[Elecraft] K3, KPA500 - PIN Diode Failure in KPA When K3 Subrx Turned on During Xmit?

[Ron D'Eau Claire]

Are you keeping your antennas grounded when they are not in use? It does not
take low humidity or wind or rain or snow to put an electric charge on your
antenna if it's well insulated. The action of the earth's atmosphere
maintains a huge charge between the ionosphere and the ground - almost
400,000 volts - in spite of a constant leakage current through the
atmosphere from the ionosphere to ground of over 1,500 amperes. Near the
ground the voltage gradient is close to 100 volts per yard of altitude.
Given some time, depending upon the number of free ions in the air, an
insulated antenna will lose electrons until is develops the voltage
associated with its height above ground. And then, "pow" when you connect
the antenna to a piece of equipment and electrons from the earth rush in.
The amount of current is proportional to the mass of the insulated antenna. 

Nowadays any metal high above the ground in buildings is connected to an
earth ground, but in the past that was not always so, and workers were
occasionally shocked when they touched metal even on a dead calm day. 

At least one Ham, troubled by almost constant high-level popping QRN,
finally traced the source to a large copper cupola roof on his home that was
not grounded. At night, he could see occasional flashes of sparks from the
roof to nearby grounded metal even though the air was still. Bonding them
together fixed it. 

Usually these currents are so small that you are not aware of them, but the
voltages can be enough to puncture sensitive semiconductor gates and
junctions. The most sensitive areas in most rigs are in the diodes used in
the SWR bridge at the antenna jack or in diodes used in T/R switches. Other
semiconductors in a typical rig may be more sensitive to damage, but they
are well protected by the intervening circuits between them and the antenna
connector that provide a d-c path to ground.  

If an antenna has been left floating, connect it to an earth ground for a
moment before you connect it to the rig. 

Good quality antenna switches provide a d-c path to bleed off the charge as
it accumulates. The KAT500, for example, provides such a d-c ground path
whenever it is turned off and for any antennas connected that are not in
use. 

73, Ron AC7AC



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of John K3TN
Sent: Friday, February 22, 2013 5:45 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] K3, KPA500 - PIN Diode Failure in KPA When K3 Subrx
Turned on During Xmit?

Here's the set up: K3 to KPA500 to KAT500, all Aux bus connections. Began to
have high attenuation on receive when the KPA500 was turned from STBY to
OPER. Determined by Elecraft to be a blown PIN diode, which makes sense. But
why did the PIN diode blow?

Could be that it just failed, could be I mistakenly QSYed and transmitted
with KPA at high power before KAT was tuned for new band. But I also
remember this scenario happening a few times before I noticed the RX
attentuation:

Running N1MM software in a CW contest. I was transmitting some N1MM message
and while transmitting went to hit the ESC key to do something and instead
hit the ` key, which in N1MM turns on the sub-RX in the K3.  In the heat of
a contest exchange, hard to remember exactly what happened but I think that
did cause a hard fault with the KPA. Not long after that the PIN diode was
blown.

Craig, the guy who repaired my KPA, said he didn't see how an inbound SUBRX
ON command to the K3 while transmitting cause lead to a condition where the
K3 is transmitting into the KPA while the KPA is in RX mode, but I'd figured
I'd cast a wider net to see if it triggers any possible thoughts. I'm also
cross-posting to the N1MM reflector.

John K3TN