[TowerTalk] [CQ-Contest] Stacking on separate towers, take off angles?

[Brian Alsop]

Guys,

I really don't know why these wildly varying multiple lobes is a big 
surprise.  Astronomers have been horizontally "stacking" antennas for 
more than 50 years.  Some of the baselines are as large as the earth's 
orbit.  Horizontal "stacking" produces an "inteferometer" effect-- that 
is multiple lobes in the azimuthal plane.  If the baseline is large, the 
angular size of the main lobe gets smaller and smaller.  Great if you're 
interested in resolving small objects in space.  However, there is the 
problem of ambiguity.  That is knowing which of the many lobes you're 
listening to.  Sure it's solvable for the astronomers.  For amateurs, 
the problem is one of putting too much power in side lobes and not into 
the main lobe.  It would take some kind of variable phasing network to 
resolve.  Radar system engineers have solved this problem many years ago 
with passively rotated and tilted arrays.

You can see a similar inteference effect by running EZNEC for a 2 meter 
horizontal antenna at 200' up.  There are a gazillion elevation lobes.


73 de Brian/K3KO

On 3/17/2013 21:36, Dan Maguire wrote:
> Although this topic has been beaten into the ground I'd like to post a clarification.  Stan Stockton and I have exchanged a few private emails and I now see how I misinterpreted his remarks.
>
> Stan proposed two towers separated by 1.5 WL.  He then suggested looking at the pattern when both antennas were rotated 45 deg off broadside.  Turns out he was thinking of *only* that particular rotation angle.  *My mistake* was making the assumption that he was interested in *all* rotation angles.
>
> If the two towers are on a North-South line, with both antennas at 1.5 WL above real/average ground (which puts the TOA at 9 deg elevation) and the towers are separated by 1.5 WL, here's a comparison of the azimuth patterns at 9 deg elevation.  The blue trace shows both antennas facing East and the red trace shows both antennas facing NE.
> http://ac6la.com/adhoc/TwoTower0vs45.gif
>
> Stan was correct in asserting that the NE facing antennas produce a pattern which is comparable to the broadside pattern.  One might even say that the pattern is better.  The max gain is down by only an insignificant 0.20 dB and there is only a single large sidelobe.
>
> To satisfy my own curiosity I then wanted to see what would happen if the spacing between the towers was changed.  In this animation the towers are still on a North-South line, both antennas are pointed NE, but the spacing between the towers is varied from 0.75 WL to 4.0 WL.  Look in the lower right corner to see the spacing (variable "S") in WL.  With most browsers you can use Esc to stop the animation and F5 to restart.
> http://ac6la.com/adhoc/Pattern_vs_Spacing.gif
>
> Another way to look at things is to plot the max gain vs the tower spacing.  In the first frame of the animation above the tower spacing is 0.75 WL (S = 0.75).  The outer ring  is frozen at 18.42 dBi and with S = 0.75 the max gain is down 3.56 dB from the outer ring, or 14.86 dBi.  In the chart below, Max Gain vs Spacing, the first data point is for S = 0.75 and the dBi value is 14.86.  Other data points show the max gain at different tower spacings.
> http://ac6la.com/adhoc/Gain_vs_Spacing.gif
>
> The S = 1.5 WL spacing is a good compromise between max gain and clean pattern.  I stand corrected.
>
> Dan, AC6LA
> http://ac6la.com/
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