dBm significant the logarithm relation to Power (mW). In case of field
strength or voltage (mV) you have to multiplication with 20.
Take a look at:
http://www.sengpielaudio.com/calculator-volt.htm
http://www.nobbi.com/conversion.htm
and very good:
http://en.wikipedia.org/wiki/Decibel
Regards,
Ralf
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Vy 73 es 55 de Ralf, DL2MRB
E-Mail:
www.hamradioboard.de