Wireless+Networking+in+the+Developing+World_Part14

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Unformatted text preview: print and be sure that your injector and wireless equipment agree on which pins and polarity should be used for power. If your wireless equipment doesn ¡ t support power over Ethernet, you can still use the unused pairs in a CAT5 cable to carry power. You can either use a passive POE injector , or simply build one yourself. These devices manually connect DC power to the unused wires on one end of the cable, and connect the other end directly to a barrel connector inserted in the device ¡ s power receptacle. A pair of passive POE devices can typically be purchased for under $20. To make your own, you will need to f nd out how much power the device re- quires to operate, and provide at least that much current and voltage, plus enough to account for loss in the Ethernet run. You don ¡ t want to supply too much power, as the resistance of the small cable can present a f re hazard. Here is an online calculator that will help you calculate the voltage drop for a given run of CAT5 : http://www.gweep.net/~sfoskett/tech/poecalc.html Once you know the proper power and electrical polarity needed to power your wireless gear, crimp a CAT5 cable only using the data wires (pairs 1-2 and 3-6). Then simply connect the transformer to pairs 4-5 (usually blue / blue-white) and 7-8 (brown / brown-white) on one end, and a matching barrel connector on the other. Mounting considerations In many cases, equipment can be located inside a building, provided there is a window with ordinary glass through which the beam can travel. Normal glass will introduce little attenuation, but tinted glass will introduce unaccept- able attenuation. This greatly simpli f es mounting, power, and weatherproof- ing problems, but is obviously only useful in populated areas. When mounting antennas on towers, it is very important to use a stand off bracket, and not mount the antennas directly to the tower. These brackets help with many functions including antenna separation, antenna alignment and protection. Stand off brackets need to be strong enough to support the weight of the an- tenna, and also hold it in place on windy days. Remember, antennas can act like small sails, and can put a lot of force on to their mounts in strong winds. When estimating wind resistance, the total surface of the antenna structure must be considered, as well as the distance from the center of the antenna to the point of attachment to the building. Large antennas such as solid dishes or high gain sectorial panels can have considerable wind load. Using a slot- ted or mesh parabolic, rather than a solid dish, will help reduce the wind load without much affect on antenna gain. Be sure that the mounting brackets Chapter 8: Building an Outdoor Node 251 and supporting structure are solid, or your antennas will become misaligned over time (or worse, fall off the tower entirely!) Mounting brackets must have enough clearance from the tower to allow for aiming, but not too much clearance that the antennas become too hard to...
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