Re: [amsat-bb] On phased arrays with azimuth and elevation beam forming

[Bob Nielsen <nielsen@xxxxxx>]

On Tue, Aug 29, 2000 at 09:18:58AM -0400, Bob Bruninga wrote:
> On Tue, 29 Aug 2000 Dquagliana@aol.com wrote:
> 
> > Recently I've read several (old) articles about the use of phased arrays
> > to create a steerable "beam" (in azimuth).    I'd like to build a phased
> > array system which I can switch in azimuth and elevation.   
> > 
> > Can someone point me to a reference, article, or web page which 
> > describes the theory behind changing the elevation of the beam ?  
> 
> I too have dreamed of this capability.  But it all boils down to having an
> electronic module that can change the phase of the RF signal going to each
> antenna.  So far, I have not found anything practical that we can afford.
> In Radar, they do it with chunks of ferrite delay lines, where the DC bias
> on the ferrite controls the phase delay through it.  This is OK at
> microwave frequencies where dimensions are small, but impractical at VHF
> and UHF?
> 
> To get an EME type gain (18 dB?) from a roof of say 2m dipoles, you would
> need say 32 of them...
> 
> But I am ALL EARS and would love to hear how we can do it on our bands!

It could be done at VHF with L-C phase shifters using varactors as
variable capacitors.  About 35 years ago, I worked at Hughes Aircraft
on a VHF repeater for the NASA ATS-1 satellite.  This had a cylindrical
array of 8 coaxial dipoles, which made the satellite look somewhat like
a huge squid.  We used varactors driven by an on-board computer to
despin the beam from this spin-stabilized satellite so that it always
pointed toward the earth.  The uplink was at ~149 MHz and the downlink
was at ~136 MHz.

The phase shift was done at low-level (after a preamp for receive and
before a few stages of amplification on transmit for each dipole). 
Getting all the initial phases aligned was quite a chore (we had some
crude phase-measurement instruments which were very level-sensitive).

Fortunately, Hewlett-Packard introduced the vector voltmeter shortly
before we were scheduled to deliver our hardware.  We ordered one, but
the lead time was too long for our needs (non-military programs didn't
get much priority in those days).  I was able to borrow a demo model
through the local H-P sales office and did all the alignment in a
couple of days.

A similar approach using varactor L-C phase shifters for an AZ/EL
steerable planar array is certainly feasible, but the cost and
complexity for a large number of elements in the array would probably
still favor a mechanically steered array.

73, Bob

-- 
Bob Nielsen, N7XY                          nielsen@oz.net
Bainbridge Island, WA                      http://www.oz.net/~nielsen
 
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