Power your access point without running a separate power cable by using free pairs on the CAT5. A number of access point manufacturers (Lucent, Symbol, and D-Link, to name three) are now offering Power over Ethernet (PoE) add-ons for their access points. A PoE module inserts DC voltage into the unused wires in a standard Ethernet cable (pairs 7-8 and 4-5). The idea is to supply the AP's power and UTP Ethernet connectivity requirements via a single Ethernet cable. This works great in areas where you may not have power easily accessible, such as a roof. This also allows you to more easily place the AP closer to the antenna, thus reducing signal loss over antenna cabling. Ethernet signal travels well over CAT5 cable; a 2.4 GHz signal doesn't do as well over antenna cabling. Also, Ethernet cabling is much cheaper than antenna cable such as LMR400. The following hack demonstrates how to build a simple PoE module pair.
Step by Step
Figure 4-30. The completed power plug.
Figure 4-31. The DC plug is mounted in the housing.
Figure 4-32. The completed PoE modules.The DC resistance of Cat5 is about 3 ohms per 100 feet per conductor, so a 250-foot cable has at least 7 ohms resistance. Most of the time, the APs draw much less than 0.8A, so you would still be above 6V at the AP. In fact, the access points typically use linear regulators to drop the voltage down to 5V on their insides, so as long as you're giving them something better than 6V at the terminals, they're likely to work. There is a good calculator online at http://www.gweep.net/~sfoskett/tech/poecalc.html that calculates the voltage drop for a given length of CAT5. Use it to estimate how much power you need to provide at one end of your cable run in order to power your access point. Terry Schmidt |