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Wifi Antenna Range, Coax Cable Length, and Signal Strength?

Discussion in 'GENERAL Wireless Discussion' started by MIY4GI, Jan 31, 2011.

  1. MIY4GI

    MIY4GI New Member

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    Do any of you guys have an idea of how a wifi antenna's range, the antenna's coax cable length, and it's signal strength, are related?

    I want to connect 2 routers together using wifi, but I need to know how much gain my directional antenna must have (assuming direct line of sight), how long the coax cable can be (assuming I use LMR-400 coax), and how close the 2 routers must be (it is definitely less than 5km or 3miles)? Also, I'll be using the router's standard output power. I just want an idea of the numbers involved.

    For example, if I have a 24dbi grid antenna, 5m (16ft) of LMR-400 coax cable, and a router that outputs 1000mW of power, what wifi range can I expect if the other router has the same setup?
     
    #1 MIY4GI, Jan 31, 2011
  2. mmillard

    mmillard Junior Member
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    The values can be calculated precisely using the rigorous, widely accepted, and classic Friis transmission formula for the wireless power transfer between two antennas.

    Here's a link to the equation on Google:
    Friis transmission equation - Wikipedia, the free encyclopedia

    I would work it out for you, but frankly, I am so tired from last night's work, I'd probably make silly mistakes. Research the missing variables (like coax losses, etc.) and then plug them in. Remember, anything RF is good to about 10dB on paper. Don't expect too much from an equation. Always be conservative in your designs.

    Oh, and not sure if it's an issue or not, but Friis doesn't consider propagation delays through cables, etc.. . Get a cable that's too slow, and it might cause the data tranmissions to collide. I somewhat doubt it, but then, I'm not really paying that close attention to what it is you are trying to do. Good luck!
     
    #2 mmillard, Jan 31, 2011
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  3. MIY4GI

    MIY4GI New Member

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    People are always saying that you should keep coax cable as short as possible, but after googling a little I found that the cable losses are insignificant compared to free space loss. What's the deal with that? I only need a 20Mbps link so how strong must a link be for that?

    Also, does free space loss get split between both receiving and sending antennas? If I have say 60dbi of gain at the antenna, and the antenna I'm connecting to also has 60dbi, then what happens if the free space loss amounts to 100dbi? What about 125dbi? Would I still get a link at 125dbi (considering 125dbi is more than 60dbi + 60dbi = 120dbi)?

    Edit:

    Thanks, I had a look at http://www.random-science-tools.com/electronics/friis.htm

    That formula is very useful. What kind of data rate can I expect if I the receiving antenna get's a signal of 10dbi?
     
    #3 MIY4GI, Jan 31, 2011
  4. TelcomJunkie

    TelcomJunkie Bad Handoff Investigator
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    I haven't run the math, but I've run faster links with a lot less power. You'll be putting out over 50dbm... depending on how well you aim the grid antennas you might actually have to turn down the power a tad to avoid overloading the opposite receiver.

    To put it in perspective, right now I've got a 30mbs link running 82 miles and it's on 5.8Ghz to boot! You're lucky that you don't have to conform with the US power output regulations!
     
    Stop hovering to collapse... Click to collapse... Hover to expand... Click to expand...
    #4 TelcomJunkie, Jan 31, 2011
  5. MIY4GI

    MIY4GI New Member

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    Hehe, yeah, but I think we got regulations in SA as well.

    What setup are you using for that 30mbps link, and what are you connecting to? 82 miles is freaking far.
     
    #5 MIY4GI, Jan 31, 2011
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