I stumbled across an interesting idea for those not wanting to lose precious dBm as discussed in the "Watts+ Gain = How many dB" thread.
I am sure some of you have already heard about it but will post for the newbies that are craving new ideas.
POE - Power over Ethernet
In a nut shell - Instead of having your AP miles from your antenna and losing db in the wiring to the antenna, have your AP right next door and run power to it from an ethernet cable.
Out of a nut shell (??)
If you have your AP inside and run cable to your Antenna on your roof, mast, nearest tree etc you lose db across the cable. Ball park the figure is about -0.5db/m. Not alot if you are running 5 m of it. However if you are getting into the 10m or more then you start dropping valuable db. Unless you are using an over powered transmitter or a ridiculously huge dbi parabolic dish then you start to drop from the magic 36dBm limit that the NZ regs allow you to transmit at.
If you are budget limited and can only afford small dbi dishes and low lowered WLAN cards then POE could be for you.
Essentialy the power for the AP is run in the same CAT 5 cable that connects the AP to your ethernet port or switch. When the cat5 cable reaches the RJ45 port at the AP then the power wires are spliced off to the power socket for the AP. The "AP to antenna" cable then runs to the antenna, at a fraction of the original distance without POE. The loss of signal in CAT 5 is a fraction of that in tradition "to Antenna" cable. I have yet to find hard data on cat 5 loss so if you find it let me know.
You can buy POE connectors that do it for you, there are even AP's that work having the power run into it on the CAT5 cable. However there are those that like doing it themselves.
It seems the whole power over ethernet thing is really engeneering connectors that are not needed. If you are running the cable up to the roof, there is no reason not to just run some speaker type cable at the same time.
There is also the issut that POE means you have to power it from the same location that the cat5e goes to, I would rather keep the power as short as possible
All in all, an admirable solution to running power over an existing cable, but pointless when you are having to install one anyway.
At 0.5db a metre, over 5 metres thats about 2.5db loss, which when you think that 3db loss is half your power, it does seem quite a lot.
(eg, 36db is 4watts, but 33db is 2watts).
I have to run a cable about 115ft across the roof, i dont really have any way to avoid this, so i'll probably be using PoE or something.. How far *can* you run PoE?
In answer to your question I researched some more sites and found a few that agreed with each other with respect to cable loss. I can only guess that since these site are not related to each other that the data is accurate.
So yes even LMR200 losses db at about 0.5/m, thats why POE is soo attractive to those having "huge lengths" like yourself!
As far as how far POE can go, i guess some maths as to the cut off Voltage your AP will take and what sort of Volt loss across cat5 cable there is. The exact figures i dont know. I guess that gives me some thing else to look at!
If you are blasting out at over 36dbm with all your cable and connector loss then it is not really a problem. For every 10m you can save in say lmr200 by running POE on Cat5 the better.
Not to mention the costs of the cable! LMR400 is about $8 a metre at the place I got a quote from, so theres about a 20 times saving going to cat 5, more then enough to pay for the tupperware container or whatever its going into to protect it from the weather.
Typical cat5 cable is about 24 gauge, which is about 25 ohms of Resistance per 1000ft, so thats about 2.5ohm's per 100ft.
Lets take the wap11 from linksys for this example.
Using the following formular: E = IR (Volts = Amps x Resistance)
We can work out how much voltage loss im going to have over my 115ft cat5 link (or in this case 100ft link).
The wap11 requires 5 volts @ 2.5amps. So
E = 2.5 * 2.5
So E = 6.25 volts lost over my 100ft link.
Therefore i need to supply 11.25volts to the cat5 to get 5 volts out the end?
Can anyone confirm/comment on this? Maths never was my thing
On second thought..
Reading further into http://www.nycwireless.net/poe/
at the bottom there is a link to a calculater to calculate power loss with PoE. Above it says:
"Important Note! The two most important inputs for power over anything are wire gauge and amps used. Wire gauge is easy to figure out (it's either 22 or 24 for common Ethernet cable) but actual input amps is much trickier. You see, most devices list their maximum current draw, not their "normal" draw. So if your access point says "6V at 1.5A" and is 100 feet away on 22 gauge wire, you would be tempted to use a 7.2V supply. But the AP probably only really pulls in half an amp or less under normal conditions, so you should be feeding it just 6.4V. You can fry your equipment by putting too much voltage into it! So what to do? Simple, just measure the normal current draw when the device is in use. Don't have the tools? Well, then, just guess. Most devices that don't have a hard disk, Pentium 4, or other power sucker actually draw very little current, making this whole calculator less useful than you might think. Guess the actual draw by reducing the listed amps by 50% to 75%. And hope for the best!"
*Wonders if he would have fried the access point using the results of the calculations in previous post*
I'm guessing my calculations are right, but i'd have to measure how much current the AP is actually drawing, and not just blindly using what it says on the data-sheet. Also i didnt take into account using more than 1 pair of wires in the cat5.
Guess you beat me to the punch on the Cat 5 volt loss. I was about to post the very same thing.
Nice one. The more info we get out there the easier it will become for new to people to have their questions answered and this wireless idea not to seem like such a daunting task.
Voltage Loss is one thing, What about Interference?
Hey I think we'll be ok for voltage loss as our CATV run is only around 10 - 20 metres, however does the 1A output from the Linksys WAP11 power adapter cause any problem with the ethernet running over the same CATV cable?
I was gonna use 2 pair for ethernet and 1 pair for POE, just running it directly without any other components?
Regards
Dan Clark
Network Manager
Scarfies.Net Ltd
I think you are referring to cross talk or interference on the cable. I am unsure if there is any noticable effect. From what i have read i cant see any.
if i remember my electronics classes correctly cables carrying dc don't create any magnetic fields, so they wouldn't cause any interference with the other cables/pairs
The main problem is the volt drop, cat5 is only small, thus has quite a high resistance
I looked at using PoE, but figured it was easy enough to run a dc cable along with the cat5, that way i wouldn't have to boost up the dc voltage to counter-act the volt drop
DC creates a magnetic field however it is not changing rapidly like AC so I dont think this would introduce any serious interference unless you have some really dodgy dc adaptor putting out a varying voltage? Correct me if im wrong.
OK so yeah PoE is now a standard, IEEE802.3af, and is designed to run up to 100m, the same as 100BaseT. It can deliver power either on the unused pairs or the TXand RX pairs.
Now be warned, a lot of gear that supports PoE, will not work with IEEE802.3af gear, in fact you can damage it. Any new gear coming out will probally support the new standard (though some venders create there own). You can also get LAN switches that provide the power down cable from the switch, with out the need of an injector.
My two cents worth - For long haul cables, try and utilise a repeater of somesort . Possibly not practical for some scenarios depending on what you use for power source and the vicinity of that power source to the repeater.
Power over Ethernet 
Linear Mode
