The Antenna
Antennas must be made for the band (or frequency) that you are using: 2.4GHz (802.11b/g) or 5.8GHz (802.11a). They should use an N connector for the cable connection. The N connector is most appropriate for the high frequency signals used by 802.11x.
Choices in antenna selection include: omni vs. directional, manufactured vs. homebrew, and amount of gain.
Typically mesh nodes will use omnidirectional antennas. This is recommended for most cases. These transmit and receive equally in any direction towards the horizon. This allows the formation of arbitrary meshes without having to aim (or re-aim after a high wind) antennas. It also means that there may be "wasted" radio energy that causes interference for nearby nodes.
You may want to choose a directional (sector or beam) antenna if you are creating a long distance point-to-point link within the mesh or if you know for sure that no nodes will ever be placed on the null side of the directional antenna. Sector antennas focus all radio energy in an area of a wide angle (e.g. 60 or 120 degrees). Beam antennas focus radio energy into an even tighter beam. Directional antennas come in many types such as Yagi, Sector, Dish, and Patch and Panel.
In a very dense network the interference caused by nearby nodes using omnidirectional antennas can cause a major drop in throughput. This problem can be addressed by replacing key omni nodes with multiple nodes with sector antennas. Collectively the nodes form 360 degree coverage and are linked via their wired ethernet cables.
There are many designs on the internet for homebrew antennas that you can make from common household items and cheap hardware using simple tools. The antennas can be pretty time intensive to build and their quality will vary greatly. The easiest designs are for directional beam antennas. Homebrew omnidirectional antennas are very difficult to build.
Every antenna will include a "gain" rating. This is a measure of how much the antenna increases the signal power in the direction that the antenna points. For omnidirectional antennas a high gain means that less power is radiated "up" or "down" and more power is radiated "outwards". For a directional antenna a high gain means less power is radiated out the "back" and more out the "front" and higher directional gain also means a tighter beam. Gain is logarithmic so every increase of 3dB gain means a doubling of power.
The best antenna cable to use for microwave applications such as 802.11x is LMR-400. The antenna cable is a major source of signal loss so it is very important to use only short runs of high quality cable with N connector ends. Installing your own ends on raw cable is very time consuming and requires special tools.
For most generic mesh applications we recommend the HyperLink Technologies HGV-2409U 2.4GHz 8dbi omnidirectional antenna. This antenna features a flared base which makes it less susceptible to wind-shear than other similar antennas from other manufacturers.
Links
Mast and Mounting Hardware
Microwave signals are extremely dependent on line of site. The higher your antenna can be the more likely other nodes are to have line of sight to it.
Generally, you can use an antenna mast on a rooftop to achieve height. If you live in a tall building you may be able to just place the node in a window. If you have access to a radio tower then you can achieve the best heights.
There are several ways to mount an antenna mast on a rooftop. For flat roofs there is a great non-destructive flat roof mounting platform that is held in place by cinderblocks. There are special mounts for gables and chimneys, as well as tripod mounts.
Any hardware designed for television antenna mounting will suffice for a mesh node antenna.
We recommend the Radio Shack ratchet style Chimney Mount (cat no 15-839) and any 5 foot antenna mast typically sold for television antennas.
Grounding and Lightning Arrest
Tall metal polls on your rooftop can be a major fire hazard as well as a risk to all electronic equipment connected to the node in any way. It is important to follow proper grounding guidelines to protect your safety and your equipment.
Lightning can travel down either of the 2 conductors in the antenna cable and they can't both be directly connected to ground. In order to protect this signal from lightning you need to install an inline gas-discharge lightning arrestor and connect that to ground.
See the ARRL Antenna Handbook for tips on grounding antennas.
Outdoor Cat-5e Cable and Power over Ethernet Injector
The cable from the node into the user's house is a CAT-5 ethernet cable which also carries the node's power. Although it is more expensive, you should get special outdoor CAT-5 cable. The outdoor cable's jacket will not break down in UV light and it is filled with a waterproof gel that prevents condensation-related corrosion of the conductors. Regular CAT-5 cable will still work, but may need replacement after a few years.
