Topband: Elimination of Treadmill RFI on 160 meters

[Jim Brown]

Tom,

You simply don't get it.  The choke is NOT a filter. It is a high 
impedance added to the common mode circuit. The common mode circuit is 
acting as an ANTENNA, either for RX or TX.  If for TX, the noise source 
is inside the box, and the current will depend upon the voltage of the 
source and behavior of the circuit as an antenna. If for RX (that is, 
the noise is received on the wiring by simple antenna action and coupled 
into the box via failure of the wiring to go the shielding enclosure) 
the current will again depend upon the behavior of the circuit as an RX 
antenna.

When we add a choke of sufficiently high resistive impedance to that 
circuit, we reduce the current at frequencies where the choke is effective.

When preparing to publish my first research on this in 2003 (to the 
AES), I found references in ancient applications notes from EU mfrs of 
ferrites showing that they clearly understood this principle.  Those 
ferrite cores molded onto cables emerging from electronic equipment are 
not filters, they are chokes. They do work in the frequency for which 
they are designed, and the only capacitance in the circuit is their own 
parallel capacitance that forms their resonant circuit -- it is the 
capacitance from one end of the core to the other via the dielectric of 
the core.

73, Jim K9YC

On Fri,1/30/2015 3:16 AM, Tom W8JI wrote:
> Any filtering or decoupling system works by the ratio of series 
> impedance to shunt impedances.
>
> A series choke is useless unless there is some value of shunt 
> impedance in the system. As a matter of fact, lack of established 
> shunt impedances is what can drive choke requirements to unrealistic 
> values. This is true in baluns, just like it is in line filters.