[TowerTalk] Tower height

[Bill Ralston]

Tom Osborne <w7why@harborside.com> wrote:

>A local is thinking of putting up a tower.  Unfortunately, to the
>N.E. is a huge hill right behind his house.  Would he be better
>off to put a tower up a little lower, with a higher angle to get
>over the hill, or go for height with a lower takeoff angle and
>hope for the best.  73

It depends. (The correct answer for any question involving RF!)

The right way to answer the question is to do some simulation using TA
(Terrain Analyzer by K6STI) or the ARRL has a program that does similar
things (I think).  You put an antenna into the actual terrain environment
you have (easily figured from a 7.5 minute topo map available from USGS),
and then see how the pattern looks at different heigths.  It shouldn't take
but a few hours to get an answer to the question. (If you are nuts like me,
you will spend 100's of hours, taking into account the terrain profile in
multiple directions, propagation statistics, world ham population
distribution, etc....)

You might be suprised how high actual radiation angles really are.  For the
most part, the answer at my QTH was "higher is better" (for DX, at least) on
all bands except 10m, and I have some hills around me.  If the hill starts
to cut out the lowest lobe of your elevation pattern, that the height is of
no value at all (and can actually hurt you, as is the case in my QTH for
10m).  If you want to mostly work U.S., I'd go with a lower antenna anyway.

If you figure out the angle to the top of the hill, here's some rough data
that should help you decide what to do:

Angle (degrees) of Elevation Pattern Peak Gain (lowest lobe) versus Antenna
Height & Band
----------------------------------------------------------------------------
---------------------------------------------------------

                           Ant Height
                 50    60    70   80   90  100
Band
40m          39    32   28   25  22    19
20m          20    17   15   13  11    10
15m          14    12   10    9    8      7
10m          10      9    8    7    6      5

For 30m, 17m, and 12m - interpolate off the above table
For 80 m, higher = better in virtually any environment!

Results are for a dipole over flat average ground. All horizontally
polarized antennas at the same height will produce similar elevation
patterns.  Local terrain will effect patterns slightly.

-- Bill N7VM
n7vm@lgcy.com
801-891-8358 work/cell
801-446-0874 home
413-254-2532 fax



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