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Originally Posted by vms
are Winegard HD769xP gain specs accurate?
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I believe that the more reputable antenna manufacturers (the likes of Winegard, Channel Master, and Antenna Direct) will publish gain specs based on actual measured data, so I do trust them to be reasonably honest.
Just keep in mind that these are only approximations to begin with. There may be unit-to-unit variations, and some people do a better job of assembling them than others. If anything is bent, broken, out of place, shorted, disconnected, painted, etc., you may end up with slightly different performance. Other nearby objects (roof, other antennas, buildings, trees, etc.) will also influence the behavior of the antenna.
Use the numbers to estimate how much signal you will have. Then give yourself some extra margin, just in case you need it.
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Other than dB gain, is there any other reason to have more antenna real estate?
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Antenna gain is probably THE most important factor in your setup. The reason antennas come in so many different sizes is because each situation is different and may have different gain requirements.
Another important antenna quality is the radiation pattern or directionality of the antenna. In general, higher gain antennas have a tighter beam width (gain and directionality are very much related to each other). This can be important when aiming the antenna, or trying to get multiple signals spread in different directions, or if you need to screen out some of the multipath signals bouncing around the environment.
Size and shape may also be important to some people. If your attic space is tight, it's good to have multiple options available so that you can strike the right balance between fit and performance.
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I read people suggesting going up in size if they're going to mount it in the attic to compensate for building loss, but it seems to me that they should go up as many sizes as they can since I think the building loss would be at least 3dB and probably closer to 10dB.
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Building loss varies by frequency and building material. TV signals can pass through most residential walls pretty easily, but nails, ducts, pipes, wiring, and other objects might get in the way a bit. Aluminum siding, wire mesh in stucco, foil-backed insulation, and similar objects can make it near impossible to get any signal through. Building losses can vary a lot, typically in the range of 1 to 20 dB. Most attics tend to be on the lower end of this range. Since every situation is different, these guidelines should be adjusted accordingly.
Note that going up in antenna size not only gives you more gain, but it also (usually) gives you a tighter antenna beam width. The tighter beam width can help combat the effects of multipath which is more likely to be present in the attic. Gain and directionality work together to improve your chances of picking up a clean signal from inside the attic.
Going too big in the attic can create its own problems too. If the size of the antenna causes it to get too close to other objects (e.g., nails, metal braces, electrical conduit, etc.), the antenna performance may start to change and you don't really get the gain improvements you were expecting. Size might also limit your ability to assemble, mount, and aim the antenna correctly.
In the end, no matter how big you go, and indoor antenna is never going to be as predictable and well understood as an outdoor antenna. Ideally, antennas work best in a completely open space (that's how they were designed to work). Indoor antennas are surrounded by objects that will block, reflect, or interfere with the signals. Nearby metallic objects can change the gain characteristics of the antenna. The surrounding environment can even change from day to day (e.g., wet/dry/snow-covered roof, moisture content in the wood, etc.). The number of variables goes up a lot. Even if attic space it not a problem, indoor antennas always come with a lot more uncertainty.