a linear supply the instantaneous input to output impedance will be
modulated at the AC line frequency, leading to the hum commonly
heard in older single-conversion receivers. In the case of a switching
supply the coupling can be modulated by the dynamic impedance of
the switching action and can cause RFI. Measurements taken by PAE
show linear and switching supplies to of fer mainly capacitive coupling,
with ~1000pF being average. At <100pF the Kx33 minimizes
this coupling and the magnitude of this common-mode RFI.
b. The magnitude of antenna system voltage on the chassis of
the rig. Well-implemented antenna systems with little imbalance
and small common-mode feedline currents are much less likely
to cause this form of RFI. When operating with a high-impedance
antenna like many end-fed long-wires, or while operating with an indoor
antenna, the use of any line-operated supply can potentially induce
antenna counterpoise currents to be coupled to the AC power system
through the power supply. Inserting a common-mode choke in the
antenna feedline will reduce or eliminate the feedline current ow. An
excellent reference on this subject can be found on K9YC’s site:
http://audiosystemsgroup.com/RFI-Ham.pdf
In order to minimize common-mode coupling through the Kx33
we offer the #33-410 Fair-Rite Ferrite Clamp-On core at our cost
to Kx33 customers at: https://proaudioeng.com/accessories/
Winding the DC power cable 6 to 8 turns through this core will
effectively increase the common-mode impedance of the DC
power cable by >3000 ohms@10MHz, effectively minimizing this
source of RFI. While the Kx33 does feature both input and output
common-mode chokes, we have had a couple of customers inquire
as to why we did not integrate a ferrite choke like this into the
design of the Kx33 itself, and the answer is size, weight, and cost.
The ferrite is over 1/2 the weight and size of the KX33, and would have
increased the cost. For 99% of all users, the RFI performance of
the Kx33 itself will be sufcient, and those users will enjoy a much
smaller, lighter power supply at lower cost.
