[TowerTalk] Ant Modeling (long)

[Tom Osborne]

Hi All

In looking over previous posts, there seems to be an awful lot of
credence to computer modeling of antennas.
I've seen things like "A three element beam with lengths of (n)
and spacing of (n) will give 7.13 dbd and 18.34 db FB.  How do we
know this?  Has someone built and tested every conceivable
spacing, length, height and checked them with a field strength
meter.  How about 4 elements on a 32 foot boom.  There are
hundreds of different configurations available.  Then after that
is figured out, What if the boom diameter is changed?  Do all the
readings change again.  Then how about raising it up 5 feet.  Do
all the readings change again?  It seems to me that there are
thousands of different configurations when you figure boom size,
element size, taper, element spacing, element length, ground
below the antenna, terrain, etc.  Surely each configuration
hasn't been verified manually.  But if I put in "2 elements on 20
on a 12 foot boom", I'd probably get a gain of 5.34 dbd and FB of
12 db or something like that.  It seems to me that in order to
say that such and such an antenna with a certain length of
elements and height has a certain amount of gain, you'd have to
measure 360 degrees around the antenna for side lobes, rear
lobes, etc, the do the same thing from 0 degrees to 90 degrees
360 degrees around the antenna to check the take-off angle.  
Where does the data that we take so religiously come from?  I
know we can model a 3 or 4 element antenna and move each element
a couple of inches or so and see different amounts of gain and
FB.
If these antennas are modeled at VHF frequencies, it would be
hard to make the same assumptions at HF.  It seems as there are
just too many variables to take the data as absolute.  If I
wanted to model my 20 meter beam, how would I take into effect
the fact that half of it overhangs my shack roof and the other
half is directly over one of my 40 meter verticals, and about 20
feet away in a tree is my 80 meter inverted "V."  Plus the ground
slopes to the N.E. at about a 25 degree angle for a half mile or
so.  The hill in back goes up another couple of hundred feet. 
How in the world would I ever be able to take into effect
everything that effects the antenna.  It just seems to me it's
hard to put in certain dimensions and get an absolute answer.  73
Tom W7WHY

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