At 10:32 PM 12/1/97, N4ZR wrote:
>
>Phil, I'm curious about the pin diode solution -- I had heard these are
>very prone to failure if the antenna SWR goes high for whatever reason -
>like absent-mindedness at 4 PM on the second day of CQWW. Is there a way
>to make them more bullet-proof? Does Ameritron do it right?
>
In general PIN diodes have a limited current carrying capacity even when
forward biased (biased ON). As the antenna system SWR increases,
particularly with a LOW Z (open circuit with a 1/4 wave of feedline, or
short 160/80 meter antennas, etc.) the current through the diode will
increase. I worked with an early high power PIN diode switch and the
ONLY failure mode was loss of the high power PIN diode when operated into
a high SWR (SWR was 3:1, amplifier was about 5 KW - don't ask) on
160 meters.
I know Tom at Ameritron learned a few things from looking at the other
PIN diode switches, including the use of multiple parallel PIN diodes
in the "amplifier out" line to handle the extra current (it's the only
critical point in the entire switch, as long as there is sufficient
cutoff bias in the "bypass" leg of the switch).
73,
... Joe Subich, W8IK ex-AD8I
<w8ik@ibm.net>
"Hindsight is an exact science."
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