[Elecraft] My KAT500 seems very sensitive to common mode RFI

[Jeff Hall, W6UX]

I have a Hex Beam and a 40m wire vertical w/ two elevated radials.  I have been fighting what appears to be a common mode RFI problem which causes the KAT500 to continuously power cycle as long as the K3 is transmitting.  Generally this only occurs when driving my KPA500 amp beyond a certain output level which differs for a given band and frequency.  I am powering the KAT500 with its own AC adapter.  The problem can occur in both bypass and auto mode.

7-10 ferrite beads on the coax attached to the KAT500 solved the power cycling problem for my hex beam antenna and I successfully operate QRO in bypass mode on all bands and modes.  However the beads did not help with the wire vertical.  I can drive 300w into my dummy load with no ferrite beads on the coax and the KAT500 is fine.

The SWR is low enough I should be able to use these antennas to run the KAT500 in bypass mode.  If I put it in Auto mode the problem is exacerbated.  I can tune the wire vertical just fine with the KAT500 in Auto mode at a tuning level of 25w from the K3.  However, if I try to transmit at 5w or more (SSB for example), the KAT500 starts to power cycle.

I have no problem using the wire vertical if it is connected directly to the K3's internal ATU.  I observed that in this configuration, I could still trip the power on the KAT500 even though it's not even connected to that antenna, so I do believe this may be a common mode RF issue in the shack.  Somehow noise is finding its way into the KAT500.  Several turns of the AC power adapter's cord around a few ferrite beads hasn't helped either.

I will at some point remove the KAT500 from the equation and see how things work by directly connecting the K3 to the KPA500.  I would like to keep it in service as I might need it at the extreme band edges of the wire vertical.   

The vertical wire is from W8AMZ.  I don't believe there is any kind of current Balun inside the feed point but I did place 10 ferrite beads over the coax at the feed point.  I'm running 100' of RG8X to this antenna.  The hex beam antenna is feed with LMR-400 and RG-213.

So I don't know if the problem is specific to my vertical wire antenna and feedline or if I should suspect a defective KAT500.

Before I take the KAT500 offline, can anyone suggest some further experiments to try?

73, Jeff W6UX