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[Towertalk] 20M Yagi ?

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Subject: [Towertalk] 20M Yagi ?
From: (Bill Tippett)
Date: Tue, 16 Jul 2002 19:51:32 +0100
WD4K wrote:
>The terrain here is typical of the mid south US area. High hills with
>gradual slopes. 

        This is exactly what I's similar to NC terrain.

>The approx 800 ground height is on a flat
>top of probably 1/2 mile square, more or less. The slopes are down 2-300
>feet within 1-3 miles away. Gradual is the best way to describe it. 

        You probably confused I4JMY mentioning 800' elevation above
sea level which means nothing.  Only elevation above surrounding
terrain is important.  Reminds me of when I was in Colorado, guys would
say "No wonder you are 5200' high I would be also."  ;-)

>The near field for JA starts sloping after 100', 

        This is very good.  From NC to JA, the predominant 20M TOA 
is ~3 degrees with very little above 10 degrees.  You have little low
TOA effect from nearby terrain, so at your effective height (assume 300'
counting the tower height), the first reflection at 2 miles is at
a peak TOA of 1.7 degrees and most of your signal will be at very
low angles where it should be.  If you play with TA, it will tell
you the optimum height to maximize your TOA for the 3 degree region.

>Eu slope starts at near 300'out and carib out 1/2 mile or so. 

        This is also very good for Europe for the same reasons I
stated originally: "From here in NC, there are two predominant angles to 
Europe on 20 meters (1-3 degrees and 7-8 degrees) and a single antenna will 
probably not cover both well."  Put the lower one at 40' and it will
have a lobe centered at 7.5 degrees.  For 1-3 degree openings, either
the high one or the stack in phase will cover 1-3 degrees.  For the
Caribbean/SA, the lower one only should work fine.

>The only near level ground is due south for a mile or
>two. Understood that the modeling needs to be done with specifics so I offer
>these very "unspecific" guesstimates as generic guides for some "what if"

        The ground doesn't have to be perfectly flat.  It only needs to
be relatively flat for the distance D from your tower which determined by:

D = H / Tan X, where H = height of highest antenna used and X = desired TOA.

If you want your TOA maximized at 8 degrees, and your highest antenna is
40', the ground needs to be relative flat for only 40 / Tan 8 = 284' from
the tower base.  TA will do all this in great detail when you enter your
actual terrain, but from what you've described, you have a very good 
situation which will naturally allow:

  1.  Relatively low angles to JA where you need them, and...
  2.  Relatively high angles to EU/Carib/SA where you need them.

If your tower were located the other way around, (level area ~100'
toward EU/SA and ~300' toward JA) it would be much more complicated.  

        After describing your actual terrain, you really only need N6BV's
TA to do an excellent job of modeling...there is no need for BOP...only
BIP which it can do.  BTW, there will be an article by NS4T in the next 
Antenna Compendium about automating the entry of terrain data which
was mentioned on this reflector in May:

I had a chance to review this for N6BV and it is very slick...but I'm
not sure the database covers non-US geographies.  Entering terrain data 
is currently the only difficult part to using TA and this new process 
does it very accurately and automatically.

                                                73,  Bill  W4ZV

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