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Re: Height above ground

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  • steveeh131047
    Bob, The answer to your question depends in part on how far the metal roof extends away from the base of the antenna, and on the take-off angles you re
    Message 1 of 10 , Sep 30, 2006
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      Bob,

      The answer to your question depends in part on how far the metal roof
      extends away from the base of the antenna, and on the take-off angles
      you're interested in. Think of the the metal roof and the sea as
      reflecting surfaces (mirrors). At the low angles of elevation you're
      probably interested in for DX (say 10 degrees or less) would the
      reflected wave from the antenna (10 degrees BELOW the horizontal) be
      reflected from the metal roof, or would it miss the edge of the roof
      and reach the sea. A bit of High School trigonometry should give you
      the answer.

      If the reflected wave will miss the roof, you're in pretty good
      shape; if not, height above the metal roof is the important factor.

      If my maths is right, at take-off angles of 10 degrees or lower, with
      the antenna 5ft above the roof, height above the sea is the
      determining height provided the roof doesn't extend more than 30 feet
      from the base of the antenna.

      But even if the "geometry" looks like low-angle signals will miss the
      roof, don't set the antenna too low or you may suffer some detuning
      effects.

      And don't forget we've only considered one direction!

      Just wish I'd got that sort of location .... any chance of a swap,
      Bob :)

      73,
      Steve G3TXQ




      --- In hex-beam@yahoogroups.com, "nasa820" <nasa820@...> wrote:
      >
      > What about installation above a metal roof building? Would it be
      > considered perfect ground? Would I be measuring the height above
      the
      > "ground" or above the roof? What about the effect 'near' a metal
      roof
      > building?
      >
      > I am looking at over 180 degrees of salt water extending out from
      > 25,000 ft to more than 75,000 feet. I'm assuming that is perfectly
      > level! I'm on a 'high bank' with the ground level about 80 feet
      above
      > the water.
      >
      > My QTH offers opportunities and challenges. The metal roof
      > potentially offers a good ground plane for a vertical, and the salt
      > water an ideal surface for a low takeoff angle. But, despite a lot
      of
      > experimenting, I've yet to really avail myself of the potential
      > benefits and get the "big kick" I think is hiding in this QTH.
      >
      > With the hex-beam, my ideas are in the direction of a relatively low
      > mount (say 5-10 feet) above the ground, but that would be 80+ feet
      > above the sea. The antenna would be about 100 feet in-shore of the
      > water line, so the question is what would have the greater
      influence,
      > the ground immediately below the antenna or the sea water in front?
      >
      > A lot of questions! Thanks in advance to the group.
      >
      > My QTH here, bottom half of the page:
      http://www.rbogash.com/ham.html
      >
      > Bob Bogash
      > W7DDD
      >
    • steveeh131047
      Bob, It depends on the geometry of your situation and the take-off angles you are interested in; a bit of High School trig will then give you an answer.
      Message 2 of 10 , Oct 1, 2006
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        Bob,

        It depends on the "geometry" of your situation and the take-off
        angles you are interested in; a bit of High School trig will then
        give you an answer. Picture the sea and the roof as two reflectors,
        and work out whether the reflected wave (coming downwards from the
        antenna) will reach the sea, or will be reflected off the roof before
        it reaches the sea.

        For example, say your antenna was 5' above the roof and the roof
        extended 30' from the base of the antenna in the direction of the
        sea. For take-off angles of 10 degrees or less the reflected wave
        will not be "shielded" by the roof and will reach the sea; so the
        effective height would be 80+. For take-off angles greater than 10
        degrees, reflection will take place from the roof so the effective
        height would be 5'.

        But don't forget that putting the antenna very low to the roof could
        result in some serious de-tuning effects. And don't forget we've only
        considered one direction!

        Sounds like a great location though ..... any chance of a swap, Bob :)

        Apologies if this is a repeat posting, the first one seems to have
        got lost.

        73,
        Steve G3TXQ


        --- In hex-beam@yahoogroups.com, "nasa820" <nasa820@...> wrote:
        >
        > What about installation above a metal roof building? Would it be
        > considered perfect ground? Would I be measuring the height above
        the
        > "ground" or above the roof? What about the effect 'near' a metal
        roof
        > building?
        >
        > I am looking at over 180 degrees of salt water extending out from
        > 25,000 ft to more than 75,000 feet. I'm assuming that is perfectly
        > level! I'm on a 'high bank' with the ground level about 80 feet
        above
        > the water.
        >
        > My QTH offers opportunities and challenges. The metal roof
        > potentially offers a good ground plane for a vertical, and the salt
        > water an ideal surface for a low takeoff angle. But, despite a lot
        of
        > experimenting, I've yet to really avail myself of the potential
        > benefits and get the "big kick" I think is hiding in this QTH.
        >
        > With the hex-beam, my ideas are in the direction of a relatively low
        > mount (say 5-10 feet) above the ground, but that would be 80+ feet
        > above the sea. The antenna would be about 100 feet in-shore of the
        > water line, so the question is what would have the greater
        influence,
        > the ground immediately below the antenna or the sea water in front?
        >
        > A lot of questions! Thanks in advance to the group.
        >
        > My QTH here, bottom half of the page:
        http://www.rbogash.com/ham.html
        >
        > Bob Bogash
        > W7DDD
        >
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