In order to send power to the node over the ethernet cable you will need a power over ethernet (PoE) injector. Many power over ethernet injectors claim to provide protection against reversing the cables (so that you are sending power back into your LAN rather than up into your node) but this is not implemented on any PoE injectors that we have used. Be careful that you connect the power side to the node and the data side to your LAN.
Information on Battery /Solar Powered WAPs
From our friends Matt Westervelt in Seattle and Elektra over in
Let me give some hints for the design.
- Consider that you may have to protect the system against theft in such a situation
- Use devices that only draw a minimum of power - every watt you waste makes the system more bulky. You end up with big solarpanels and huge batteries.
But before I actually start describing the design of a solar system let me share some initial thoughts about such a desaster situation like in New Orleans: In a real pinch you dont need a solar system. If you want to provide desperately needed infrastructure during a desaster situation a solar system is luxury - if most cars and trucks are wrecked you dont have to bother about ruining a set of batteries within a month by discharging them a little too deep for a while. In a disaster (read: life or death situation) you may not need to think about where to get solar panels/ a full equipped and well designed solar system. If it takes four weeks for the mains power to be restored you could collect a bunch of charged batteries out of wrecked vehicles (in New Orleans there must be plenty of them) and connect them together. All you need is some fuses, cable and a (digital) voltmeter to find batteries in good condition. And a police guy that protects you from being shot by his colleagues for looting while you confiscate some parts from wrecks. That may sound silly but I dont think it is. A good and charged truck battery contains 1500 wh at least. Take 4 of them for each node and a system with low power AP will run for a month! And last but not least: I truly share the enthusiasm about wifi and what you can do with it. But in a desaster situation where people desperately have to call for help and need means of basic communication while landline phones and GSM fail the easier and feasible way is to resort to VHF/Shortwave Transmitters - many of such devices may be around, but I dont know how popular/widespread Citizen Band Radios nowadays really are. You dont need to set up a complicated infrastructure, just the transmitter, a battery and a simple antenna - and there you go. From a high point you ll have a range of 50 kilometers easily, and 40 to 120 channels. Now some thoughts on designing a autonomous solar system: I have no clue about the weather situation there. If you want a system with 100 % uptime you have to calculate the system with some safety margin. Such a system is consisted by: Solarpanel(s)---> Cable----> Charging Regulator (Preferably with MaximumPowerPointTracking)---> Cable ----> Fuse -----> Cable ----> Lead-Acid-Battery (Could be of Sealed-Lead-Acid-Type) Consumers (your Accesspoint, Wireless Router or whatever) will be connected to the charging regulator. Most charging regulators come with a protection that protects the battery from being discharged too much - lead-acid batteries degrade dramatically if you discharge them beneath 11 Volts. In fact this should never happen. A system that is designed to last for many years keeps the battery always at least 50 -70 % full. (Depending on battery type, 70% is a good value if you simply take a cheap battery from a car or truck) Batteries designed for solar systems can go down to 50% charge without wearing out quickly. But if you design a system for a desaster zone you can live with the fact that a battery may be worn out after a year or even within two months. Thus you may go far below this value. What you dont want is running out of power so the system switches off. Lets do a example calculation: First we want to know how much power is consumed. Preferrably we want a device that runs on DC directly, with 12 Volts that most batteries provide. (Most autonomous solar systems work at 12 or 24 volts.) Something link a Linksys WRT54 or a Meshcube. In fact most APs have a switched mode voltage regulator inside and thus will work in such a voltage range. Just open it and have a look if there are two relatively big capacitors and a inductor near the DC-Input. If they are present you have a switched mode input, maximum input voltage should be somewhat below the voltage printed on the capacitors. Usually 16 or 25 volts. The Linksys is great - it runs at any voltage between 5 and 20 volts. Although the radio could be better... Meshcube works at 8 to 20 volts, and you can plug in really good wifi cards (one to three of them!) I assume the WRT54 consumes about 7 watts - that is not measured, just a rough estimation, maybe someone comes up with measurements if this device would be the choice. We need that service 24 hours a day - so the device will draw 24h * 7 watts = 168 watthours. Lead acid batteries have approx. 