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when looking at my NM value, I use dBi numbers to add to it, right? (not dBd?).
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Actually, you need to use dBd. dBi is often used for marketing purposes because the average consumer doesn't realize that the numbers have a built-in 2.15 dB offset.
Broadcasters and the FCC use dBd as their reference. For example, if an broadcaster is licensed for a 1000 kW transmitter, that means 1000 kW relative to a 0 dBd antenna. Our reports follow the same principle, showing you the theoretical signal power (or EM field strength) "at a point in space" available to a 0 dBd antenna.
You'll find that in most real-world broadcast situations, people use 0 dBd as their reference. That's because it's something you can actually build and calibrate against. You can't build an isotropic antenna. Isotropic antenna gain is nice from a theoretical math (and computer modeling) standpoint, but it's not that useful in the day-to-day activities of a broadcast engineer.
It used to be that antenna companies like Winegard and Channel Master published their gain specs in dBd all the time. More recently, you see a mix of dBd and dBi specs which just makes things more confusing for the average user.