[Elecraft] KPA-1500 SWR fault above 700-800 watts on 160m

[Jim Brown]

On 9/3/2020 4:21 AM, CUTTER DAVID via Elecraft wrote:
> I use K9YC-style chokes, eg several turns on 240-31 core, or, if it's really bad, several turns on several cores using 7" diameter turns (from memory, check his cookbook).  I'm told this is cheaper and better than anything you can buy.  Of course I know nothing about your layout but it has always helped me to ground the coax before it comes into the house.

Hi David,

Note that, based on extensive research in 2017 and 2018, I no longer 
recommend the older style of choke that you're using, and I revised the 
RFI tutorial to reflect that. I published both that revision and the new 
Cookbook late in 2018.

k9yc.com/publish.htm

The problem with the older style chokes (multiple turns of coax on 
multiple #31 cores) is that 1) any "scrambling" of the winding reduces 
choking impedance, and 2) variations in winding style (diameter, 
spacing) make the chokes non-repeatable.

The new choke designs ARE repeatable, AND take into account the wide 
manufacturing tolerances of all ferrite parts, which can significantly 
affect the choking impedance and where the chokes are tuned. The work I 
did in 2017 and 2018 included characterizing nearly 200 #31 cores 
obtained from multiple vendors over about ten years, selecting cores at 
the limits of those tolerances, winding and measuring hundreds of chokes 
on those cores, tabulating the results, and making recommendations for 
each band and each transmission line type based on worst case results 
for each design. Chokes wound following the new 2018 Cookbook should 
provide at least the specified response on any #31 core you buy. Note 
that #31 mix is made ONLY by Fair-Rite.

Another point about measuring common mode current. As you have observed, 
it varies along the line, simply because in the common mode circuit, the 
feedline is part of the antenna, and both voltage and current vary along 
the line conforming to wavelength relationships at the operating 
frequency. A good choke forces a current minimum at the point where it 
is inserted. How common mode current varies along the line depends on 
the quality of the choke, the electrical length of the outer conductor 
of the coax (VF~0.98), and how the shield is terminated on the other end 
(grounded or floating using a link coupled tuner).

A choke is most useful at the antenna feedpoint (up in the air, or at 
the feed end of a vertical or other long wire), which tends to minimize 
common mode current all along the line, because it "disconnects" the 
feedline from the antenna. A choke at any other point is FAR less 
effective -- all it does force a current minimum at the point it is 
inserted, leaving the feedline connected to the antenna, allowing common 
mode current between the choke and the antenna, and coupling any noise 
current to the antenna.

For this reason, the first choke in any system should ALWAYS be at the 
antenna IF the antenna is matched to the feedline. The power handling of 
my chokes are ONLY applicable if the feedline is matched at the point 
where they are inserted. A choke inserted into a poorly matched or 
un-matched line is much more likely to overheat and fail. Dissipation 
(heating) is addressed in the 2018 Cookbook.

73, Jim K9YC