KAT100 diodes

[w9cf]

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

[Don Wilhelm-4]

  Kevin,

I cannot comment on your proposed diode substitution.  A long time ago,
the 1N5711 was chosen for the K2 wattmeters (actually for the original
diode detector in the K2) because it tested more constant with respect
to frequency than all others tested.  I would suggest that you test the
response of your proposed replacement diode over the range of 160
through 10 meters before making a decision.

I would suggest that you put a coax switch in-line between the K2 and
the antenna, one of those inexpensive 2 or 3 position switches for
CATV/GAME/VCR type switches will do the job fine, some have UHF
connectors.  That will easily allow you to switch the K2 to a dummy load.

I also highly recommend that you make certain you have a DC path across
all your antennas.  A 100 uHy choke with sufficient current carrying
capacity or a high value resistor will bleed off any static that may
develop on the feedline.

My reason for both these suggestions may not be obvious, so let me
explain.  Let's say your feedline has developed a static charge, without
a bleed resistor or inductor, that charge can remain on the feedline
long after the event that caused the static.

Now when you connect that feedline to the K2, consider that the
connector is a PL-259, so observe that the center conductor is connected
first and the shield is connected last (that is different if the
connector is a BNC or type N where the shield contacts first).  Given
that the center conductor contacts first, then entire charge of any
static that is present on the feedline will be discharged through the K2
- note than grounding the K2 will not help.  That is just a
characteristic of the UHF connector mechanism.  All the more reason to
provide a DC bleed path across each of your antennas.

73,
Don W3FPR


On 7/10/2011 6:23 AM, w9cf wrote:

> 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
>

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[w9cf]

Thanks Don,

I do plan to evaluate the protected diodes (or better diodes if someone
suggestions them) before using them.

I do have DC paths for the antennas and an antenna switch at home, but
I've been using the K2 here in Europe while traveling with temporary
antennas and an antenna switch is an extra piece of gear that I don't
want to have to carry on the airplane. While disconnecting the antenna would
certainly have kept this from happening, I don't think a lack of a DC
path on my antenna had much to do with it since my KPA100 Rev. C has a DC
path to ground at the antenna input, and I didn't disconnect the antenna.
It's funny, but even with antennas with a DC path to ground I touch the
center conductor to ground before attaching connectors.  That's probably
from years living in Arizona where its often so dry I have to do that
even when connecting phone plugs to my stereo or TV or I'm likely
to zap something.

My hope for the KAT100 is to have a belt and suspenders approach in
the future. That is, I will try to disconnect the antenna, but if I
forget, I would like to have a circuit that can take a small amount of
discharge without destroying the diodes. Since unprotected Schottky
diodes are known to be susceptible to reverse voltage ESD damage,
I thought slightly changing the circuit might improve things while
leaving the accuracy unchanged.

Thanks again and 73,
Kevin, w9cf