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

[Jeff Hall]

Thanks everyone for their replies to my problem.  I read Jim Brown's paper this afternoon and have ordered some mix 31 Big Clamps.  I'll try putting one at each end of the 40m coax feedline (RG8X) with the prescribed number of turns.
-Jeff 

     On Saturday, February 14, 2015 4:22 PM, Michael Walker <va3mw at portcredit.net> wrote:
   

 There are some great articles on common mode chokes
http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

The other is K9YC's cookbook.
Since I bought a VNA, I have been able to play with the choking value of some random chokes.  Very interesting.
Mike va3mw

On Sat, Feb 14, 2015 at 6:57 PM, Harry Yingst via Elecraft <elecraft at mailman.qth.net> wrote:

There must be something special about 40 meters and micro-controllers.
My Aquarium Controller used to do a Reset when I transmitted on 40.The cure for me was to wrap the control cable leads through Some Snap on chokes
I just installed my KAT500 today and considered taking thepower lead and wrapping it around a snap on choke a few times.


      From: "Jeff Hall, W6UX" <w6ux at ymail.com>
 To: Elecraft Reflector <elecraft at mailman.qth.net>
 Sent: Saturday, February 14, 2015 12:48 PM
 Subject: [Elecraft] My KAT500 seems very sensitive to common mode RFI

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

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