Jim Lux wrote:
>At 07:53 PM 12/8/2005, K4SAV wrote:
>>The two largest factors affecting the take-off angle of a horizontal
>>dipole are the height above ground and the terrain surrounding your
>>antenna. NEC based and similar programs will give you information on
>>antenna efficiency and take-off angle over flat ground but they won't
>>handle the terrain question. The only program I know of that handles the
>>terrain is HFTA, but there may be others that I don't know about. If
>>you live in rugged terrain, or have sloping ground close by, it is
>>possible that this can have a larger effect than the height directly
>>under the antenna.
>
>Jerry brings up a very good point.. NEC will give you the antenna pattern,
>but doesn't deal with far field effects. Not only uneven terrain, but
>varying soil properties, not to mention houses, trees, etc.
>
>This too has been studied by hundreds, if not thousands, and there are
>modeling programs to deal with it. HFTA handles horizontally polarized
>antennas with uneven terrain with uniform soil properties. There are other
>codes for more complex problems. Jim Breakall used NEC-BSC from Ohio State
>to deal with the variable soil properties AND uneven terrain.
>
>I think Jerry is basically right that there aren't any programs suitable
>for amateur radio use other than HFTA for this problem. The other codes
>are either wretchedly expensive and require a lot of work to enter the
>model, or are just little pieces of the overall analysis, and you'd have a
>heap of software work in front of you to do it all.
>
If Bob (who started this thread) buys the ARRL Antenna Handbook to get a
copy of HFTA, he'll find that author N6BV approaches the whole problem
in a radically different way.
Instead of jumping straight into antenna engineering, N6BV is much more
strategic about it.
1. Which band? Where do you want to work to? So, what takeoff angle do
you NEED?
The answer to that is determined entirely by propagation - not by the
antenna at all. At any given moment, a particular propagation path from
A to B will only require ONE wave angle. However, this angle will vary
over time, and a strategic goal like "big into Europe" implies a range
of different destinations, so there will actually be a range of takeoff
angles you'll need to cover.
2. How can I deliver the most signal at that range of needed takeoff
angles? The answer to that comes in two parts:
i. The effect of antenna height and terrain.
Ii. The type of antenna.
Your location and the height of your tower are much more important than
the antenna you put on it. That isn't to say the antenna is unimportant
- quite the opposite, antenna engineering is exactly as important as we
all know it to be.
What we've been neglecting is that terrain, antenna height and your
strategic goals (the parts of the world you want to be "big into") are
all EVEN MORE important.
The idea of engineering the whole antenna system to meet the needs of
propagation is a relatively new one for amateurs. Until recently we
didn't have the tools for propagation and terrain analysis, so we have
tended to downplay how important they are. But the professionals have
known about this for generations, and now we have those tools on our
desktops. Most of the work has already been done for us in the HFTA
program and its propagation data files.
OK, we're still amateurs. We don't have unlimited resources, and most of
us aren't able to choose our station location. We all have to make
compromises. But the more limited our resources are, the more important
it is to think clearly about the very best way to use them.
Those two chapters of the latest Antenna Handbook are well worth
reading... and after doing a few runs with HFTA, reading over and over
again. It isn't easy stuff, but it'll turn your thinking completely
around.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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