[hpsdr] Beam Steering

[Joe Martin K5SO]

All,

To hopefully minimize confusion, let me make a few points relative to  
Frank's initial observations with the dual Mercury program.

1)  There is a big difference between a steering null and a null that  
is forced via IQ phase shifting.  Understanding exactly what that  
differences is and how it manifests itself in our observed "signal  
strengths" in the dual Mercury program is precisely what I'm studying  
at the moment.

2)  In Frank's on-the-air example, the station he was observing, WTIC,  
operates on 1080 KHz and he was using an antenna spacing of about 6.5  
meters.  That yields a D/lambda ratio of about 0.02; which is a tiny  
ratio.  From the calculated patterns I posted in the PDF file earlier  
you can see that when the D/lambda ratio is very small the resultant  
pattern is nearly omni-directional in all cases.  Thus, it is no  
wonder that the observed "steering" null is so small in his observation.

At first it is perhaps a bit confusing that even with a perfectly omni- 
directional antenna pattern (antennas colocated) we can put a  
tremendous null on the signal with the controls in our program but we  
should understand that that null is quite different from what I refer  
to as a "steering" null, i.e., a null that is associated with the  
physical directional pattern of the antenna array.  If the antenna  
pattern of the array has only a marginal null in its shape, that's  
what you'll see in terms of steering.  However, that being said, you  
can still use the "null adjust" control to put a whopping null on any  
signal you chose by shifting the phase of the IQ stream until such a  
null occurs.  As it happens, we can "steer" the antenna pattern by  
appropriate phasing of the IQ stream and we can also shift the streams  
to such a degree that interesting effects occur, including producing  
strong nulls that have nothing to do with the antenna pattern.

We (me especially, at least) have a lot to learn with regard to the  
new, powerful phasing that is now available to us in this HPSDR  
project, but that's what makes all this interesting, at least for me.   
Such dramatic phasing was not generally possible before, I think; at  
least not in ham radio systems.  I'm sure that comment will draw fire  
but it's certainly true for me, at least.

My advice to you is to play with the program and learn by using it,  
both on the air and in a controlled laboratory situation if you like.   
This is new ground for most of us hams and seeing how it can work will  
become clearer as we gain experience by using it.  The program is  
intended to be a tool by which we can do precisely that, in addition  
to serving the usual, and several, diversity reception modes of  
operation.

73,  Joe K5SO





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