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fun with wifi and antennas

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Here are some things that all of you know. 802.11 series of standards from IEEE define some information and specifications for wireless networks implementation. The frequencies of the most commonly used wireless networks are:

Network      Frequency
-----------------------
802.11a       5 GHz
802.11b       2.4 GHz
802.11g       2.4 GHz
802.11n       5 GHz or 2.4 GHz

The first standard of the above does not have any significant problems with interference since its frequency is not being used very often by other devices. On the other hand, because of the extremely small wavelength of this frequency the signal cannot reach ranges greater than those of a few tens of meters. However, 802.11b/g and possibly 802.11n use the so called FHSS (Frequency-Hopping Spread Spectrum) which is used on numerous wireless devices including cordless phones, remote controls etc. FHSS can moves between various frequencies, wifi operates on DSSS (Direct Sequence Spread Spectrum) but since FHSS can change arbitrarily frequencies, 802.11b/g can have interference with some devices including some cordless phones. To avoid this, you can try changing channel of your 802.11 network since each channel has a frequency seperation of 5 MHz.

I believe everyone has built at least once an antenna to extend the range of a device. It is a really simple task that might decrease your signal noise and increase your signal strength, but depending on your construction it may also increase your possibilities of diying from cacner ;p Anyway, we only care about capturing 802.11 networks so I’m not going to discuss any consequences this might have. So, what options do we have?
Parabolic antennas and reflectors
Omni-directional antenna
Directional cantenna
If you’re about to built a cantenna since this is the most simple and cheap one, you might also need a calculator like this one. I’m sure that by now you’ll be thinking that these all are already known.. But have a look at these two:
Cell phone antenna
– Refectors using mirrors
This is my personally favorite one. I was trying something else and realized that when I was moving a mirror close to the antenna of an access point the signal was being changed. I can’t explain this without asking you to perform an experiment by yourself.. 2,4 GHz which is the electromagnetic spectrum used by 802.11 is by far a lower frequency in comparison to the visible spectrum which is both wave and particles depending on the approach. However, I believe that this reflection has to do with the silver or aluminum coat with tin used at the back of the mirrors. As the SHF (Super High Frequency) signals can get reflected on metallic surfuces. Just try this, take a mirror and place it in front of your antenna at your AP, oriented at the opposite location of your receiver (latop or desktop, or whatever). Normally, you’ll see a notable decrease of the signal. Next, try the opposite where the mirror is behind the antenna oriented to reflect the signal to your receiver.. Neat, isn’t it? :D

Anyhow, these where just some random information on wifi. If you have any more I’d be glad to hear them. :)

Written by xorl

February 3, 2009 at 16:17

Posted in fun, tips, Uncategorized

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