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

[Vic Rosenthal 4X6GP/K2VCO]

Don't know what kind of ferrite beads you are using, but many are 
intended for VHF use and are not helpful at HF. Also, it is usually 
ineffective to just put a bead on the coax -- you should wrap multiple 
turns through a large toroid. Good ferrite materials for HF are #31 for 
1.8-7 mHz and #43 for 14-30 mHz. Either one would probably work fine for 
both antennas.

> 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

-- 
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/