At 11:21 AM 5/9/2005, k1ttt@arrl.net wrote:
>the 'appearance' of what they do is the reason for the names.
>
>the how is the harder part. because the elements are different lengths it
>affects both the phase and the magnitude of the current on the parasitic
>elements. the longer or shorter they are the further from resonance at
>the driven frequency they are, this affects the magnitude and phase of the
>induced current so that the elements produce fields that add in one
>direction and cancel in the other. the best way to learn about this is to
>model a 2 element yagi and watch the current in the parisitic element as
>you sweep the frequency over a wide range so that the parasitic changes
>from a 'director' to a 'reflector'... watch both the magnitude and phase
>of the current in the element relative to the driven current and figure
>out how it adds to the produce the pattern.
>
>of course the other factor that affects the magnitude and phase of the
>current is the distance from each of the other elements. this is the even
>harder part to simulate because each element's currents depend on all the
>other element's currents and the distance away each element is.
Easy (albeit tedious) to simulate, hard to conceptualize.
It's just a big matrix equation which represents the interaction of element
i with element j. (or, as in MoM codes, little tiny segment i of element n
interacting with little tiny segment j of element m. my last NEC run had
2000 odd segments, and it took about a minute to calculate all the
interactions (i.e. fill that 2000x2000 matrix with numbers). The problem is
that the raw numbers don't really provide much conceptual understanding.
There ARE some very nifty modeling programs that visualize the E and M
fields within the antenna (there's a picture on the cover of one of the
ARRL Antenna Compendia that shows the field inside a Yagi), and playing
with one of those while changing parameters DOES really help you understand
what's going on, especially if you look at losses in highly coupled (short
spacing) arrays. Makes that whole "Q" vs "Maximum Gain" thing much more
understandable. Unfortunately, these nifty programs are neither free nor
easy to use, in the general case. There ARE some really good animations
out on the web (where I can't remember the URLs) that show the energy
moving among the elements of a Yagi, etc.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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