I believe Zener diodes have an internal capacitance that may render them problematic for this purpose. But - I am not sure of this.
Lee KX4TT
On Tuesday, 9 November 2021, 11:50:57 am GMT-5, Evan Hand <elhandjr@...> wrote:
Fred,
Protection of the IC5 is needed.? Seems like the BS170s are easy enough to fail, especially in a portable operation where the antenna might not be ideal.? They are also relatively easy to replace.? Better would be a way to protect the BS170s.? Maybe with Zener clamp diodes there.? That would protect from a high SWR voltage spike killing the BS170s and then taking out the 74ACT08.? Current limiting the input could help with the output shorted condition.? Both would be good things to add if portable is the goal.? The current limiting could be external to the QDX.
As with you, I have not really analyzed the issue.
73
Evan
AC9TU
Lee