Thread: Confused
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Old 2-Sep-2010, 5:08 AM   #6
Join Date: Dec 2009
Posts: 632
Originally Posted by acutshall View Post
-So on the chart (signal power) is represented by NM and (noise power) is represented by Pwr, thus SNR?
-Do I subtract one from the other to get the net signal?
The Noise Margin (NM) column is all that you really need. It already takes into account all the terrain, distance, Earth curvature, and SNR into account. This number needs to stay above zero in order for you to have a usable signal.

The Power (Pwr) column is just there for information purposes. When signals are too strong (at a level that might overload some electronics), it might be good to look at this number. However, in most cases, you don't really care what the actual power levels are.

The analysis will tell you Noise Margin "in the air" at your house, based on the address or coordinates you provide. The NM does not know what kind of antenna you have, the length of cable you have, the number of signal splits you have, etc. To calculate how much "net" NM you have, you need to add/subtract the gains/losses that are specific to your setup. Antenna gain gets added to the NM, everything else (cable losses, splitters, etc.) gets subtracted from the NM.

The channel list is color coded to give you a general idea of which channels are strong, medium, weak, or really hard to get. The color coding takes into account what you might find in "typicial" installations. An indoor (set-top) type antenna is probably good enough to pick up the "green" channels. An attic antenna is probably good enough to pick up the "yellow" and higher channels. A rooftop antenna is probably good enough to pick up the "red" and higher channels. Channels in the "gray" zone are very weak and will probably require extreme measures to get.

-The goal is to have a net of zero or above( plus number) or the other way around?
After factoring in all of the specifics of your setup (antenna gain, cable loss, etc.), you want to end up with a positive NM.

-Is the "some additional margin by recommending designing to an NM of +10already ." reflected on the charts
The NM values in the chart are "in the air" at your location. The tools do not account for anything that is specifically installed by you (antenna, cables, splitters, amps, etc.). You need to adjust the number to account for the details of your particular setup.

The "net" NM number you end up with tell you how much margin you have for natural signal fluctuations. Signals will naturally change throughout the day, under varying weather conditions, and from one season to the next. The more "excess" Noise Margin you have, the better off you will be. This allows your setup to tolerate more of these random fluctuations before having any channel drop-outs. Having an extra 10 dB of NM is generally considered good enough to have very reliable reception.

It might be more clear to me if you were able to provide a numerical example.
Here's a post that works through some numerical examples:

Last edited by mtownsend; 2-Sep-2010 at 7:45 PM. Reason: Fixed spelling error
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