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There is a legitimate use of dB without a reference point. An attenuator attenuates by -20dB, not by -20dBm.

This is right and all... But this usage still leads to confusion about what you are measuring your filter by.

There are filters we measure on power, there are filters we measure on signal amplitude, and "signal amplitude" can be ambiguous on some contexts too. There should be a way to specify this one better.

Well, dB is fully specified in that regard. It's always power. You can calculate the voltage gain from it under certain assumptions, and under normal assumptions you get that factor 2. But a -20dB attenuator will always reduce the power by a factor of 100.

There is also antenna gain in decibels.

dBi

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