90% discharging efficiency. 168 wh / 0.9 = 187 wh At 12 Volt the current in ampere would be 187 wh / 12 volts = 15.55 amperehours Now lets assume we get a bad weather situation while we harvest *nothing* from the sun because we have shitty weather with cloudy sky for one week. 15.5 amperehours/day * 7 days = 109 amperehours 109 ah * 12 Volt = 1308 watthours If we allow our battery to get discharged from 100 % to 30% charge thus consuming 70 % of the capacity in such a rare situation (depending on the area where we are going to build the system) we need 109 ah / 0.7 = 156 ah storage capacity A truck battery is available with this size. That would be the cheapest source. (Maybe you find a wrecked truck with working batteries, that the owner wants to donate or lend for a good purpose). The amount of energy that you can harvest with a solarsystem depends on the area where you are and the time of the year. A well designed system should be able to fully recharge the battery within a few days in good weather conditions while delivering power to the AP . Note that the charging efficiency is 90% - so you lose again 10% of the solar power. Usually you'll find information about the energy of the sun radiation from administrative bodies competent for weather. They collect such information over the years and can tell you what to expect for each time of the year. Simulation and calculation programs for solar systems are available, PVSOL being one commercial (and expensive) program, but it is only available for
Constructing a Low-Resource Node
While rooftop nodes are more manageable, more efficient, more durable, and more reliable, their cost of a few hundreds of dollars can be prohibitive to many communities. Community networks built on a budget may wish to make use of recycled desktop computers. This is possible with CUWiNware's CD (ISO) image.
Typically a low resource node is placed in an attic to minimize the cable run to an antenna on the rooftop. A low resource node may also be placed near a window with an antenna in a tall building or a building very close to another CUWiNware node.
Most hardware issues from the Rugged Outdoor node apply to the indoor low resource node. Here are notes on additional considerations.
CPU and Motherboard Considerations
While the CUWiNware software will operate on most 486 or better platforms, most old consumer-grade 486 and original Pentium desktop machines have such old motherboards that PCI Wireless NICs or PCI/PCMCIA bridge cards will not function properly. Check to see if your wireless card requires PCI 2.0 and, if so, whether the motherboard of the desktop you are using has a PCI 2.0 compliant motherboard.
Your options for a wireless NIC are to use a PCI wireless card, a USB wireless card, or a PCI/PCMCIA bridge card with a PCMCIA wireless NIC. It is probably easiest and cheapest to find a compatible PCI card.
It is easiest if the BIOS on your low resource node supports booting from CD-ROM. Some very old motherboards do not support this. If you can't boot from CD-ROM, it is possible to bootstrap with a special floppy disk which will then hand over control to the CD image.
An attic-based node will probably not have a keyboard attached to it. You'll want to disable the requirement in the BIOS that a keyboard be attached. Otherwise every time the node is turned off and back on it will hang on boot while it waits for a keyboard to be connected.
Other Considerations
A low resource node has many moving parts, this is what makes it less reliable. You will need a CD-ROM drive, optionally a floppy drive (if you can only boot from floppy or if you want the node to remember configuration changes across reboots), an wired ethernet NIC, and a wireless NIC. There is no need for a keyboard, monitor, mouse, or hard drive.
If you wish to reduce the amount of moving parts you can install an IDE compact flash drive instead of a CD-ROM and Floppy.
Console
n order to interact directly with the node you will need a serial console. Some messages may show up on a monitor connected to the node but the only way to interact with the node is not through the keyboard but through a terminal program on the serial port. The settings are 19200 N81. You will however need to use a monitor and keyboard to access the BIOS settings when you first set up the node.
Antenna Options
In addition to the high gain antennas that are typically used with outdoor nodes, it is possible for an indoor node that is very close to another CUWiNware node to simply use the low gain omnidirectional "rubber duck" antenna that comes with the wireless card, especially if it can be put in a window facing the other nearby node.
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