[Elecraft] (OT) - Unplanned Discovery

[Howard Hoyt]

Hi Stan,

One thing we have learned in supplying thousands of Kx33s to hams is the 
importance of minimizing or eliminating common-mode currents in the 
antenna system.  The transverse mode RFI output of the Kx33 is extremely 
low and has never been the cause of noise to our customers, but if the 
antenna system has significant common-mode potential, it will always 
seek a current path through whatever is attached to the rig...through 
the power supply, your body, as well as grounds.  The outside of the 
coax shield, the rig, the power supply and AC mains are all in a series 
path for common-mode currents.

The most direct cure is to place a high common-mode impedance in the 
antenna feedline.  The DC power lead on the Kx33 is smaller in diameter 
than most RF coax lines, so it may be easier to form a choke on that 
cable.  You mentioned "YES, I HAD CHOKES ON ALL SUPPLIES, CABLES, 
WIRE."  The exact core material used and number of turns is important in 
constructing common-mode chokes.  Jim Brown, K9YC has excellent 
tutorials on this subject: http://audiosystemsgroup.com/RFI-Ham.pdf

Another measure which will shunt common mode currents away from other 
paths is a low impedance ground on the rig.

Something else I want to mention, as it is an issue which has cropped up 
recently: There are several sellers on both eBay and Amazon stating "Pro 
Audio Engineering  Kx33 power supply" in their ads, but no supplies 
other than those sold directly from us are the actual Kx33.  We know 
this because we worked with one such customer to resolve noise issues 
and it turned out the unit he had was not a Kx33.  eBay and Amazon both 
state they can not do anything, since the other vendors are claiming 
they use our part number and name only to state an equivalency, which 
they most assuredly are not.  Our Kx33 has less than 70 pF of input to 
output coupling vs. 1000 pF or more for most other supplies, making it 
far easier to choke common-mode currents.  This was the most difficult 
spec of the supply to arrive at, but considering we knew it could be 
used with temporary antenna systems with common-mode potential it was 
important to achieve.

As we unconditionally guarantee everything we sell, please contact us at 
info at proaudioeng.com and I am sure we can help you eliminate this noise 
or refund your purchase price.

Cheers & 73,

Howard Hoyt - WA4PSC
www.proaudioeng.com