[Elecraft] Prevent Snow/Dirt Static damage to KPA100 after upgrade

[Kenneth A. Christiansen]

40 years ago we had a windy dust storm in Bismarck ND and I lost the RF 
amplifier in my new solid state Heathkit receiver three times that day 
before I realized the dirt and high winds were putting a static charge 
on my antenna. I found there was a capacitor between the antenna and the 
RF amp and the static would arc across the capacitor  and burn out the 
RF amp. Of course tube type receivers did not have this problem. I found 
out a 100K resistor from the antenna to ground would prevent this and 
not load down the antenna.

Several years ago my friend I each bought a new IC-746PRO. The first 
time we had a blizzard with 50 MPH winds and snow my friend and I both 
burned out our new rigs with snow static trying to check into the ND 
Weather Net. The K2 survived just fine. The IC-746PROs were both fixed 
under warranty but it still cost both of us shipping. I found out the 
IC-746PROs did not have any path to discharge the antenna charge from 
the snow static.

I recently installed the KPA100 Upgrade and one of the steps was to 
remove R10 the 100uh choke between the antenna jack and ground. There is 
no other path to ground and I have read on the reflector about people 
having their SWR and Power measuring diodes go bad and it was easy to 
see why. I plan to use this rig to check into the ND Weather net this 
winter so knew I needed to fix the problem.

MFJ on my remote tuner uses a 40 K resistor  to protect their circuit. 
That is less than the 100 K I have used over the years so it should 
protect even better.

I put in the following change to fix my problem. I bought four 10K half 
watt resistors in a package from Radio Shack for $1.99. I soldered them 
in series and soldered one end to the center pint of the antenna jack J2 
and the other end to ground. I calculate that the resistors would 
withstand a 16 to 1 VSWR and I do not think the KPA100 would be able to 
put out 100 watts to a 16 to 1 VSWR so the resistors should be safe. 40K 
should protect the diodes from any problem and under normal conditions 
should dissipate 0.2 watts.

There is an easy way to protect the amplifier without making a 
modification to the amplifier it self. Buy a PL259 T adapter and a PL259 
plug. Solder one end of the 40 K series resistor to the center of the 
PL259 and the other end to the shell. The resistors can be covered by 
tape to prevent an accidental RF burn. Screw the PL259 /40K assembly to 
one side of the T adapter. Put the other two ports of the T adapter in 
series with the coax to the antenna or the coax to the 50 ohm side of a 
tuner.

The one remaining precaution is to touch both sides of the antenna lead 
to ground just before hooking them up to discharge any static 
electricity on the antenna. This fix will not protect the equipment from 
a lightning strike but it does allow me to use my rig to check into the 
ND Weather net during a blizzard.

I hope this can save someone else from having rig trouble after a high 
wind snow or dust storm.

73 for now

Ken  W0CZ