Hi Mike,

Well, I'm glad you asked (for more data).  The curves are uglier, having much wider
swings than I had thought possible, and they show that much poorer isolation (than
previously thought) is possible at certain frequencies.

This week we have managed to take swept data at ZA values of 4, 15, and 19.6 deg,
and I captured trace data for all beams/pols at each ZA.  As we worked through the
process, I also wrote down the highest peak on each trace, partly as a contingency
and partly to help find the worst case(s) without having to plot up all the curves from
all 42 (so far) measurements.

BTW, many thanks to Anthony and Felix for their help and for putting in long hours
in the dome.

The worst offender (so far) turns out to be beam 3B, at a ZA of 4 deg.  See the
attached plot, which also shows curves for ZAs of 15 and 19.6 deg. Note that there
is a fair degree of similarity in the curves, especially in the regions of strong
response.

I also plotted a green dot on this graph, showing the value taken with the simpler
measurement a couple of weeks ago.  Note that it disagrees with all three curves of
the present work by roughly 5 dB, which I find to be a disconcertingly large
disagreement.  Part of this probably arises from the fact that this time, I took more
care to account for every little thing.  However, it could also signify poor repeatability
of the sliding shutter, or it could indicate that I've made a blunder somewhere.  I'll be
looking into that possibility very carefully!

Note that the worst peak value indicates a safety margin of only about 4 dB, which is
rather scary.

One thing I'd like to try is to have somebody take a long stick and poke at the shutter
while I view a live trace on the VNA, to see how sensitive the thing is.  Perhaps we can
do this tomorow (Friday), but it means that I (and one other person) would have to miss
the scheduled staff  lunchtime discussion will the APRPP folks.

I've been thinking of possible ways to improve the performance, and the only thing that
comes to mind is attaching some kind of attenuating sheet to the shutter surface, but
arranging it so that it does not cover the choke "slots".  I suppose that it would want to
be some kind of ferrite-loaded rubber or paint.

Meanwhile, enjoy.

Dana