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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
