The formula for honeycomb panels agrees with the one I have found in a shielding design guide, provided by a commercial shielding products manufacturer. (If you want to know which one, contact me directly.) The last term in the equation is similar to that for multiple vent hole openings, where the thickness (depth) is much smaller than the opening. In this case, the depth is greater than the opening, but you still have to account for multiple apertures. You can't ignore it. If you think about it, each hole acts as an antenna source on the other side of the panel. The effect of each one individually may be small, but collectively, they add up. The result is (obviously) an overall reduction in shielding effectiveness. The advantage of honeycomb, or any waveguide below cutoff, is that the attenuation is very high below the cutoff frequency. Even with multiple apertures, the shielding should still be very high below the cutoff frequency. DB
> ---------- > From: [email protected][SMTP:[email protected]] > Reply To: [email protected] > Sent: Friday, October 20, 2000 7:31 AM > To: Paolo Roncone; [email protected] > Subject: Re:Holes, waveguides and honeycombs > > > forwarding for [email protected] > > ____________________Reply Separator____________________ > Subject: Holes, waveguides and honeycombs > Author: Paolo Roncone <[email protected]> > Date: 10/20/00 12:03 PM > > Group, > > We are working on a couple of designs of telecom gear contained in metal > s.u.b.-racks and we have to meet emission limits up to 40 GHz. > I need some advice on the workings of waveguides below cut-off and > honeycombs, because we have to include ventilation openings without > possibly degrading the shielding effectiveness. > First, I made an inquiry on textbook formulas for circular and rectangular > waveguide cut-off frequencies. I was happy to find consistency among three > different sources (I found the same formulas although rearranged in > different fashions). > From Ott's "Noise reduction techniques in electronic systems" I found: > > fc = [6.9/d] GHz for circular waveguides > > fc = [5.9/l] GHz for rectangular waveguides > > where fc = cutoff frequency > d = diameter of circular section (inches) > l = longer side of rectangular section (inches) > > Now my question is: what about honeycomb panels ? > Can I use the same formulas for honeycombs ? Here the single cells are > neither circular nor rectangular. Can I still apply these formulas with > good accuracy ? If not, anyone knows of other formulas that apply in this > case? > As for attenuation (shielding effectivenes) of one single waveguide > opening, if the frequency is well below cutoff , this is proportional to > the ratio of length/diameter of the waveguide. The recommended ratio is 2:1 > to 4:1 in order to get good attenuation. > > Now, I just found a formula for attenuation of honeycomb panels as function > of frequency, length-to-width ratio of each cell and also number of cells: > > S [dB] = 20log(fc/f) + 27.3(t/W) - 10log(n) (f < fc/10) > > where: > > S [dB] = Shielding Effectiveness in dB > fc = cutoff frequency of waveguide > f = frequency > t = cell length (or thickness) > W = cell section width > n = number of cells in honeycomb panel > > I have no problems with the first two terms in the above equation. As for > the third term, that means that increasing the number of cells (n) in the > honeycomb panel degrades the shielding effectiveness of the panel (ex. > 1000 cells means 30 dBs lost). > Before finding this formula I had a feeling that due to the skin-effect > each honeycomb cell could be treated as a single cell. > So far I wasn't able to find other formulas for honeycomb panels. So I'd > like to have some feedback on this. > I hope to get some useful directions. > > Thank you in advance,> > > Paolo Roncone > Cisco Photonics, Italy > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
