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
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