K2 - Unstable power output on 40m (digital modes)

Posted by Brian - N5BCN on Oct 25, 2015; 5:45am
URL: http://elecraft.85.s1.nabble.com/K2-Unstable-power-output-on-40m-digital-modes-tp7609533.html

I'm having an issue with unstable power output using digital modes on 40m above about 20 watts.  All other bands seem to be ok.

My antenna setup is as follows: a loop consisting of about 245 feet of wire, feed line consists of about 21 feet of 450 ohm ladderline to an Elecraft BL2 balun, then about 3 feet of coax to KAT100/KPA100/K2.  I've tried the balun in 1:1 and 4:1 configs with no apparent change.  The feed point of the loop is in the attic above the shack, but the majority of the wire is outside running along a wood fence.

No problems tuning any band including 40m.  When using digital modes, power output and ALC action (no bars) appears normal on 10 through 30m, and even works ok on 80m.  Haven't tried 160m.

However, when attempting digital modes (using a Signalink) on 40m, power output and ALC action is "all over the place" and generally folds back to a couple of watts.  The output also seems to be extremely "touchy" and sensitive to the TX drive setting on the Signalink that's not apparent on other bands.  I don't see this craziness using a dummy load, so I'm suspecting maybe I'm getting RF feedback only on 40m for some reason.

I'm not sure what to try to mitigate this.  The variables I could change would be to increase/decrease the ladderline length, try a more robust balun, or maybe try a counterpoise wire tied to the balun's ground lug?  Any ideas and suggestions would be most appreciated, I sure would like to be able to use 40m!

73 Brian, N5BCN