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What's the approximate range achievable by a 2.4 GHz signal of 1 watt in semi-urban areas? I can see 2 km from our rooftop. Will a 2.4 GHz signal go that far, i.e. if I send WiFi signals from our rooftop is it possible to catch them on my phone 2 km away?

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The frequency and the power is not sufficient information to determine range. What is the noise floor? Do you have directional antennas? What's the path loss? What SNR does your modulation require to function? –  Phil Frost Feb 16 at 23:14
    

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This is a strange sort of question, as in reality the signal will go infinitely far (effectively) however you are really asking at what distance might a receiver be able to pick up the signal.

In testing a 2.4GHz signal with a 100mW omni antenna, the furthest distance I could receive a signal with less than 5% retries (802.11b kit) was 2 miles with a 100mW 3dB receiver. I could manage over 5 miles with a directional 10dB antenna, but had some trouble aiming it accurately.

Your phone is going to have challenges at that range, but if you have a specific location, you could use a directional antenna on your rooftop aimed at that location. Even a basic Huber Suhner running at 1W could make that work.

(disclaimer - I used to test 2.4GHz radio kit from Symbol, Telxon, Motorola and Cisco. I can't post the data tables, but more than happy to give indications of what might work)

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It's unlikely that the transceiver at the fixed location will hear the relatively tiny signal from your phone 2km away well enough to establish a link.

You need to have more power on both sides to make it work.

Even then, you might need specialized, large antennas or dishes.

Of course if there are no restrictions then you should be able to establish such a link with huge dishes on either end at a fraction of that power.

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Try using the Friis equation for questions such as this. See http://en.wikipedia.org/wiki/Friis_transmission_equation for more information.

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Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. That way, should the linked page ever change or become invalid for any reason, the answer will still be useful to visitors to Amateur Radio. –  Michael Kjörling Jun 4 at 11:12
    
Friis equation isn't going to be very helpful, since it gives us free space values, which are in this case the best-case values, and the question is about semi-urban areas which can make a significant impact to the propagation of signal. –  AndrejaKo Jun 5 at 6:41

Your question is unanswerable with out detailed information, most of which is not obtainable. There are many issues that limit the distance on 802.11 "WiFi" communications, some of which are:

  • The transmitter power of the devices AT BOTH ENDS of the link
  • The antenna system gain at both ends of the link
  • The ambient RF noise level at both ends of the link
  • The "timeout" value set in software at both ends of the link

Having higher transmit power at one end will only help if the ambient RF noise level at the OTHER end of the link is high because both ends must hear the other for communication to occur. Having a 1 watt transmitter at one end and a cell-phone at the other won't (usually) give you any better results than the 50 milliwatts or so out of a normal access point. Note also that "bi-directional amplifiers" seldom give any better response (except in the case where the amp is located at the antenna and is used to overcome feed line losses). Antenna gain is one thing under your control that WILL help out, since a high antenna helps receive as much as it helps transmit power. Better antennae make better links! The ambient RF noise level "is what it is" and is seldom under your control. The "timeout" value is the amount of time the system waits before giving up on a reply to a message and is usually set low enough that it will be the "hard limit" on how far you can make a link work. It is, with rare exception, not something that you can get access to. The actual value varies by device and almost never published for consumer devices. Experimentation is about your only way to get an answer. Beg, borrow, or buy a wireless router or access point that can connect to an external antenna. B, b, or b a high gain antenna. Connect them together and place them on a high point, turn on your wifi, turn off your Mobile Network connection, dial up Pandora and start walking. When you stop hearing music that's as far as it works.

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