Why would there be an extra reflector at the end and be a circle? couldnt they get away with just having a single pole at the end of the 4 branchs already on the antenna, or does that enable the cicular polarization sensitivity?
You could add an extra yagi reflector to each of the two beams instead of the
screen reflector. The screen works over a wide bandwidth (as long as it is big
enough) and is effective regardless of the polarization. Similarly, a loop tuned
as a reflector will work for both polarizations, though it will be more limited in
bandwidth. In fact, because loop elements will work for any polarization you
can use a quad antenna and feed the driven element in two places to get
circular polarization. Like this: http://www.homingin.com/dualfeed.html#toc
Why would they choose to use a reflector instead of the other options? Might
make a slight difference in performance. Might be required to meet some sort
of F/B ratio requirement. Might enable them to sell it for a higher price to the
military because it looks more complicated.
That's true of most antennas: we look at a lot of options and then choose one,
possibly on the basis of some parameter (and the assumption that the specifications
are accurate). Sometimes just because we like the look of it, or we get a good
deal on one, or it is within our ability to construct, when there are many other
options that would work just as well.