火曜日, 12月 27, 2005

RF power





















The transmitting power output is estimated at two different points of a wireless system. The first point is called the intentional radiator (IR). IR includes the radio transmitter and all cabling and connectors but excludes the antenna used. The second point is the power actually irradiated from the antenna, designated as the equivalent isotropically radiated power (EIRP). Both IR and EIRP outputs are legally regulated by the FCC in the US(see part 47 CFR, chapter 1, Section 15.247) . To measure both power and the emitted energy and the recieving sensitivity of your wireless device, watts (more often mW or decibels). Power gain caused by antennas and amplifiers as well as power loss caused by distance, obstacles, electrical resistance of cables, connectors, lighting protectors, splitters, and attenuators is estimated in decibels or, to be more precise, dBm. The m in dBm signifies the reference to 1mW: 1 mW = 0 dBm. Antenna power gain is estimated in dBi (i stands for isotropic), which is used in the same way with the dBm in RF power calculations. Decibels have a logarithmic relationship with watts: PdBm = 10log pmW. In simple terms, every three dB change would double or halve the power and every 10dB difference would increase or decrease the power by an order of magnitude. The receiving sensitivity of your wireless devices will be affected in the same way. To calculate the EIRP value of your wireless kit, simply sum all your dBm values of devices and connectors involved. For example a standard wardrivers rig consisting of 20 dBm (100mW) PCMCIA client card, 2 dBm loss long pigtail connector, and 5 dBi gain magnetic mount omnidirectional antenna would have 20-2+5=23 dBi or 200 mW power output. Note that each 6 dBi increase in EIRP doubles the transmission or reception range (so called 6 dB rule).

online RF power calculators
zytrax, eccommwireless, cgsnetwork, vwlowen, satcomresources.

However, if you deal with wireless networking on a regular basis, it is vital to familiarize yourself with RF power calculations and be able to perform basic calculations of mW /dBm conversions and EIRP output in field conditions without any tools or tables available.

Wi-Foo 3:33-34

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