[TowerTalk] Phasing Help

[Jim Lux]

At 12:50 PM 5/11/2005, Bill Tippett wrote:
>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

For folks that haven't seen the charts.
Blue is all 3
Red is bottom two
Green is bottom one

Are you sure the advantage comes from moving the main lobe, or from 
suppressing the signals (and noise) coming other angles.  The main lobe 
doesn't move a whole lot, being roughly 5 or 6 degrees TOA all the time. 
What really changes a lot between configurations is the gain at 10-15 
degrees.  For instance comparing the blue and red curves, on the main lobe, 
you're only seeing a 1 dB change in gain, which is pretty small. However, 
the gain at 11 degrees changes by some 6-7 dB, which is pretty substantial, 
especially if you're close to the noise floor, or if the guy on the other 
end is.

Likewise, comparing the red and blue curves against the green curve, the 
green pattern strongly suppresses signals coming in low (1-2 degrees).  The 
shape at 5-6 degrees isn't much different than the others, maybe peaking at 
7 instead of 5.

What really changes between the various configurations is how much other 
stuff you get, besides the main lobe.  Red adds another 5-6 band of TOAs, 
Green adds yet another 5-6 degree band.  It's more a "beam spoiling" or 
"focussing" more than a "beam moving" operation.