[TowerTalk] Antenna Heights and EZNEC

[David Robbins]

> I like these questions....I am a "show me the real data" kinda guy.
> 
> This debate reminds me of the story from Medieval times when a group
of
> Monks debated for a couple of years how many teeth a young horse had.
Some
> argued the number was 40, other 42. A young Monk who had only been in
the
> Monastery for a few years had a epiphany when he asked the question,
> "Since
> we do not really know how many teeth a young horse has, let us go
forth,
> find a young horse and count his teeth."
> 
> The young Monk was defrocked for being too worldly.
> 
> My point: why not put a 10m antenna at 200ft in a area where the HAAT
is
> reasonable, the ground has good reflectivity and take the
measurements?
> 
> Or why don't we go and make some measurements at the stations of some
of
> the excellent stations mentioned here previously. In other words,
answer
> these well versed question of RLVZ? This would surely make a ground
> breaking article from which all could read, study, and benefit.

Unfortunately in the real world you are not going to find a station
where you could take those measurements that is not encumbered with
stuff that would make your measurements questionable.  Things like other
towers, other antennas, power lines, buildings, hills, rivers, the
ocean, etc, etc, etc.  and even if you did find a reasonable place where
someone had put a single 10m antenna at 200' with nothing around it how
would you make the measurements?  Days of helicopter flights up and down
and around and around?  Then prove the helicopter wasn't disrupting the
pattern you were trying to measure?

Now, once you have made such a single set of measurements, what do they
prove?  You get a pattern for one antenna on one tower over one specific
topography.  The worst part is that the pattern you measure would change
when the ground froze or thawed, when another antenna was put on the
tower, when antennas on the tower rotated relative to each other, when
the neighbor moved his RV, etc, etc, etc, ad nauseum per real world.

That is why so much trouble is taken to set up antenna test ranges and
why every antenna manufacturer doesn't have one of their own.  And even
if a manufacturer has a range and provides patterns for their antennas
you can not assume that the same antenna at the same height will provide
the same pattern for you since your ground and surroundings are
different.  Doing real world measurements of antenna patterns is tough,
and in the end you have just one pattern over one type of ground at one
height.

This is one of the reasons that numerical methods of modeling antennas
were developed.  With numerical models you can test the same antenna
over a variety of grounds and at a variety of heights and even add some
real world interactions with obstructs and other stuff if you have the
computer power and time to do it.  For those who really understand the
limitations of the numerical methods they are a great tool for comparing
various options for a given site... and in the real world this is really
what is the most important for most of us.  i.e. I know that a given
antenna in my terrain is predicted as having 12dbd gain in a given
direction at a specific takeoff angle... do I really care if it is 11dbd
or 13dbd, no, not really.  now if I want to know how much better or
worse that antenna will be if I move it up or down 30' I plug that into
the model and see what it says.  Can I go measure this and prove that
its right?  No, but if I know and understand the model limitations I can
believe that the antenna probably does something close to what is
predicted.  If the model says there is a null at a 15 degree takeoff
angle I can try adding another antenna and moving it up or down to fill
in the null.  Do I know that it works in the real world?  No, not for
sure, but its better than putting the single antenna up and wondering
for years why I have trouble working dx at certain times when the guy
down the road doesn't.  Can I measure it?  Not without major investments
in time and money.  Would I prove anything if I could measure it?  Only
that the numerical model was close or not for my specific case.



David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 



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