[RFI] RFI suppression cores for 147 MHz

[Jim Brown]

On 3/1/2013 2:09 PM, Dale wrote:
> 1) It MUST have a full circumferential grounding contact to the chassis of the equipment into which the cable gets connected.  Except for direct hard wiring, this does not happen with the conventional RJ-45 jack, "shielded" or not; 2) The net path to chassis at each end MUST be low enough in impedance to effectively route the currents induced into the shield to the chassis.  The "rule of thumb" for effectively low Z is 1/8th to 1/20th wavelength at the highest frequency of interest, with shorter path length preferred.

Hi Dale,

A couple of thoughts.  First, a circumferential connection is less 
critical if the length of the interruption of the shield and the 
interruption of the twist is VERY short,  As I see it, it's a question 
of degree -- the shorter the opening as a fraction of a wavelength, the 
greater the shielding effectiveness. Second, if the shield wanders 
around the circuit board before it gets to the shielding enclosure, 
there's a Pin One Problem that will couple RF both ways to/from the 
shield to the equipment. Third, if there's an effective shield contact 
at one end but not the other, noise from the equipment with the good 
connection may be suppressed pretty well, whereas noise from the other 
end will not be. Fourth, I suspect that  some (much?) of the VHF trash 
that escapes from UTP (ordinary CAT5/6/7) may be differential mode. One 
way to figure this out is to use a LOT of #43 cores on both ends and 
observe the resulting noise reduction.  Those cores are killing common 
mode, not differential mode.  I suspect that both are present.

SO -- IMO, having the shield go to the shielding enclosure by a very 
short path is generally far more critical than whether the connection is 
circumferential.

And this particular discussion is specifically addressing 100-200 MHz, 
which is where #43 is most effective. Above that, another material like 
Fair-Rite #61 and other materials from other companies are the weapon of 
choice.

For those new to the Pin One Problem, there are several tutorials on my 
website that address it in considerable detail. And in that context, 
it's important to realize that there are Pin One - Like Problems 
associated with ANY cable shield, and with the Green wire to the Mains 
Power outlet.

73, Jim K9YC