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

[Don Wilhelm]

Brian,

The Elecraft BL2 is a good common mode choke.  It is rated at 250 watts 
into a *matched* load.
Your load certainly is not matched, and I do not know how to de-rate the 
power for any given load.
Check to see if the balun core heats - if so, you could be operating the 
BL2 outside its range of capability.
Note that I am only making a guess here.

One thing that you could do is to remove the coax from the tuner and 
measure the impedance of the entire antenna system with an antenna 
analyzer.  That will give you a better idea of whether to use the 1:1 or 
4:1 position on the balun.

Another thing to consider is the position on the antenna feedline in 
relationship to the radiator - the feedline should come away from the 
radiator at right angles for as far as possible.  If it does not, energy 
from the radiator can couple onto the feedline and create common mode 
current that is difficult to choke off - in that case, the choking 
impedance of the BL2 may not be sufficient and you may have to add 
additional common mode chokes.

As a quick test for common mode current on 40 meters, yes try tying a 33 
foot counterpoise wire to the ground post on the BL2 - keep the far end 
isolated and insulated because it will have a high RF voltage on it.  If 
it makes a difference, then you will have to get rid of the common mode 
current on the feedline.

73,
Don W3FPR

On 10/25/2015 11:09 AM, Brian - N5BCN via Elecraft wrote:
> Don,
>
> Yes, things settle down with a dummy load, so it looks like I'm dealing with
> RF feedback.
>
> Per the original post, I'm using an Elecraft BL2 balun at the
> coax/ladderline junction.  Shouldn't this act as the common mode choke that
> you suggest or are you suggesting to place a common mode choke in addition
> to the balun?
>
> Interesting idea regarding the KPA100 latest version since 40m is the
> troublesome band.  I purchased the KPA100 around 2011-2012, so I would think
> it has the latest hardware but I'll double check when I get a chance.
>
> 73 Brian, N5BCN
>
>
>
> --
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