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5dB
Omni-Directional (Doughnut-shaped response)
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Wireless networking is becoming an easy and economical alternative to running wires around your home or office. It also opens up new possibilities for connecting buildings which are up to several kilometres apart. There are currently three standards upon which wireless networking devices are built. The table summarises some of the features for each
The following discussion applies mainly to the 802.11b standard. Range You will see specifications for different brands of wireless networking devices quoting wildly different ranges. Take these claims with a pinch of salt. Unless the manufacturers have got something very wrong, or are operating at illegal power output levels, then products from different sources will all behave similarly if they are built on the same standard. There are two types of application in which wireless networking is used: internal and inter-building. Indoors Radio waves travel in straight lines and at 2.4GHz and above do not penetrate obstacles very well. Some surfaces reflect the signals quite well whilst others tend to absorb them. Water, which comprises most of you, is particularly good at absorbing the energy, so you will find that putting your hand over an antenna can reduce the signal substantially. (Your hand won't warm up very much because output power is limited to 66mW in Europe.) As a general rule 802.11b devices will usually cover a house quite well but there are no guarantees. The signal passes better through wooden floors and ceilings than through brick walls, and has no chance at all through concrete or stone. The use of an access point in the loft connected to a directional antenna pointing down from the rafters has proved an effective way to get full coverage in a typical house. For a more restrictive range the built-in antennas often work very well. Your choice of wireless network adapter may be significant. If your mini tower PC stands on the floor with it's back to a radiator, you can't a expect built-in adapter with integral aerial to work very well. Or if you tend to sit with your notebooks aerial sticking out of the left hand side and your access point is down the corridor to your right, then the computer itself will screen the signal. Try an adapter with an external aerial that can see over your keyboard. Outdoors The following table is a guide to the distances you might expect to achieve using a standard 66mW access point with a given antenna gain at both ends. A reciever sensitivity of about -83dB is assumed.
Antenna Gain Q. How does an antenna produce gain? A. By focusing the available radio energy in one direction. Q. OK, so how can an 'omni-directional' antenna have gain? A. Because it radiates around itself in a disc pattern, stealing power from above and below. The above Q&As should help you get a feel for the radiation pattern for different types of antennas. A 0dB antenna radiates equally over a complete sphere. Bearing in mind that 3dB represents a doubling of radiated power, you could imagine a 3dB directional antenna radiating its signal into one half of that sphere. A 3dB omni-directional antenna would have a radiation pattern in the shape of a sphere with a cone removed from the top and bottom. Diversity Receivers The wavelength of 2.4GHz radio waves is about 12 cm. Because the signals reflect readily off many surfaces, there will often be a pattern of patches around the room where reflected signals cancel out the direct signals leading to 'dead zones'. If your antenna is in such a dead zone you get no signal. However, for every dead zone there will be a 'double-power' zone. Wouldn't it be nice if you could have a second antenna, just a few centimetres away? There would be a good chance of this second antenna sitting in a better reception zone. A system with two aerials in this arrangement is called a diversity receiver. Most wireless devices are diversity receivers with the notable exception of PCI cards. If you need to use a PCI card, an external antenna, which you can move around to get a good signal is a good idea. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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