On Thu, 26 Jun 2008 16:13:05 +0100, Steve Hunt wrote:
>I wasn't arguing that modelling is better than empirical measurement,
>but that modelling is probably better than innumerate answers. It is
>certainly a useful "first step" in developing a new antenna, and it
has
>saved me a lot of shoe leather :)
Jerry and Steve have said this rather well for us techie types, but I'm
gonna underline it for those aren't so techie.
OF COURSE actual meausrement of the performance of an antenna (or any
other system) is better than a computer model. BUT -- MEANINGFUL
measurement of an antenna is BLOODY DIFFICULT, EXPENSIVE, and TIME
CONSUMING. At a minimum, you need a controlled test range and a means
of moving an RX antenna around it in three dimensions in a controlled
way. Maybe you could hire a copter and a pilot and a test rig for a day
or two to do that? Not with my credit card!
>One other point - it seems to me that the same "unknowns" in an
>antenna's environment which might make modelling untrustworthy, might
>equally make a fellow Ham's practical experiences irrelevant. In other
>words "just because it works for him doesn't mean it will work for
me".
Again, EXACTLY RIGHT. The missing data for the model are those things
you forget about when you build the antenna, or are too complex for the
program to compute. NEC works on flat earth to produce the DIRECTIONAL
PATTERN (and field strength) for the antenna. It is VERY good at that
if you plug in all the conductive stuff AROUND the antenna. As an
example of this, take a look at pages 61-63 of
http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
which talks about an NEC model of how the pattern of an 80M vertical is
distorted by feedlines to other antennas nearby. You want to talk about
theory and practice -- soon after I moved here I got the vertical
working fine, then added a couple of more high dipoles for other bands
that ran past the vertical and up to about 100 ft. On-air performance
was a lot worse, so I suspected those new feedlines. I stuck ferrite
chokes on them near the ground, and it killed the interaction. The NEC
model predicted both the interaction and the validity of the fix. But I
had to be observant enough to plug in the spatial positions for those
other feedlines.
There is other very good software that predicts the VERTICAL pattern of
horizontally polarized antennas at varying heights over REAL terrain in
the FAR FIELD of the antenna. In other words, it tells you how YOUR
antenna is going to work differently from the FLAT LAND NEC model.
You download terrain data from USGS sites on the internet. This
software, called HFTA (High Frequency Terrain Analysis), was written by
N6BV, editor of the ARRL Antenna Book and a member of our local ham
club. He's a pretty smart guy, and the software WORKS. A bunch of our
guys (myself included) have used it to decide how high to put our
antennas depending on our terrain. We live in the mountains of CA, and
the terrain here is anything but flat. Without exception, they guys who
have used it say it gives very good answers to what happens in their
part of the real world. It tells us, for example, how our antennas will
react with a steep slope up or down from our QTH. I don't know of any
comparable software for vertically polarized antennas.
The important thing to realize here is that the difference between the
loop that the original poster asked about and a decent dipole wasn't a
lot, and you are NOT going to reliably see those differences in
ordinary ham QSOs, even with lots of them and lots of thought. To see
those differences, you need the copter, and a damned good engineer
running the experiment.
Ah, you say, why not hang both antennas and switch between them? Lots
of guys have done that, including me. My experience has been that the
difference between similar good antennas is often less than the result
of selective fading. That is, a signal may peak at one antenna while it
is dipping at another antenna some distance away.
73,
Jim Brown K9YC
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