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In a standard Ethernet cat 5 cable there are 8 wires and only 4 are
used to make a connection. The unused set of four provides DC power to
the bridge and is called Power Over Ethernet or POE. The wiring
instructions and fabrication of the POE interface box can be
found at the following url http://www.nycwireless.net/poe
The input to this POE box is dc power and the Ethernet connection from
your computer. The DC power comes from the AP or Bridge supplied power
adapter. The output of the POE box is a cat-5 Ethernet cable spanning
up to 300 feet in length. This cable runs to your nema enclosure and
now combines the DC power and Ethernet. This therefore means that
there is only one cable going to the elevated box.
If your access point or bridge is compatible with the IEEE 802.3af
standard one simply plugs the cat-5 cable into the AP or Bridge
Ethernet connector. If you do not have a compatible AP or Bridge one
simply splits off the DC power and wires it directly to the PCB's dc
input power terminals.
The key factor regarding the decision to mount the bridge near the top
of the mast relates to the fact that at the 2.4 GHz frequency of the
802.11g WiFi bridge, a significant signal loss in the transmit and
receive modes will be experienced with a transmission line in the 60
foot range. When one compares the losses found in a standard WiFi
access point with an integrated antenna the total cable loss values
may be considered zero. This is because the antenna attached to the
access point is about 1 inch from the transceiver.
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Do not consider
installing a high gain mast top 802.11g antenna and locate the
bridge or access point down below. |
You therefore will be forced to use a coax cable at least 60 feet in
length and will therefore experience significant signal losses. These
cable losses reduce the signal energy between the radio base station
and the antenna. For example a low loss antenna cable has a loss of
0.23 dB per foot at 2.4 GHz. Standard loss cable is often closer to 1
dB per foot. For cable runs less than roughly 10 feet the default
value of 3 dB can be used assuming you are using a top quality (and
expensive) coaxial cable type. The bottom line here is why use a
coaxial cable where significant signal losses will be present versus
using a cat-5 cable to achieve the desired antenna height. The theory
supporting the significant signal loss at 2.4 GHz over a 60-foot
coaxial cable is as follows. The electrical resistance is in a cable
is the result of opposition to the movement of electrons. The power
output of a cable can be derived from Ohm's and Watt's laws when the
voltage is not alternating (DC current.) When a signal is alternating
(at, for example, 2.4 GHz) the moving electrons tend to push away from
the core of the conducting cable and move towards the outside of the
cable. This is called the skin effect. In essence, it's as though the
cable had less cross-sectional area than the area that is actually
present. Skin effect causes the current to occupy a smaller
cross-sectional area. Consequently, the relative resistance to current
flow is greater for alternating current than for direct current. This
solution works equally well irrespective of the use of a desktop
or laptop computer within your vessel.
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