It is common practice (including throughout this book) to state the signal strength of transmitted signals in dBm, even though this makes no physical sense. dBm is a unit of electrical power, a ratio of the signal power to one milliwatt. Power is defined only within a circuit. After transmission from an antenna, signals are rigorously defined only in terms of field strength. The correct units are volts per meter (or more often microvolts per meter). However, in many communication theory applications, it is extremely convenient to define a transmitted signal at some point in space in terms of dBm. That definition really assumes the situation shown in Figure B-1, in which an ideal unity gain antenna is located at the point in space being considered. The signal power in dBm is then the output of that ideal antenna at that location.

Inspec keywords: electromagnetic field theory; antennas

Other keywords: ether waves; electrical power; ideal unity gain antenna; signal strength; communication theory; signal power ratio; transmitted signals; field strength

Subjects: Electric and magnetic fields; Electromagnetic waves: theory