Using outdoor antenna with AP - should I leave take off indoor antenna #2?

Discussion in 'Wireless Internet' started by MonkeyOmen, Sep 15, 2004.

  1. MonkeyOmen

    MonkeyOmen Guest

    I've got a 3Com 7250 802.11g access point, which comes with two small
    5" antennas that screw into the unit.

    I also bought an 8 dBi omnidirectional outdoor antenna and 20'
    low-loss cable so I could mount the antenna outdoors. This
    antenna/cable replace one of the two original 5" antennas.

    Should I leave the 2nd small antenna attached, or remove it? Why or
    why not?

    Thank you.


    Also, while I'm here, one other question, about antenna gain and
    range.

    My understanding is that a 3 dBi increase indicates a doubling of
    power, but effective power drops as the square of distance. To double
    range you need 4x the power at the antenna, or a 6 dBi gain. Is this
    correct?
     
    MonkeyOmen, Sep 15, 2004
    #1
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  2. If you have no need for indoor coverage, then remove the 2nd antenna.
    If you need coverage in an area best covered by the 2nd antenna, leave
    it connected.
    Most current access point software has a setting to enable or disable
    one antenna.
    A few access points are really weird with how they handle the
    diversity antennas. Some will receive on both antennas but only
    transmit on one. If you start getting weird results, switch to the
    other antenna port.
    Correct. 3dB increase in radiated power will double the coverage area
    (area of the circle in square feet), but only increase the range
    (radius) 1.4 times. 6dB increase in radiate power will give 4x the
    coverage area and double the range. Think:
    Area=Pi*Radius^2

    Drivel: Note that for every increase in antenna gain in one
    direction, you take away signal from some other direction. Antennas
    to not manufacture additional RF. Only transmitters do that. The
    "taken away" part can be in the up and down direction, which is
    usually useless, so there's an obvious benifit. However, if you
    happen to be a user in these areas, you're got a problem. This is
    what happens when someone puts a very high gain omni antenna on the
    roof, and finds that they can no longer connect from directly below
    the antenna. All the signal is going towards the horizon.
     
    Jeff Liebermann, Sep 15, 2004
    #2
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  3. MonkeyOmen

    MonkeyOmen Guest

    Thanks for the response, and for devoting so much of your time to
    answering questions on this newsgroup. Your posts are very
    informative.
     
    MonkeyOmen, Sep 16, 2004
    #3
  4. Thanks. Sometimes, my answers are amazing accurate.

    I managed to forget one obscure point. If you're using WDS (wireless
    distribution something) to make your access point act as a store and
    forward repeater, do NOT use both antennas. Disconnect one of them.
    The problem is that the diversity antenna juggling algorithm
    apparently works on the basis of the last successfully received
    packet. With WDS, the packets can alternate between antennas as it
    receives on one antenna and forwards on the other. The result is the
    worlds worst performance because of the intentional delay between
    switching antennas.
     
    Jeff Liebermann, Sep 16, 2004
    #4
  5. MonkeyOmen

    Mike Guest

    Hi Jeff,

    Thanks for the great info! I have a Cisco AP350 with a T1 coming into my
    house. I would like to provide access for my neighbors. What do you
    recommend for an outdoor antenna? My house sits on a high spot, but the
    problem is a lot of tall tree and thick woods between houses.

    TIA,
    Mike
     
    Mike, Sep 17, 2004
    #5
  6. Dunno. The best I can do is offer some general clues and rhetorical
    questions.

    Mini rant: No numbers = no specific answers.

    What are the distances involved?
    How much difference in vertical elevation?
    Is this going to be a neighborhood LAN (everyone joins in) or are you
    going to do this point to point, one house at a time?
    Is price an issue?
    Are there wired alternatives?

    Trees and woods are a serious problem. 2.4Ghz does not go through
    anything with water inside. If you have a view of the *TOPS* of the
    trees, you can possible mount the client radios (for the neighbors) in
    the tops of the trees, and run PoE (power over ethernet) to the
    treetop. The major problem with this is maintenance. Climbing a
    100ft tree, in the middle of a storm, to remove an ant colony, was not
    my idea of fun. Basically, you need to get line of sight somehow.
    Perhaps this would be a good excuse to do some tree thinning or
    logging.

    I helped with a neighborhood LAN similar to yours, where the view from
    the central hilltop was nothing but tree tops. Looking from above,
    that's all you see. However, trees tend to not have branches at
    ground level, so there was a few between the trunks at ground level.
    The access points were moved to ground level and located so there was
    some semblence of line of sight between the tree trunks. I had to use
    multiple access points on the central hub to cover all the neighbors,
    but it did work.

    Another way of going through trees is to use 900MHz. We had quite a
    bit of experience with Metricom in the area. 900MHz is blocked by
    trees, but nowhere as bad as 2.4GHz. The problem is that 900MHz
    equipment is difficult to find. The available speeds are also limited
    to about 1.5-3.0Mbits/sec depending on manufacturer. The band is also
    full of interference from other devices. The only currently
    manufactured 900MHz hardware I know about is Motorola Canopy system.

    One thing you shouldn't do is install a very high gain omnidirectional
    antenna. The problem is that the vertical radiation angle is very
    narrow. From even a modest hill, you will be sending the signal over
    everyone's head. Think multiple access points, sectored antennas,
    panels, dishes, or anything directional.

    I live in an overgrown redwood/fir/oak forest. We have a crude 2.4GHz
    neighborhood LAN that barely works through the trees. However, we
    also have a buried coax and fiber distribution system that works well
    enough. You can go about 1000ft point to point with CAT5 or coax
    between two switched ports at 10baseT-HDX. Some drivel I wrote on the
    subject:
    http://www.google.com/groups?selm=
    http://www.google.com/groups?selm=
     
    Jeff Liebermann, Sep 17, 2004
    #6
  7. MonkeyOmen

    Mike Guest

    Thanks Jeff for the excellent response. Sorry for not providing more info.
    Please see above.

    Thanks again.

    Mike
     
    Mike, Sep 17, 2004
    #7
  8. Please don't reply to posting like that. Put your answers on seperate
    lines so I don't have to surgically seperate my drivel from your
    answers.

    One mile is a bit rough though the trees. See:
    http://trevormarshall.com/lapierre.htm
    for how one person did an 1100ft link using high gain antennas through
    the trees. Despite the increased gain, he still was only able to get
    33% signal strength.

    50 ft of vertical elevation is not going to make much difference at
    5,280 ft. Therefore, you don't have to worry about downtilt or
    beamwidth.

    A neighborhood LAN usually requires an omni antenna or multiple radios
    with panel (patch or sector) antennas. However, at one mile, you're
    close to the limits of what can be done with cheap commodity routers.
    I suggest a 12dBi omni antenna on your roof, your access point (not
    router) mounted at the antenna to eliminate lossy coax, and panel or
    patch antennas with 8dBi gain or more at the neighbors. However, if
    there are thick trees in the way, and no line of sight, forget it. It
    won't work and if it does work initially, it won't stay working when
    the trees get wet or grow leafs.

    If DSL is coming, you're going to have a difficult time getting the
    neighbors to pay for the hardware. Little things add up quickly with
    wireless. We had the same problem in my area, where SBC kept
    promising DSL, but letting the delivery date slide. We could sell
    WISP service, but only to those that didn't know that DSL was coming.

    Running your own wire is an alternative. It's not that difficult in
    rural areas. Forget it in the cities.
     
    Jeff Liebermann, Sep 18, 2004
    #8
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