Re: Antenna computor programs and pitfalls
- From: Jim Lux <james.p.lux@xxxxxxxxxxxx>
- Date: Fri, 09 Mar 2007 09:50:53 -0800
art wrote:
The most basic of all programs for antennas come from Roy
They do nothing but number crunching like a calculator and
will give you an answer close to what other programs provide
but not the same. The program does NOT help the user in any
way other than give you an answer regarding the performance
of what you provide.
Which, is, of course, the "sine qua non" for a *modeling* program.. it should take your model and tell you what the performance will be.
It does NOT give you any help as to where
you could benefit in any way. When you move beyond the most
basic of antenna programes you can obtain help fr4om the
programs in that you don't have to specify actual dimensions
which may be useles because you can alow those dimensions
to be variable to allow the computor to guide you in the right
direction
to meet your desires.
This is an "antenna designing" program. Many such programs will use one or more "antenna modeling" programs as part of their operation, although, it's not by any means universal.
The cost of these type programs are similar
to eznec but can go up as high as a couple of thousand dollars
tho most amateurs should be satisfied with the cheapest versions
"antenna design" is a fairly wide field, and people literally spend their life becoming good at one small part of it. There's a lot of judgement and skill in antenna design, particularly when it comes to things like mechanical/electrical tradeoffs and manufacturability. There are tools designed to address one niche or another (e.g. there's programs that are designed to optimize electrical performance microstrip patch arrays, there's programs that are designed to optimize Yagi-Udas, etc.)
Invariably, such tools are (at least originally) designed to be used by a person who will do the "higher level" trades (Do I use a rectangular or circular array of patches? What mechanical tolerances can I hold in manfuacturing? Can I hold a 5 meter by 4 meter array flat enough to actually work at Ka-band?)
There is some work on integrating all of these, but it's still baby steps (for instance, taking a Solidworks model and turning it into a meshed grid for modeling, or trying to integrate electrical, optical, and mechanical models for large dishes (e.g. IMOS))
Some programs are designed around the yagi only for simplification.
yes.. back before computers got cheap, people worked out clever analytical models for certain classes of antennas.. arrays of parallel straight thin elements would be one that's particularly amenable to such analysis. No surprise that as computers came to be more common, such models would be first ones to be implemented.
These ofcourse need to be avoided since they are based on the
yagi being unbeatable.
Not at all. it's that people had equations for Yagis (based on empirical experience that Yagis worked and met at least some of the requirements), and people tend to want to work with what is familiar. If for no other reason than you can compare the output of the modeling code (or the optimization code) with something you've actually built and see if it matches (aka validation).
So if a choice has to be made then programs
with variable dimension abilities together with a sufficient large
number of pulses are by far superior toi any other computor program.
Well, sure.. if you're going to any sort of Finite Element analysis (of which the method of moments methods are just one subset), more elements is better. But there's issues and concerns there, too: computational resources is one, roundoff and numerical precision is another. Start looking at models with hundreds of thousands of very tiny pieces, and it becomes quite the numerical analysis/computer science challenge to effectively compute it. And there are people working on it. I'm aware of several efforts to implement some MoM and FDTD codes on large (1000 processor) cluster computers. (say you want to simulate an entire ship, airplane, or spacecraft)
None of these programs agree with each other because of built in
errors
Error is the wrong word here (although technically correct), because it is perjorative and implies that there is a fundamental bug, which is generally not the case. All modeling codes are inaccurate to some degree, partly because of the limited fidelity of the model input (surely you don't want to spend the time to put in the actual atomic composition of the elements) and partly because of a deliberate tradeoff between speed and uncertainty (most people would rather have an answer in a few minutes accurate to 1% than an answer next week accurate to 0.01%)
As a side point some programs provide errors because the user
doesn't understand the thinking behind garbage in and garbage out
because there is no oversight with respect to programmers error.
This is true of any modeling code. Better codes DO some reasonableness checks for nonphysical structures and such. But, just like using a chainsaw to saw down trees more rapidly than using a handsaw, there's some assumption that the user has some skill.
.
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