• Its pretty hard to describe, but I have two clients in a IBSS, they each have a directional and a omni antenna on.

    The omni on one radio is mounted about 8?¡é?€?? away from the cement wall and it has a metal pipe 12?¡é?€?? behind it.
    The second radio?¡é?€??s antenna is mounted about 6?¡é?€?? off the metal rail car.

  • All:

    Is anyone else thinking "Fresnel zone"?


    Besides visual line of sight 802.11 communication requires RF line of sight. It is not enough to place your eyeball at one station and see the antenna of the other station. The Fresnel zone between the two stations needs to be clear as well.

    I hope this helps. Thanks. /criss

  • This problem sounds like Fresnel Zone blockage more so than reflection now that we have more facts. As you may not be able to change the distance try changing the frequency to see if that helps.

  • Within the 2.4 GHz ISM band I doubt the frequency can be changed enough to make a useful difference in the width of the Fresnel zone. I think our friend is trying to shoot RF energy down a 300' hallway of steel and concrete.

  • Correctomundo, Criss. 100' sounds pretty good for what he's doing! My hunch is that the pipes don't matter as much as the dang WALLS! The Fresnel Zone at 300' is about 5.5 feet in radius. 60% of that is about 3.3 feet. If the antenna is only 8" off the wall, then DING, show him what he's won, Johnny, Fresnel Zone obstruction is the answer.

    Either move the antenna at least 3.3 feet away from the walls and ceiling (probably not possible) or live with the decreased range caused by obstructing the Fresnel Zone. Third options, anyone?

  • Short of installing an additional AP in the middle of the tunnel, run leaky coax along the track.

    Been there, done that . . .


  • Thanks again all.

    Fresnel could be it.

    I have not experienced the fresnel being a impact at short distances. I have defiantly seen it be a issue at 1 plus miles.

    Am I also hearing that cement walls may cause reflections?


  • Hey guys,

    I have a tough time with this being a Fresnel zone problem. The cement is not completely obstructing the Fresnel zone. Some signal will get through the cement.

    Besides that, the tunnel he describes is almost invariably wider that 3.3 ft. The fresnel zone size next to the AP is probably closer to 6".

    Most likely this is a reflection that is out of phase with the original signal. If you need to get a point to point signal in a situation like that, use higher directional antenna and low power. The directional antennas will help reduce reflection. You don't need lots of power, so that is why you will lower the output power. High power is bad in reflective environments.

    Let us know.


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