[Elecraft] Power Supplies

[Howard Hoyt]

Hi Kevin,

>>As for common mode currents in the antenna... would that
>>only be an issue during transmit?

Except for very special cases, antenna systems display reciprocity, and 
show very similar or identical current distributions at the frequency of 
interest in transmit and receive, although the magnitudes are obviously 
very different.  In a well-balanced antenna system the currents in the 
feedline are equal and of opposite phase, and in the case of a coax 
feedine the resulting fields are contained within the coax, i.e. there 
will be no current flowing on the outside of the coax shield.  Antenna 
system imbalances at the feedpoint will cause the imbalance current to 
flow on the outside of the coax shield and radiate in transmit and 
affect the antenna pattern.  In receive the exact same imbalance will 
exist and affect the pattern identically.

In both transmit and receive, these common-mode currents will be 
conducted to the chassis of the rig and anything attached to it. They 
also capacitively couple through the power supply, you, and anything 
else touching or near the rig.  When these currents couple through the 
power supply to the AC line they effectively make the AC power system 
part of the antenna and couple any noise present in the AC mains to the 
receiver.  As the antenna currents pass through the supply they can also 
be modulated by the input-output impedance of the supply which varies at 
the rate of rectification, so the supply can add its own noise to these 
currents.  Interestingly enough many people report stronger reception of 
the desired signals along with the increased noise, certainly proving 
the common-mode currents become part of the antenna system.  Breaking 
this current path with a common-mode choke will greatly reduce or 
effectively eliminate this current and noise.  For HF chokes we agree 
with Jim Brown's recommendations and we supply mix 31 cores for the 
purpose.  Proper grounding at the rig can also reduce the AC mains coupling.

After selling thousands of these Kx33 supplies we have learned a lot 
about the nature of most "power supply" RFI.  We have found very few 
instances where any supply was causing RFI by transverse conduction (RF 
riding on the DC output) or radiation (proximity of the supply to the 
receive antenna).  In the almost all cases, antenna system imbalances 
and the resulting common-mode currents were inducing RFI in the manner 
described above.  I'd be glad to send you a ferrite core to try, contact 
me off-line.

I hope this helps,

Howard Hoyt  - WA4PSC
www,proaudioeng.com