> If the 1N4007 sort of did the job, probably a 1 watt or ten watt PIN
> diode would do a major part of the improvement the 1KW PIN diode does
> and would be more available.
The Microsemi UM2110 was selected for its voltage handling capability (1 KV
DC), rather than its power handling.
Recall that the very first Omni Sixes suffered from poor port isolation (~
30 dB). Then, TT began using PIN diodes until they began receiving reports
of diode failures in the T/R circuit. I believe TT then went back to the
1N4007 diodes in all subsequent production units. This occurred after
publication of the January, 1993 QST Product review. The Product review
discuss TT's problems and redesign of this circuit.
I assume TT could have used a more robust PIN diode, but at some point, a
compromise had to be struck between cost and reliability. Presumably, the
1N4007 met TT's criteria.
It would be interesting to review the T/R circuit in the Omni VII to see if
1N4007 diodes are still used. And if so, what other circuit refinements
were implemented that resulted in such excellent port isolation.
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