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RE: variable phase delay / phased arrays



I'll try to answer a couple of questions at once:

Dan, N8FGV asked:

> What would be the effect of thermal noise from the surrounding
environment?
> Dish designers try to prevent the dish feed from illuminating beyond the
rim
> of the dish so that the feed only sees the cold sky. If the patch elements
> with 120 degree effective beamwidth are mounted on a plane reflector they
can
> see 180 degrees of sky and the surrounding environment, trees, buildings,
even
> the ground if the plane is tilted. Will this thermal noise be
de-correlated in
> the signal processing, or will it raise the background noise in ways that
> cannot be processed out?

The voltage outputs from the antennas gets added. the contributions from
outside the desired beam get cancelled. The sensitivity to any noise sources
where the voltages add up (i.e. in the antenna beam) increases and outside
that area it decreases.

I'll offer a thermodynamic argument: Lets place an n element antenna array
inside an isothermal enclosure (like the 2.7K Black Body background). Each
element would generate a signal proportional to the temperature of the
chamber. We add them together.  Now place a single antenna inside the
chamber; it also generates some power. If the n-element array generated more
power, then we could run an engine "pumping" power from it into the single
element. Voila! We have succeeded in developing perpetual motion!

Doug, KA2UPW asked

> Suppose I have an "n by n" patch array at my ground
> station.  Suppose I want to receive the satellite as
> it moves across the sky using my "n by n" patch array.
>
> Instead of phasing the LO to the mixer for each patch,
> could one instead use a single LO that is fed
> to each of the mixers, and do all of the phase
> processing on the signal output from the mixer (say at
> audio with DSPs) instead of transforming the phase
> of the LO? Wouldn't this be equivalent?

Certainly you could use A/D converters to phase and adjust at IF -- in fact
that's what I suggested for the spacecraft. But to properly add the signal
from the n x n array elements, you need to have n^2 A/D converters, one per
antenna. Your note mentioned "audio" so I gather you were thinking about
using a Sound Blaster as the A/D -- that's OK, but it would require n^2 (or
maybe n^2/2) boards to be plugged into the computer. I think you should
focus your thinking on other A/D approaches.

Keep the ideas coming!

73, Tom


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