Quote:
Originally Posted by TheEmrys
I guess I'm trying to understand the relationship between nm(dB) and PWR(dBm). Do I add them together to get >0?
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From the FAQ:
The Noise Margin (NM) of each channel "in the air" at your location is 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.
Simply put, ignore the receive power. Add your antenna gain to the NM value, subtract the noise figure of your preamp, and subtract distribution losses that add to the system noise figure. (The system losses are variable and use a process called the Friis equation, but that's a bit beyond the question asked.) The result must be greater than zero, but +10 db of margin is a good goal.
The use of a preamp depends on the total signal level available at your location. In your case, the signals are strong enough without a preamp, so your use of a preamp is compensating for some other losses, perhaps due to low antenna gain and/or trees and/or neighboring houses and/or multipath and/or distribution losses and/or cable problems and/or inadequate receiver performance.