At 08:04 AM 7/29/2006, Bill wrote:
>Nice response Jim. One comment regarding the statement that "The
>composite wave strength may be greater or less than that from the driven
>element alone. . ." This may be true in the sense that the field strength
>is greater in some parts of the spherical spectrum and weaker in others.
>There is still a total field strength that cannot be multiplied without
>violating the first law of thermodynamics.
yes.. I like the "squishy water balloon of constant volume" analogy for
antenna patterns. You can poke and prod it into a bizarre shape, but the
overall power remains the same.
> The design of the antenna array is to purposely distort that field so
> that there is a higher radiated strength in a chosen direction or
> directions and the trick is to understand how to do the distorting to
> achieve the desired goal. That is probably what the books with all the
> tirg were saying. Understanding fields was on of the tougher subjects,
> for me, at least, in engineering school and translates into an incredibly
> complex subject when applied to antennas.
I think it's, to a certain extent, coming up with a conceptual model that
matches how your particular brain thinks. There's a wide variety of ways to
teach/explain these sorts of things, and some click for some people, and
others click for others. It's all the more tricky when you're dealing with
something that's not directly visible or tangible. Think of the "rubber
sheet" analogy for curvature of space due to gravity.
> Fortunately finite element methods have in many ways superseded pure
> math in analyzing antennas as evidenced by the popularity of the NEC type
> programs and their ease of use for those not familiar with the complex
> math involved in field analysis. The unfortunate part of finite element
> analysis is that it does not convey the underlying principles
> involved. That is one of the reasons why your cogent comments are so
> much appreciated by old retreads like me.
One of the huge advantages of FEM codes is that they can be used to draw
gerat pictures that make things visible.. 3D rendered antenna patterns are
great for getting past the "pattern in the horizontal plane polar
plot" They also are great for qualitatively evaluating antenna designs..
you can SEE at a glance where the current is high, (esp in surrounding
But you run the risk of viewing the "conceptual model" as being something
that actually exists, when it might just be a conceptual construct that
allows to work with it. A good example is something like conceptualizing
the flow in a power/transmission line as either:
a) two waves propagating in opposite directions
b) active and reactive power each flowing in one direction or the other
Two different conceptual models for exactly the same physical reality.
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