[Elecraft] Sudden jump in SWR when power above 80 watts

[Ken]

On Feb 18, 2012, at 5:43 PM, Don Wilhelm wrote:

> Rick and all,
> 
> Look for something in the antenna system that is arcing over above the 
> 80 watt level (or whatever level the problem starts).  Check every 
> junction in your antenna system.  Your workaround may be OK, but the 
> real fix may increase your antenna efficiency.


That's where I first started.  I replaced the ancient twin lead with new open wire.  No change.  I replaced the 4:1 balun with a new 1:1 balun.  No change.

I replaced the coax to the balun.  No change.  I inspected the center insulator, that's a block of quarter inch PVC with everything separated at least two inches.  I checked the end insulators.  They look fine and they are tied to Dacron line.   The ONLY thing I didn't change is the antenna wire ;-)   

The antenna is in free space with the closest object being the antenna mast a foot away.

What did stop the problem is changing rigs (to a TS-440) or changing the antenna tuner (to the Palstar).   What's in common with the problem?  The K3. 



> 
> The other situation that will cause "bad behavior" above a certain power 
> level (and on a particular band) is common mode RF coming in from the 
> antenna and its feedline.  The SWR sensing circuits may "go crazy" in 
> the presence of too much RF voltage.  In this case, the use of common 
> mode chokes at the antenna, and possibly at other points along the 
> feedline may be what it takes to "tame the beast".


Now that sounds like a possibility.   

Ken WA8JXM