At 02:20 PM 5/11/05, Jim Lux wrote:
>So, while the main beam doesn't move much, the null does move. That moving
>null is probably the advantage of various array stacking schemes (i.e. the
>whole Top/Bottom/BIP/BOP thing doesn't move the elevation angle of the
>main lobe much (some 6 degrees in the example I modeled), but it does move
>the null (and second order sidelobes) a lot. Particularly if you want
>that nice sharp null to replace the main lobe (switching from BIP to BOP,
>for instance), 30 degrees of phase shift might result in significant changes.
>This is fundamentally why I think that the value in stacks is the fact
>that you can change things at all (in an elevation plane sense), not that
>you can actually predict it. It gives you a knob to turn quickly, and
>your ears can quickly determine which of the 4 settings works best, at
>that frequency, that time, and that configuration.
<SNIP>
>This is fundamentally why I think that the value in stacks is the fact
>that you can change things at all (in an elevation plane sense), not that
>you can actually predict it. It gives you a knob to turn quickly, and
>your ears can quickly determine which of the 4 settings works best, at
>that frequency, that time, and that configuration.
The advantage of stacks (I'm more familiar
with 3 than 2), is matching the primary lobe TOA's
of your system to the incoming signal. Nulls and
secondary lobes are of little interest since most occur
at angles well above TOA's of incoming signals.
http://users.vnet.net/btippett/terrain_&_toa's.htm (2nd graph)
Although I have 7 combinations of the 3 Yagi's,
only 3 combinations are normally used...All 3,
Bottom 2, Bottom only. In fact, once I learned
which TOA's are likely at which times in the
openings, I'm normally switching between just
2 at any given point in the opening:
Early - All 3 or Top 2 (low angles)
Middle - All 3 or Bottom 2 (moderate angles)
Late - Bottom 2 or Bottom only (high angles)
73, Bill W4ZV
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