## Re: Height above ground

Expand Messages
• 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
• 0 Attachment
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

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
>
• 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
• 0 Attachment
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
>
Your message has been successfully submitted and would be delivered to recipients shortly.