[Elecraft] Power Supplies

[Howard Hoyt]

Hi Kevin,

It is very possible that what you are experiencing is noise induced by 
common-mode currents from your antenna system.  These currents take any 
path connected to the chassis of the rig including ANY AC mains 
connected power supply, regardless of whether it is a switcher or 
linear.  The 10~100 nF average input-to-output capacitance of AC 
operated power supplies provides a low-impedance path from the DC output 
to the AC mains at ham frequencies.  We have done a LOT of testing of 
this characteristic here at PAE in order to separate conducted RFI from 
RFI caused by common-mode current.   After identifying this as a problem 
which is usually worse in portable antennas we took extreme pains to 
reduce this I/O capacitance with the Kx33 and we were able to achieve 
less than 100pF, making the Kx33 much less conducive to allowing 
common-mode noise.

Elimination of common-mode currents has many benefits including cleaning 
up the pickup and radiation pattern of the antenna, elimination of 
receive noise, and reducing or eliminating RFI-induced events like alarm 
system triggering (ask me about that one).  When using a superhet 
receiver, baseband AC mains hum is not detected as it can be with a 
direct-conversion (DC) receiver.  This problem plagued early DC 
receivers like the Heathkit HW-7 which often buzzed like a bee when 
operated from a linear 12V supply.Fortunately this current can be 
greatly reduced by the use of common-mode choking on the antenna 
feedline, DC power lead or both. As has been stated here innumerable 
times, common-mode choking should be done on all antennas at the 
feedpoint and optionally but beneficially at the shack end as well.  An 
excellent reference on this subject has been written by our own Jim 
Brown K9YC and can be viewed at:

http://audiosystemsgroup.com/RFI-Ham.pdf

Cheers & 73,
Howard Hoyt - WA4PSC
www.proaudioeng.com