The impact of trees on a 2.4GHz link?

Discussion in 'Wireless Internet' started by Tim G, Nov 17, 2003.

  1. Tim G

    Tim G Guest

    Hi Everyone

    I would like to connect two buildings which are no more than 500 metres
    apart to share Internet access.

    The only potential obstruction between the buildings is a single line of
    tall trees running directly across the line of sight (ie there would only be
    a single tree in the way) about half way between the two buildings. Winter
    won't be a problem but I'm worried what affect this might have when the
    leaves are out, as all the information I've found so far is a bit sketchy on
    what level of impact trees have on a 2.4Ghz signal.

    So far my research has led me to attempt some "link budget" calculations and
    I can get some promising figures (about 30 (dB?) - lots of spare signal
    strength?) using high-gain directional antennae. Would this be enough to
    "break through" the trees or are things more complicated than this?

    I'd appreciated any advice or thoughts you have as this is my first serious
    wireless project beyond plugging in an AP and hoping for the best :)

    Tim.
     
    Tim G, Nov 17, 2003
    #1
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  2. Tim G

    Rick Guest

    RF propagation can be difficult to predict when talking about
    obstructions like trees. Especially with low output devices like
    wireless LAN. Different shapes of foilage will attenuate your signal
    at different rates. (i.e. pine needles/elm leaves)

    My advice is to see if you can borrow a couple of directional antenae
    next spring and just give it a go... Good luck.
     
    Rick, Nov 17, 2003
    #2
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  3. Tim G

    Tim G Guest

    Hi Rick

    Thanks for the advice, I should have known anything involving nature would
    be unpredictable :)

    Tim.
     
    Tim G, Nov 18, 2003
    #3
  4. Tim G

    Glenn Elmore Guest

    Tim
    As has been noted, there is indeed a great deal of variation in
    transmission among different types of foliage. There's additional
    variation related to water content, precipitation collecting on them
    etc. A VERY coarse rule of thumb for a median value might be something
    on the order of .25 dB/foot of foliage. This is so coarse and general as
    hardly to be useful but perhaps it is still better than nothing.
    In addition to the average excess attenuation, there is probably
    going to be considerable variation due to moisture so just because it
    works with leaves when things are dry it doesn't mean you won't lose
    another 10 dB when it rains.
    However, if you truly only have a single tree thick obstruction on an
    otherwise cleanly LOS path, at 500 meters you probably will be in pretty
    good shape with ~20 dB gain on each end. You may actually be able to
    work with less than that but how much less is an open question any
    you'll probably just have to try it. You might be able to start with a
    single high gain antenna on the least accessible end and then add
    antenna to the other end until it is "good enough".

    Good luck

    Glenn Elmore
     
    Glenn Elmore, Nov 19, 2003
    #4
  5. Tim G

    Tim G Guest

    Hi Glenn

    Thanks for the information. The more people I 'talk' to about this the more
    obvious it becomes that it will be down to trial and error in the end.

    I had settled on 16 dB gain antennae at both ends (they look like the
    biggest and most expensive I can get away with!) but now with what you've
    said I'm tempted to put a 24 dB antenna on one end 'just in case'. Putting
    a stronger antenna at the less accessible end of the link makes a lot of
    sense, thanks for the advice.

    It will probably be a month or so before all this is in place, but I'll post
    the results of the project in here when it's finished. Although it will
    only be anecdotal information it might be of use to someone else.

    Tim.

    --
    My real e-mail address is tim218 before the at followed by supermail.org.uk.

     
    Tim G, Nov 20, 2003
    #5
  6. Hi Tim,

    I just installed a link very similar to the one your describe. I have
    two rows of 50 foot tall White Pines in the path. The overall path is
    approximately 400 meters. I am using parabolic grid antennas (15db
    gain). I was concerned about the attenuation of the trees. However,
    I have I am getting a very strong signal between my two WAP54G access
    points. Just be sure you use good antennas and low loss coax.

    I am amazed how well my link is working. I am getting no packet loss
    and 1.6 ms ping times. I am using it to stream security video to my
    house from an out buiding on my property.

    Fred
     
    Fred J Keller, Nov 26, 2003
    #6
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