1. It appears that the s21 testing was done using the 50 ohm
source impedance of the VNA. This test doesn't really
put the amplifier through its paces. Instead, put a
small capacitor in series with the input to the amplifier.
The capacitor should be equivalent to the whip
capacitance, something like 20 pF, IIRC.
2. Phase mismatch is more likely to be the problem than
amplitude mismatch.
3. Unit 2 appears to be the odd man out in terms of phase,
looking at the s11 plot. It appears to be directly
attributable to the input filter being slightly higher in frequency.
I suspect when you remeasure s21 with the series capacitor,
you will see a phase mismatch on it. If unit 2 is indeed
the odd man out, then performance will depend on which
position it happens to be installed in. Do you always install
the amplifiers in the same positions?
4. The performance of this array is something of a crapshoot,
because it depends on the tolerances of the input filter
components. The fact that one instance of the array works well
doesn't necessarily carry over to other instances of it.
Another crapshoot occurs if you randomly move the amplifiers
around.
5. A ideal high input impedance voltage follower should
have an s21 of 6 dB. Perhaps 1 dB, being 5 dB down, is
"normal" for these amplifiers. OK if they're matched.
Rick N6RK
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