[Towertalk] 20M Yagi ?

Bill Tippett btippett@alum.mit.edu
Tue, 16 Jul 2002 16:49:15 +0100

I4JMY wrote:

>In case of a 200/300' real drop, and 800' hill top location, it's quite 
>possible that the first reflection is so far that the equivalent height 
>of the antenna/s from ground is 800'.

        I interpreted this to mean 800' above sea level and 2-300'
above surrounding terrain.  Assuming this, the effective height is
2-300' plus the tower height.  The immediate terrain near the 
antenna is very important and this is why there is no substitute 
for a model.  If for example you wanted a 10 degree ground reflection
and had one antenna at 40', that would occur at 214' from the tower,
for an antenna at 30' it would occur at 170', and for one at 20' it would
be at 113' (assuming flat terrain).  It might be possible to locate the 
tower on a flat area of the hilltop to achieve this.

        Both antennas out-of-phase might be another viable solution
since I believe this would also give a high takeoff angle.  I just took 
a quick look at two of my KLM's in free space and the BOP TOA peaks at
24 degrees azimuth, so it might be possible to choose a different phase 
angle to steer the maximum lobe to whatever TOA is desired.  For example,
if the top one leads the bottom by 90 degrees, the TOA peak drops to 12 
degrees.  As I said before this is a complex problem and there is no 
substitute for looking at it problem with some good models assuming the
actual terrain in question.  

                                                73,  Bill  W4ZV