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Old 14-Oct-2011, 7:55 PM   #6
Join Date: Dec 2009
Posts: 632
when looking at my NM value, I use dBi numbers to add to it, right? (not dBd?).
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.
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