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antennas

  • 25-05-2004 12:38pm
    #1
    davidclayton
    Registered Users, Registered Users 2 Posts: 1,218 ✭✭✭


    a router/ap i'm looking at has a 2 dBi gain iccle antenna,
    and you can replace it with an external 9 dBi outdoor antenna if you want.

    what is the relationship of dBi?
    is it 4.5 times stronger?
    or does it work on a logarithmic scale?
    [as with speaker wattage]

    dave:rolleyes:


Comments

  • #2
    lynchie
    Registered Users, Registered Users 2 Posts: 2,007 ✭✭✭lynchie


    Its logarithmic based the same as watts. A 30dbi antenna pumps out 1W while a 20dbi antenna pumps out 100mW.


  • #3
    davidclayton
    Registered Users, Registered Users 2 Posts: 1,218 ✭✭✭davidclayton


    i;m still confused...

    so its way more than 4.5 times
    more powerful???

    dave


  • #4
    oscarBravo
    Technology & Internet Moderators Posts: 28,820 Mod ✭✭✭✭oscarBravo


    It's not so much "more powerful" as "more focused" - antenna gain can't increase the power of the transmitter, but concentrates it into specific directions. A high-gain omni works by reducing the signal available above and below it, and increasing the signal roughly in a horizontal plane around it.

    Every 3dB of gain doubles your signal strength. A 9dB antenna is 5 times more useful (in its peak direction) than a 2dB [ 2^((9-2)/3) ] unless my understanding is way off.


  • #5
    davidclayton
    Registered Users, Registered Users 2 Posts: 1,218 ✭✭✭davidclayton


    i need the vertical signal
    [to service 3 floors]
    so it's not a good idea to get that antenna then??

    dave


  • #6
    oscarBravo
    Technology & Internet Moderators Posts: 28,820 Mod ✭✭✭✭oscarBravo


    Originally posted by davidclayton
    i need the vertical signal
    [to service 3 floors]
    so it's not a good idea to get that antenna then??
    Not if it's an omni, no. If it's a planar type of antenna, then it achieves gain by focusing the signal in a particular direction. A 9dB planar (like this one) has a beamwidth of 50x75 degrees. Mounted at the top of a building, pointing straight down, it should give you the coverage you need.

    That depends on the building's construction, of course - timber floors tend not to be a problem, but concrete is a disaster.


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