What is Power over Ethernet (PoE)?
Getting power up to an access point (AP) can be a real problem and a significant expense. Luckily there is a new standard for power transmission over ethernet cables, 802.3af or PoE. PoE allows the same CAT5 ethernet cable to carry both data and electrical power. PoE uses the two idle connectors to carry DC power through the cable.
The concept to remember is to keep the coax cable between the AP and its antenna to an absolute minimum. Less than three feet is best. It's easier and cheaper to send ethernet data over long runs of cable rather than microwave signals. PoE allows you to send power to the AP right through the same CAT5 cable that carries the ethernet data to the AP.
Power Over Ethernet Calculator allows you to calculate the voltage drop and required power supply for homebrew Power over Ethernet hardware.