RE: [amsat-bb] Lock on Tracking

["Tom Clark, W3IWI" <w3iwi@xxxxxxxx>]

Close, but not quite:

> I can tell you how NASA and others do this at microwave (2
> ghz) and higher. They call it auto tracking. They split the
> feedhorn into four equal quadrants using what is called an
> orthogonal mode junction.  The signal strength in each quad
> is continually measured and compared to the others. The dish
> is automatically adjusted in an attempt to keep all four
> quads equal. The computer knows which way to adjust the dish
> based on which quad has the most signal.

Let's say that you label the 4 quadrant feeds up, down, left and right and
that the antenna's tracks in the up-down and left-right directions. You take
feeds up and down and form the electrical difference. When the up-down
pointing is correct, then the up minus down voltage goes thru a null and the
sign of the difference can tell you which way to move to come back to the
correct position.

Now do the same thing with the east-west pair to pint in the other axis.

Finally take all 4 feeds and add them together to generate the real signal
you want to receive.

In most cases the add & subtract networks are implemented in low-loss
microwave waveguide. The 4 feeds make three outputs (sum, l/r difference &
u/d difference) which feed three low-noise amplifiers.

73, Tom

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