Interpreting Noise Margin in the TV Fool Report

This is the chart I made to give you an idea of what the NM(dB) column in the TV Fool report means in easy to understand terms. There are all sorts of ideas on what are strong and weak signals. I would hope this chart will be used as a standard in the conversation.
Above is a typical TV Fool Report. The NM(dB) column is the subject of this page. The Path column is also mentioned. Below is an expanded explanation of each row in the signal strength chart at the top.

Technical notes on TV Fool Noise Margin - TV Fool NM is the value at the antenna terminals and assumes an antenna gain of 0 dB. This is the highest noise margin in the system. A preamp increases the noise thus lowering the NM by the noise figure of the preamp. Without a preamp coax losses attenuate the signal and the TV tuner noise figure increases the noise, both contributing to a lower noise margin. The only way to increase the noise margin over the TV Fool value is to use an antenna with gain.



<-20dB These stations are impossibly weak to receive and there is no antenna system that can guarantee reception. TV Fool often shows these stations as "Tropo" which means tropospheric ducting is required for reception. This is highly unreliable. TV Fool is most inaccurate with weak signals and sometimes these stations can be received. In general you should ignore these stations if they appear on your report. A low noise preamp is required for any chance of reception. A distribution amp should be used if more than 1 TV will be connected to the antenna.
-20 to -10dB These stations are extremely weak to receive and even the best antenna system cannot guarantee reception but they're not quite as hopeless as the Impossibly Weak category. TV Fool usually shows these stations as "2 edge" which means the signal has to refract over 2 hills or mountains. These paths are easily affected by temperature inversions that make for large swings in signal strength. TV Fool is most inaccurate with weak signals and it may be possible to receive these stations. Reception is not likely to be 100% of the time and in general you should not count on these stations for recording. A low noise preamp is required for any chance of reception. A distribution amp should be used if more than 1 TV will be connected to the antenna.
-10 to 0dB These stations are very weak to receive and it will take a very good antenna system to get consistent reception. I have a number of stations in this category that are my "local" stations and I receive them >99% of the time. Compromise antennas mounted in the attic and especially indoor antennas cannot be expected to receive these stations. Stations in this category are usually 1 or 2 edge paths so reception is not likely to be 100% of the time and in general you should expect some periods of no reception. A low noise preamp is required for best reception. A distribution amp should be used if more than 1 TV will be connected to the antenna.
0 to +10dB These stations are weak to receive but with a good outdoor antenna system reception should be near 100%. There may be some dropouts under certain atmospheric conditions since these are still mostly 2 edge signals. An attic mounted antenna may be good enough for reception in some cases if multipath is not a problem but more dropouts will be seen. A preamp is required for best reception. A distribution amp should be used if more than 1 TV will be connected to the antenna.
+10 to +20dB These stations have adequate signal strength but will still require an outdoor antenna system for reception near 100%. There may be the occasional dropout under extreme atmospheric conditions. More of these signals should be 1 edge than 2 edge which means more stable reception. An attic mounted antenna may be good enough for reception of more stations if multipath is not a problem but you're still likely to experience dropouts. A preamp is recommended for these stations. A distribution amp should be used if more than 1 TV will be connected to the antenna.
+20 to +30dB These stations have good signal strength and any outdoor antenna system should give 100% reception. A dropout should be the extreme exception. Stations in this range typically have 1 edge paths as shown in the TV Fool chart unless the station is extremely low power. Such stations can be LOS. An attic mounted antenna should be enough for reception of these stations if multipath is not a problem but you're still likely to experience some dropouts. A preamp and distribution amp may be helpful for these stations.
+30 to +40dB These stations have strong signals and any outdoor antenna system should give 100% reception. There should not be dropouts. Stations in this range typically have 1 edge paths as shown in the TV Fool chart unless the station is low power then it's probably LOS. An attic mounted antenna should be good enough for reception of these stations if multipath is not a problem. These stations are strong enough for indoor antennas. Poor reception of these stations on indoor antennas is the result of multipath or an antenna that doesn't cover VHF and UHF. If there are also some weak stations to be received, a low gain preamp with high overload tolerance can be used. A distribution amp can used if there are more than 2 TVs.
+40 to +50dB These stations have very strong signals and any outdoor antenna system will give 100% reception. There should be no dropouts. Stations in this range typically have to be LOS. An attic mounted antenna is good enough for reception of these stations if multipath is not a problem. These stations are strong enough for indoor antennas. Poor reception of these stations on indoor antennas is the result of multipath or an antenna that doesn't cover VHF and UHF. If there are also some weak stations to be received, a low gain preamp with high overload tolerance can be used. A better choice is a distribution amp with no preamp.
+50 to +60dB These stations have extremely strong signals and any antenna system will give 100% reception with no dropouts. Stations in this range have to be LOS. Poor reception of these stations is the result of multipath. No preamp should be used with stations this strong as the preamp and/or TV may be overloaded. No distribution amp should be required.
+60 to +70dB These stations are so strong that they might overload the TV and impact reception of weaker stations. Any antenna will give 100% reception with no dropouts. Stations in this range have to be LOS. Poor reception of these stations is the result of multipath. No preamp should be used with these stations under any circumstances. No distribution amp is required. An attenuator ahead of the TV can be helpful to reduce overload if that appears to be a problem.
>+70dB These stations are so strong that they will likely overload the TV and impact reception of weaker stations. Any antenna will give 100% reception with no dropouts. Stations in this range have to be LOS. Poor reception of these stations is the result of multipath. No preamp should be used with these stations under any circumstances. No distribution amp is required. An attenuator ahead of the TV can be helpful to reduce overload problems.

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