The W8AMZ is a 1/4 wave vertical with two radials. This means that all of the return currents are expected to go through those two wires. Except there is a third that is very appealing to the antenna - the coax shield. You might try adding more radials. That will distribute the return current among more radials, which will also reduce the return current on the outside of the coax. In addition, choking at the feed point is very important, as is choking at the shack entry point.
Quarter-wave verticals are very difficult to tame when it comes to return currents and coax. It is doable, but patience and lots of experimenting are required. The reason you see it with high power is quite simply that there is more power and the currents are much higher. Remember, the power is the square of the current, so the problem goes up exponentially with power. The KAT500 is actually no more sensitive than any other ATU to shield current problems, but we ask so much more of it than a 150 watt ATU. Getting rid of the shield currents before they get to the KAT500 and making sure the SWR on the antenna coax going to the KAT is within proper range is vital to keeping everyone happy. 73, JackB, W6FB > On Feb 14, 2015, at 6:46 PM, Jeff Hall <w...@ymail.com> wrote: > > I have several turns of the AC adapter cord through two snap on beads. I > don't have anything on the AUX cables at this time, but that is something I > should probably check. I ordered 6 of the Mix 31 Big Clamp toroids from > KF7P. For the 40m vertical I'll place one choke at the feedpoint and see if > the problem is resolved. If not, I'll place another choke at the KAT500. If > I still have a problem, another choke will be placed at the output of the > KPA500. If that still doesn't work, another choke will be placed at the > output of the K3. Once I have suppressed the common mode RF on the vertical, > I'll try replacing the string of ferrite beads on my hex beam's coax with a > Big Clamp choke at the KAT500. > -Jeff W6UX > > On Saturday, February 14, 2015 6:25 PM, Harry Yingst <hlyin...@yahoo.com> > wrote: > > > Do you have chokes on the power, AUX, and Serial Cables to the KAT500? > > > > > > From: Jeff Hall <w...@ymail.com> > To: Michael Walker <va...@portcredit.net>; Harry Yingst <hlyin...@yahoo.com> > Cc: Elecraft Reflector <elecraft@mailman.qth.net> > Sent: Saturday, February 14, 2015 9:22 PM > Subject: Re: [Elecraft] My KAT500 seems very sensitive to common mode RFI > > 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 > <va...@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@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" <w...@ymail.com> > To: Elecraft Reflector <elecraft@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 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to hlyin...@yahoo.com > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to va...@portcredit.net > > > > > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to jackbrin...@me.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
