In a message dated 2/12/2001 07:24:19 Eastern Standard Time, i4jmy@iol.it
writes:
> Antenna modeling, at least to determine maximal gain (phisical of the
> array) is more accurate than any test environment.
> A test environment is a useful tool when all the external parameters
> are under control and/or some antenna "feature" is unknown (read trap
> losses) and one knows how to read what's actually measured.
> The basical limitation with modeling is the approximation of real
> devices and the unavoidable simplification of the external evironment.
> The test field limits, expecially when performed out of an anechoical
> chamber, are the existing and uncontrollable parameters (ground wave,
> side modes of propagation, reflections, etc.) or the impossibility to
> warrant short and long term calibrations (instruments) or to measure a
> resultant pattern (of direct wave and ground reflections).
> Moreover, comparing HF lobes at angles (generally zero elevation) where
> the lobe attenuation is already noticeable (to minimize reality) and
> unwawnted modes or reflections are dominant, we may easily get false
> results.
> Using an accurate antenna modeling in a free space situation we can
> know for sure what's the maximum gain obtainable by any antenna array
> and we can compare antennas in a theoretical (but real) plain field.
> Unfortunately that gain will be hardly obtained in practice and nothing
> beyond real is actually true.
>
>
(Lets change the silly subject line, let me worry about manufacturers :-)
True in some cases (mostly horizontal antennas?), but don't forget that:
1. We use antennas in the real environment and there are so many factors and
variables that influence and participate in the antenna performance, like
ground conditions, surroundings, hardware used, etc. The final say in the
antenna performance is what all those factors do to enhance or detracts from
the antenna performance. That can be only verified by real hardware models in
situation approximating final environment where the antenna is going to be
used.
2. Modeling software is getting better, but is not perfect and not gospel by
any means. Like wordprocessor never made anybody great writer, antenna
software will not guarantee great antenna designs. I have seen things happen
that are still hard to explain.
3. Free space might be fine for some abstract comparisons. Take vertical
antenna close to the ground, like quarter wave radiator with single radial
sloping on an angle. Run pattern on it in free space, average ground and over
salt water. You will see completely different pictures. Free space picture is
completely useless to consider, and you will see quite different performance,
different pattern, over poor and decent or perfect ground. Now you use that
element to form arrays, and you will get your completely different results
(and how well does that reflect reality on the beach?). As far as using free
space modeling to develop verticals (especially for salty grounds) that is of
no use, purely hypothetical.
This is how I can get close to 2 dB gain from the "no good" Quad loop.
There are some other interesting properties that EZNEC is helping to uncover.
But let me not jump the gun, I am just in process of investigating various
configurations.
Interesting difference between horizontal and vertical antennas over
average and salt water ground is that horizontal antennas will gain about
half dB, while verticals get around 5 - 6 dB boost down to horizon, which
horizontals will never make (only in free space?). Horizontals already used
the boost from ground reflections in forming their FF pattern, while
verticals get it "eaten" by poor ground, but have it "restored" by good
ground and radials.
When developing Razors I realized the importance of environment and I
tried to model the conditions around antennas as closely as I could. When the
2m models were scaled to 15m, the "error" was about 50 kHz in resonant
frequency, which I considered very good and confirmation of decent
correlation.
As far as developing real (vertical) antennas for the salt water "ground"
using free space models is useless and dangerous (could cause one to be
called "folklorist" because he tried to use real life situation vs. "software
bible" :-)
Two plus two is four, even if calculator says otherwise.
73 Yuri
da loop defender
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