http://www.tvfool.com/index.php?opti...57#how_to_read
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This is the predicted Noise Margin (NM) of each channel "in the air" at your location, specified in dB. You must add/subtract any gains/losses you get from your antenna, building penetration, amps, cables, splitters, and other factors present in your situation. Hypothetically speaking, you need to end up with an NM value above 0 in order to pick up a station.
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Every atom in the universe has electrons in motion, every atom produces a tiny amount of radio frequency energy. As a result there are always random radio waves 'in the air'. All of the parts of an antenna system and the TV connect to the antenna also generate small amounts of radio waves. As a desired signal gets extremely weak, the tiny amount of noise generated by atoms of the antenna will be more able to interfere with reliable reception. If the signal arriving at the antenna is too weak, the the noise generated by atoms in the antenna can be stronger than the signal received.
Noise margin as used here at TV Fool takes into account the expected level of 'background noise' produced in the antenna versus the minimum amount of desired signal needed to receive the TV signal.
If you see a listing on your TV Fool report that predicts a NM of 0.1 dB it means that an antenna with no gain or loss connected directly to an excellent tuner would in theory be on the extreme edge of just able to receive a usable signal. Any bit of noise or interference would interrupt the successful reception of the signal. In this scenario, installation of an antenna with gain would increase the chance of reliable reception.
Some TV tuners are better than others. One measure (that is very hard to find published) of tuner quality is it's noise figure. Lower NF is desired because it indicates less noise is generated by the tuner itself.
Preamplifiers and distribution amplifiers produce noise because like everything else in the universe, are made of atoms. One measure of the quality of an amplifier is it's noise figure (just like the tuner). Amplifiers amplify both desired and undesired signal. They can't tell the difference. In fact amplifiers amplify the noise generated inside the transistors and other parts of the amplifier. A good amplifier will have a low noise figure.
When you're dealing with weak signals (low NM) it would seem obvious that taking steps to increase the signal level would be a good thing. Not as obvious though, taking steps to avoid adding noise is just as valuable.
Antennas are able to provide gain when designed to be sensitive in one direction more than another. This gain does not change the tiny amount of noise generated in the antenna. This means that an antenna with gain, will deliver more desired signal to the coax, and if the antenna is directional, it will be less sensitive to interfering signals from directions other than the one it's aimed at. The result will be greater difference between the desired signal and noise, which is an improvement of noise margin. An antenna with 10 dB gain will increase the received signal 10 dB compared to an antenna with no gain (0 dB). The antenna noise does not increase, so, the signal to noise ratio and noise margin would increase by 10 dB also... more if the directional nature of the gain antenna resulted in less noise received.
An amplifier will increase the strength of a signal but will also amplify noise that comes in through the antenna. Then, in addition, the amplifier will add the noise produced by itself. The end result is less difference between the desired signal and undesired signal (noise). This results in a lower margin (difference) between desired signal and noise. In the case of a signal with a NM of 0.1 db in the air, using the zero gain antenna and a 10 dB amplifier with a noise figure of 3 dB would result in a signal power increase of 10 dB and a noise power increase of 13 dB (the 10 dB amplifier gain and the 3 dB additional noise produced by the amplifier). The resulting noise margin would then be -2.9 dB which would end any hope of seeing that signal.