Broadcast wifi on empty tv frequencies

^{From left to right: VHF filter, converter, PC motherboard and WiFi card, power supply}

American student Ryan Guerra (Ryan Guerra) built a working device to broadcast a WiFi signal over a distance of more than 1600 meters.

The self-made design (pictured above) consists of a standard WiFi card, a computer under Linux and an Alcatel Lucent experimental converter, which lowers the frequency from 2.4 GHz (20 MHz channel width) to 563 MHz (unused TV channel, 5 MHz width). At the output to the converter is connected a regular TV antenna, which can be installed outside the house.


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The width of the radiation pattern of this antenna is about 60 ° in the horizontal plane, so that anyone who enters this sector can receive the signal. Since the channel has shrunk by a factor of four from 20 MHz to 5 MHz, the data rate is also proportionally reduced, which is natural.

As experience has shown, lowering the frequency provides better signal reception in poor visibility conditions, for example, through dense foliage of trees. In real conditions, the signal was successfully transmitted over a distance of more than a mile (1609 m), although there was no direct line of sight.

Of course, for the technology to work, a similar self-made design must be on the transmission side and on the receiving side. This may be necessary if your home is a kilometer away from the free hotspot, and you still want to work with WiFi. Then you can put the transmitter in the hotspot zone - and receive a signal from a far distance.

It should be noted that Ryan Guerra is not a simple student, but a member of a university research group that last year won a grant from the US National Science Society in the amount of $ 1.8 million to develop broadband data transmission technologies in unused areas between TV channels ( white spaces )

This is not the first attempt to use telechastoty for data transmission. For example, a year and a half ago, in the small American town of Claudville (pc. Virginia), an urban network was created on unused telefrequencies, which is considered to be the first such project in urban settings. There were other small experiments: for example, the same “lokalka” was tested on the Microsoft campus.

In such a system, the most difficult is the coordination of the frequencies of all transmitting devices so that they do not interfere with each other. If you use the native Wi-Fi standard Carrier Sense Multiple Access (CSMA), then with a large number of devices the network bandwidth will be significantly reduced. Perhaps, for "WiFi on steroids" will have to introduce new protocols (by the way, Microsoft has been working on it for a long time).