The terrain modeling program HFTA, written by N6BV and available free
with 20th edition of the ARRL Antenna Book, will plot the takeoff lobe
for various antennas over whatever terrain profile you specify. It also
has a neat feature that compares that takeoff lobe versus the optimum
takeoff angles for various DX paths as calculated by IONCAP over a full
11 year sunspot cycle. The program is quick and easy to use, and since
you live in Michigan, I assumed you had reasonably flat ground with a 3
element yagi and did some runs for you.
HFTA says that a 3 element yagi 50 feet high over flat ground will have
a broad takeoff lobe centered at 20 degrees elevation. At 70 feet high
the lobe is slightly narrower in the vertical plane with a peak at 14
degrees. Here are the predicted average relative (disclaimer .. read
the previous three words a few times) signal strengths (70 feet versus
50 feet) for the major DX headings from Michigan:
Europe 1.1 db
South Africa 2.0 db
South America 1.4 db
South Pacific 2.2 db
Japan 2.1 db
Asia 2.3 db
There are a lot of assumptions and possible errors behind these
predictions, but as averages they should give you some idea of what you
might expect. The longer paths will, of course, benefit more from the
additional height as shown above.
73,
Dave AB7E
trife@sirus.com wrote:
> Secondarily, am I likely to see much improvement in performance for the extra
> 20 feet of antenna height, particularly on 20m?
>
>
> Thanks/73
>
> Tim
> KK9T
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|