[RFI] RFI suppression cores for 147 MHz

[Jim Brown]

On 3/1/2013 5:09 PM, Dale wrote:
> Jim,
>
> Thank you for the comments.  The reason I stressed the full circumferential shield termination within the connector (usually within a back shell or termination clamp/ring system) is because that is what is required to meet MIL-STD-461 (and some DO-160 civilian aircraft) radiated emission requirements for many aerospace systems.  With so many hams having problems with LAN and digi equipment interference issues due to having their antennas close to (or even inside) the house/shack, I am quite certain that there are plenty of instances that actually need RFI suppression to the same degree as the military systems.  This would be a lesser problem if people could place their antennas tens to hundreds of feet away from their computer and network systems.  I can assure you that failure to maintain a full circumferential, low-Z shield termination on cables has been the cause of many system test failures.  There is also a real need to make certain that pc card ground planes have low-Z current paths to chassis; that's part of a good design.
>
> YES, the path for shield grounding MUST be to the chassis of the associated equipment.  In this case, "grounding" can mean "common or chassis return", as opposed to "earth ground".  A lot of ITE equipment operates with a 2-wire line cord, so there is no "earth" ground to be found.  However, when one considers the loop area of the RFI-conducting path, the primary concept that matters is total length (and net Z) between the device chassis (which contains the noise makers) and the cable shield.  Having a connected peripheral device in which the LAN signal does not tie to chassis via a short, direct path is nothing more than having a probe antenna sticking out at the end of the coax.

The word "ground" is again causing some confusion, and I choose to avoid 
it.  An EARTH connection is not part of the cause of this problem nor is 
it part of a solution. Two key issues are part of this connection. 
First, connecting the shield to the shielding enclosure makes the shield 
continuous, and it MUST be continuous to be effective. Second, any 
shield current must NOT enter the equipment. For most equipment, the 
shielding enclosure, along with other connections to the shielding 
enclosure, serves as a sink for shield current. As I'm sure you know, in 
some systems, a PROPER "ground" layer on a circuit board can serve in 
place of a shielding enclosure -- I'm thinking of microstrip or 
stripline with no breaks in the ground layer(s) under traces, and no 
breaks between ANY external connections to the ground layer(s).

> The test frequency range for radiated emission MIL-STD-461 (and DO-160) testing is typically 30 MHz to 1 GHz.  In most cases, the most important portion of the spectrum needing help is 30 MHz to about 300 MHz, and the stated 100 to 200 MHz certainly falls within that as a subset.

Yes, but to put this in context, pass/fail could be the difference 
between 40 dB of attenuation and 60 dB, and for this problem, 40 dB 
might be enough (or not). :)

> As a side note, I am active on all of the bands listed, but except for my HT, all antennas are at least 50 to 100 feet from the LAN cables

My issue in Chicago was with a talkie inside the 2nd floor office in a 
wood frame building trying to copy 2M repeaters in the 10-15 mile 
range.  I was not aware of issues with a 6dBi omni vertical that was 30 
ft or so away and 20-30 ft higher. OTOH, this was in a mixed 
residential/industrial neighborhood, with fairly high ambient noise.

73, Jim K9YC