[RFI] Common Mode Line Filter

[Jim Brown]

On 8/11/2014 8:59 AM, Pete Smith N4ZR wrote:
> I have an inexpensive Qualtek computer input line filter that is rated 
> 250 VAC at 3 amps.According to the schematic on the side, the third 
> wire is grounded to the chassis on both sides of the filter.  Would 
> such an arrangement make for better filtering by solving the pin 1 
> problem, for the device using it? 

Yes, it certainly would, Pete, provided that the length of the wiring 
between the noisy equipment and the filter was very short. Ideally, such 
a filter should be built into the equipment, and there are commercially 
available filters with a standard line cord and chassis mount. Again, 
though, care must be taken that the connection to the shielding 
enclosure is actually made -- the most common failures being paint in 
the way, or connectors mounted to a PCB whose shields don't get bonded 
to the chassis at the point of entry.  Most ham rigs have that issue 
with connectors for control lines. Although most screw the RF connectors 
down to the chassis, they forget all the rest.  The Elecraft outboard 
XV-series transverters fail to bond the RF inputs and outputs -- the BNC 
connectors are insulated from the chassis, and the two I own (for 2M and 
220MHz)  have stability issues.

73, Jim K9YC