KAT100 diodes

Posted by w9cf on Jul 10, 2011; 10:23am
URL: http://elecraft.85.s1.nabble.com/KAT100-diodes-tp6567545.html

I neglected to disconnect my antenna, and a not so nearby lightning
storm fried the 1N5711 diodes in my KAT100. From reading the reflector,
this seems to be a somewhat common occurence, but I have not seen
any suggestions about circuit changes. Looking at the data sheet
for the 1N5711, it does not have a PN guard ring. While I realize I
should disconnect my antenna when not in use, sometimes I forget, and
therefore I was considering replacing the 1N5711 diodes with something
like a SD101A (Mouser part number 625-SD101A) which does have a guard
ring. My understanding is that the protection pn junction goes into
avalanche mode at a voltage below the maximum reverse voltage of the
Schottky diode protecting the Schottky junction from small ESD events.
The SD101A seems to be slower, but with otherwise similar characteristics
compared to the 1N5711, and therefore adequate to 10 meters.

IN5711, Max reverse voltage = 70 V, capacitance 2 pF, recovery time 100
ps at 5mA.

SD101A, Max reverse voltage = 60 V, capacitance 2 pF, recovery time 1
ns at 5mA.

My questions are:

1.Is this a reasonable substitution or is there a better choice?

2. If D1 and D2 go into reverse avalanche mode from an ESD event, it
could put a large negative voltage on U5, so I was thinking that in
addition, I would bridge C33 and C34 with similar diodes, which would
normally be reverse biased (and therefore forward biased if D1 and D2
go into avalanche mode, protecting U5. Is there something I am missing?

I have no experience in ESD protection, so any advice by people more
knowledgeable on whether this would be likely to help the KAT100 survive
in the future when there is a lightning storm several miles away would
be appreciated.

Thanks and 73,
Kevin w9cf