I'm new to wireless field and have a question regarding RF math. How do I obtain the dB gain/loss of cables and...
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antennas? Cables and cable connectors induce signal loss, while antennas are designed to introduce signal gain. These products cite a loss or gain rating in product specs. A loss of -3 dB cuts power (mW) in half, while a gain of +3 dB doubles power. A -10 dB loss reduces power to one tenth, while a +10 dB gain increase power ten times. The actual formula is ( Pdbm = 10logPnW ), but using 3's and 10's can approximate signal gain/loss without a logarithmic calculator.
For example, suppose you have an AP that outputs 100 mW. The first -3 dB connector cuts power in half (50 mW). The second -3 dB connector cuts power in half again (25 mW). A +6 dBi antenna doubles power twice (25 x 2 x 2 = 100 mW). Cable loss, typically rated in dB per 100 feet, must also be factored in -- as must any amps, attenuators, and splitters in the path between the AP and antenna. The result of this equation is known as Equivalent Isotropic Radiated Power (EIRP): the power output at the antenna. Remember that there is also Freespace Loss introduced as the signal propagates through the air between the sender and receiver.
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