[TowerTalk] closeness to commercial building roof top

[n4kg@juno.com]

HUH?  What do you mean by "second" reflection?

The second reflection will hit the earth AFTER if refracts from
the ionosphere, anywhere from roughly 1000 to 4000 miles away.

The antenna pattern is determined by the vector combination 
of the incident wave from the antenna and the FIRST reflection 
from the earth (or whatever reflecting surface it hits such as a 
conductive roof).  (Assuming flat terrain)

Angles of interest for DX are from 1 to 30 degrees.  If the line 1 
degree below the horizon hits ground then the effective height 
of your antenna is equal to the building height plus support height,
assuming flat ground.  If the line 30 degrees below the horizon
hits the (conductive) roof, then the effective height of the antenna
is equal to the support height (10 ft) at that angle.  

To find what angles are affected, draw a line from the center
of the antenna to the edge of the roof.  Determine the angle
that represents below the horizon.  ALL angles from that 
number and higher will be reflected from the roof and for
those angles, the effective height of the antenna will equal
the antenna support height (10 ft).

Antennas overlooking a ridge will have a double peak in
the vertical pattern.  The lowest peak will occur at the angle 
which corresponds to  an effective height equal to the tower 
support height plus the height of the ridge (or building).
 
The second peak will occur at the angle which corresponds
to an effective height equal to the tower support height.

Higher angle peaks will also occur following the usual ground
reflection patterns for an effective height equal to the tower height.

Of course, this ASSUMES the roof is conductive.  You will
need to determine the metal content and density of the roof
to know whether it is a good reflector or not.

I must admit that I am a little confused by your two posts.
In the first, you talk about a support height of 70 ft, 60 ft
from the building and 10 ft of mast.   In the second (attached)
you talk about a 70 ft tower on top of the 60 ft building for
a total support height of 130 ft.  Two VERY different scenarios.

de  Tom  N4KG

#  #  #  #  #  #  #


On Wed, 3 May 2000 09:55:59 -0500 brunet@us.ibm.com writes:
> 
> 
> 
> How important is the first reflection vs the subsequent ones.  With 
> a 60'
> building if we put the tower at 70' above the roof and centered the 
> tower
> such that the first 30 degree reflection hit the roof but the second 
> one
> hit the ground at a height of 130' from the top of the tower how 
> would that
> impact the performance?
> 
> Thanks, Pete ws4g
> 

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