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7325 Views 7 Replies Latest reply: Apr 19, 2014 11:58 PM by papalyle RSS
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Apr 8, 2014 10:00 AM

Long distance wireless communications

I'm thinking about studying wireless communications to see if I can implement wifi or bluetooth for long ranges, around 50 to 100 miles.  Is this even theoretically possible?

  • davecuthbert Apprentice 87 posts since
    Feb 18, 2014
    Currently Being Moderated
    Apr 8, 2014 12:58 PM (in response to Riley)
    Re: Long distance wireless communications

    Given the 1 watt FCC power limit and the wide receiver bandwidth (17 MHz for 802.11b if I remember correctly) 50 miles is impractical.

     

    Given enough transmit (ERP) effective radiated power (think transmitter power and antenna gain) and a receive antenna having gain it is theoretically possible. An example is analog UHF television. With an ERP of 1 MW, a receiver bandwidth of 6 MHz, and a good receive antenna a range of 50 miles is practical. This gives us a rough idea of what it would take to achieve 50 miles with 2450 MHz WiFi.

    • papalyle Novice 55 posts since
      Apr 6, 2014
      Currently Being Moderated
      Apr 10, 2014 10:25 PM (in response to davecuthbert)
      Re: Long distance wireless communications

      Dear Sir,

       

      Sounds like a tough project!

       

      papalyle

      • davecuthbert Apprentice 87 posts since
        Feb 18, 2014
        Currently Being Moderated
        Apr 17, 2014 6:40 PM (in response to Riley)
        Re: Long distance wireless communications

        Let's be more rigorous in our analysis. This is not an exact analysis but it's much better than the previous one. Fig. 1 in the paper (link below) shows that at a distance of 50 miles (80 km), with the RX and TX antennas 20 meters above ground, and 1 kW ERP the field strength at the RX location is 15 dBuV/m. Note that this graph is for 600 MHz but we'll use it anyway at 2450 MHz.

         

        The AF (Antenna Factor) of a dipole receive RX at 2450 MHz (Wi Fi band) is 38 dB. That means that its output into 50 ohms is -38 dB relative to the field strength. For 15 dBuV/m the dipole output is 15 - 38  -23 dBuV. Converting to dBm (for a 50 ohm system) and we have -23 - 107 = -130 dBm. Let's use a RX antenna having a gain of 20 dBd. The signal we have to work with is now -110 dBm in the TX. But to comply with FCC regulations we can't have an ERP of 1 kW, it can be only 1 watt. So, our -110 dBm is not -140 dBm.

         

        Is -140 dBm enough for a WiFi link? The thermal noise floor at room temperature is -174 dBm. The noise power in the 17 MHz RX bandwidth is 72 dB more, giving noise of -102 dBm. Add to this a 3 dB RX noise figure and a requirement of 12 dB S/N ratio and the signal required to complete the link is -87 dBm.

         

        We need -87 dBm but we have only -140 dBm. We are 53 dB short. The antenna gain is already 20 dBd and an antenna with a gain of 73 dBd at 2450 MHz is not practical. Raising the antennas to 1200 meters gains about 30 dB and now the required RX antenna gain is a doable 43 dBd.

         

         

        http://www.qucosa.de/fileadmin/data/qucosa/documents/5521/data/WFMN07_II_B3.pdf

  • papalyle Novice 55 posts since
    Apr 6, 2014
    Currently Being Moderated
    Apr 18, 2014 7:36 PM (in response to Riley)
    Long distance wireless communications

    If I had to guess, I would say this was a pretty darn good estimation, All my microwave was as a military tech in the Army. 50 miles isn't so much of a problem which is my main reason for endorsing the above analysis (gut feel). In Viet Nam we shot 96 voice and data channels across 300 mies of enemy territory. I t required 10 kw of power to the antenna

    .  The antenna, parabolic dish, were 300 foot in diameter. What you are are talking about is most probably completely doable with modern technology.

     

    papalyle

     

     

     

     

     

    som

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