Rf/emi

Discussion in 'The Projects Forum' started by eeboy, Jan 16, 2008.

  1. eeboy

    Thread Starter Active Member

    Sep 27, 2007
    90
    1
    I am designing a sheet metal enclosure for some passive components contained inside a matching network. The inductor gets quite hot in this application and so I want to move some air through the enclosure. I am ready to drop some slots or holes in the side but I've got no idea how to size the aperatures properly so that my EMI emissions will be minimal. The fundemental frequency for the network is 27.12 MHz. Any suggestions?

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, your wavelength is just over 41.15 feet ... getting down to the 12th harmonic, the wavelength would be 0.120558, so if you used a 3/16" drill (0.1875") that wouldn't be close to resonant. 1/8 would be a bit too close.
     
  3. eeboy

    Thread Starter Active Member

    Sep 27, 2007
    90
    1
    So the idea is to avoid multiples of the wavelength? This means the enclosure is ineffective for multiples beyond the 12th harmonic? I would assume the magnitude at the 12th harmonic is very small anyways so this may be a moot point.

    For my understanding... if I was to create a slot instead of a circle the opening is no longer one fixed dimension (diametor of circle) but could be any dimension between the diameter of the slot and the length of the slot. So, if the diameter is constrained to .100" and the length of the slot is made .150" (edge to edge) there exists a plane in which the aperture appears as .120" which is the wavelength of the 12th harmonic. Make sense? How does this play into attenuation of the 12th harmonic?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can also make much larger holes and use aluminum screen mesh to make an RF shield across the hole/s.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's a table of dimensions, in inches. The first column is the harmonic, 2nd the wavelength in inches, 3rd is 1/4 wavelength, 4th is 5/8 wavelength. Best to steer away from dimensions that are close.
    Code ( (Unknown Language)):
    1.  
    2. 7  3.857853982  0.964463496 2.411158739
    3. 8  1.928926991  0.482231748 1.205579369
    4. 9  0.964463496  0.241115874 0.602789685
    5. 10 0.482231748  0.120557937 0.301394842
    6. 11 0.241115874  0.060278968 0.150697421
    7. 12 0.120557937  0.030139484 0.075348711
    8.  
    I'm a bit hazy on the details of waveguide, but many years ago I worked on X-band radars. From the center of one side to the center of the other side of the waveguide was exactly 1/4 wavelength of the center of the "channel" we were using, in the 9 GHz range. Of course, if you put an ohmmeter on it, it was a dead short. However, at the frequencies it was designed for, it was an excellent conduit of high powered RF. The waveguide was roughly twice as wide as it was high; nearly 1" wide and 1/2" tall. I don't recall measuring it. But, if your slots were narrow and not very long, I don't see them as being much of a problem in regards to RFI emission.

    At a former employer, they built what was supposed to be an EMI/RFI shielded "cage" for testing purposes. They used copper screen wire and copper sheet, grounding the whole thing to several copper rods driven deep into the earth. However, the enclosure turned out to be very electrically noisy; likely something to do with the copper screen wire and it's impedance at high frequencies. Best to not depend on screen wire for blocking EMI/RFI.
     
  6. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Long slots and seams can also let in a lot of noise. Multiple smaller holes would be better.
     
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