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Hawking Technology, for instance, sells this WiFi amplifier. If I understand correctly, it extends your external wifi card’s antenna with a device which lets you receive a better signal. So basically it is using the same antenna as before.

From what I remember of my old physics classes, an antenna is a passive device which receives an electromagnetic signal. If the booster is connected to the same antenna as before, how could the received signal be better? For instance, I can’t see how there could be less errors: I would think that the errors had been created by obstacles the EM wave goes through on its way to the antenna : what I mean is, the signal has already been altered when it reaches the antenna, so a booster is useless in that case.

Am I wrong about that ? Am I missing something ?

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All these amplifiers do is amplify the original signal. That of course is a very simple explaination of what they actually do. Basically instead of your WiFi router getting the signal this device will recieve it then broadcast its own signal. –  Ramhound Jun 14 '12 at 11:49
    
@Ramhound my English is pretty bad but I could have figured that from their name. So the only difference is that the computer sees a higher signal power? Isn’t that completely useless? I thought the power of the signal was just a way to judge its quality (i.e. average number of errors) : if the quality is still poor then what is it good for? –  qdii Jun 14 '12 at 11:53
    
@Ramhound Wait, I’m afraid you are mistaken. The device I was talking about is not a repeater. I mean, it connects between the computer and the computer’s antenna. –  qdii Jun 14 '12 at 11:54

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Just on the link you pointed there is a "How it works" link.

enter image description here

enter image description here

If the booster is connected to the same antenna as before, how could the received signal be better?

It boosts the signal that feeds the antenna, I mean, instead of going 10 power unit(just exampling, it is actually messed on miliWatts or dbm) to the antena, the amplifier now boosts it to 20 power unit. This means that the antena now is irradiating the double of the signal is was irradiating before(in this case 3dB up).

Also, according to this reference:

A cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier (BDA) as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal amplifier and an internal rebroadcast antenna. These are similar to the cellular broadcast towers used for broadcasting by the network providers, but are much smaller, usually intended for use in one building. Modern cellular repeater amplifiers rebroadcast cellular signals inside the building. The systems usually use an external, directional antenna to collect the best cellular signal, which is then transmitted to an amplifier unit which amplifies the signal, and retransmits it locally, providing significantly improved signal strength. The more advanced models often also allow multiple cell phones to use the same repeater at the same time, so are suitable for commercial as well as home use.

Also, something about technical theory about amplifiers:

All models will include a signal amplifier. Even the cheaper home-use models (typically band selective) now provide 20dB - 50dB gain and many of the more expensive models provide over 50dB. Excellent high-power models (not home usage - smart and expensive technology of the operators) offering gain around 100dBm (ICE function is welcomed as an improvement of the radio isolation between donor and service antenna). However, since the decibel scale is measured on a logarithmic scale a 30dB gain represents a one thousandfold signal power increase - meaning the total amplification of a repeater with greater than around 50dB is likely to be useless without a good, well aligned antenna. This is due to the difficulty of filtering the correct signal out from the background noise, which will be amplified equally, and the limiting maximum signal power of the amplifier (for picorepeaters typically from around 5 dBm (3.2 mW)). Standard GSM channel selective repeater (operated by telecommunication operators for coverage of large areas and big buildings) has output power around 2W, high power repeaters (e.g., NodeG from Andrew) offering output power around 10W). The power gain is calculated by the following equation:

enter image description here

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Does it mean that, to be useful, the device should be applied both on the emitter and receiver’s antenna? –  qdii Jun 14 '12 at 13:07
    
No, the amplifier just works on the emitter side, the only way to improve the "gain" on the receiver side is changing the antenna to another more "directive" one, I mean, with more gain in db(the higher values here are better). You could use the same antenna on the emitter side to to improve link performance. Another receiver improve technique is using another radio, I mean, the lower is the receiver limiar, the better it would be. Both techniques would improve your receive gain, but this device only works on the emitter side. –  Diogo Jun 14 '12 at 13:16
    
Actually you could use an amplificator on the receiver side, but this would be another theory, I mean, to boost receiver signal you would be problems on filtering noise on reveive side. It is usually made on most of reveiver nowadays by default, it is not a manual/custumer improvement such as this booster does on amplifier side. –  Diogo Jun 14 '12 at 13:20
    
But the data goes both ways. I mean, at some point, the receiver’s antenna will have to emit data too. If the amplifier is only on the AP side, won’t there be a problem when the PC sends data not far enough? –  qdii Jun 14 '12 at 15:47
    
This is true, that is another point of your question. It makes sense when you point your receiver as an emitter. However, the equipment's site says "Increase both Transmitting and Receiving Power", so probabily it deal with the receiver boost as a pre-amplifier of the received signal that goes to the detector. –  Diogo Jun 14 '12 at 17:07

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