antennas, arranged in an array, do the same thing, intentionally or otherwise! A nifty little piece of software by Karl L. Barrus demonstrates how combinations of omnidirectional antennas create directional patterns. The directional patterns shown in the graph varies with the number, spacing and electrical phase of the radiators. For more info, look at this page with good material about Yagi antenna stacking in general, or maybe an article about TV (and of course FM as well) stacjubg by James Kluge who wrote this on stacking (courtesy Mike's FMDX page).

    Connecting Multiple Identical Antenna for Reduced Interference.

  1. You can connect identical antennas together to reject signals from a particular direction. Scala has good papers on this in their catalog. See their papers here.

    Good paper on details of signal distribution.
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