Tom,
Well, you're pretty close to Tennessee. Ground wave propagation is fairly
short distances. Let's say around 30 to 150 miles or so. And when you
talk about pointing antenna's toward Europe and Japan you are talking
about long distances requiring radio wave take off and arrival angles of
around 5 to 40 degrees. What about the in between distances like
Tennessee?
Your best bet to reach those closer distances is to send your radio wave
at a very steep angle and let it bounce almost straight back to the earth.
That is called NVIS (Near Vertical Incidence Skywave). A good explanation
can be found at:
http://en.wikipedia.org/wiki/Near_Vertical_Incidence_Skywave
The 70 ft tower and OptiBeam sounds great! Wish I could afford that
system. You should be very happy with it for working Europe and Japan.
But it won't do much for reaching Tennessee as your signal will shoot
right over it. If you do a bit of research on what antennas people suggest
for using during the NAQP, a contest within North America, you'll see
many say to use lower antennas. Some even talk about lowering theirs
from 70 ft or so down to 30 ft. That gives them a higher angle of radiation
allowing for stronger signal strengths inside North America.
The low horizontal loop is a NVIS antenna that also gives some DX
capabilities and would compliment your tower plans.
You might spend some time looking at a program called HTFA. It is
offered by the ARRL. Dean Straw, N6BV. did some great work which
can predict how your proposed system will work toward reaching
different parts of the world. I found a program called MicroDEM on the
Internet and used it to generate terrain profiles which can be imported
into HTFA. I wrote an article on it and Dean put it in the ARRL Antenna
Compendium Volume 7 and has included HTFA and instructions for
using MicroDEM in the more recent ARRL Antenna Books.
Using the program I found out I don't really gain much going from
50 ft to 70 ft with my yagi. So, I opted to save time and money and
stay at 50 ft until I can go up to around 100 or 120 ft. You might find
out you need to go to 90 ft or higher to get a strong signal where
you want it to go. Or you may find 50 ft would be better. Or maybe
your proposed 70 ft is just right. If you learn to use the program it
will give you a good feel as to how you can expect your system to
perform. I've heard good things about its reliability.
Kevan
N4XL
_____
From: Tommy [mailto:aldermant at windstream.net]
Sent: Sunday, September 13, 2009 2:35 PM
To: 'knason'
Cc: secc at contesting.com
Subject: RE: [SECC TNQP N3FY
Hi Kevan,
I'm sorry but I don't think I get the connection between a NVIS antenna and
my station as it is?
At the present time I have two 40m EDZ antennas, one pointed toward Eu and
the second on pointed toward JA. In addition, I have a two element 20 m EDZ
yagi pointed into Eu. At the present time I am in the planning stages on a
70' tower with an OptiBeam 2-el 40m yagi and a 16-el tri-bander. How does a
NVIS fit into this scheme? I seem to be missing your point Kevan?
73,
Tom - W4BQF
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