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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband
