forwarding for: <[email protected]>
____________________Reply Separator____________________ Subject: RE: Holes, waveguides and honeycombs Author: "Brench; Colin" <[email protected]> List-Post: [email protected] Date: 10/20/00 11:35 AM Hi, here are some clarifications that might help you in your design. Shielding effectiveness is a measure specifically geared to a plane wave, that means the source is a long way from the shield and there is no direct interaction between them. This is seldom true, as a result I have found that the shielding behavior is better than expected 99 times out of 100. The effect of the number of holes is also a tough call. Really this should be read as the number of equally illuminated holes from each source rather than the total number of holes. Looking into this often shows that only a small portion of the overall vent area needs to be considered for each source. Shielding performance goes up drastically with the thickness. A w/t of 1:1 gives 27dB 2:1= 54 and 4:1 over 100dB "per hole". I have designs which are so far only required to work to around 20GHz worst case and use 2:1 (3mm holes 6mm thick). Probably the most common size has a 4:1 ratio. If you really find that you need more shielding performance then honeycomb is available with two other constructions; The first consists of two thin honeycombs joined together with a half cell offset, I'm not sure how this behaves. The second has the honeycomb at an angle of 45, so that the thickness remains small but the honeycomb thickness is increased by 40%. Again this can be stacked for even better performance. However, that is pretty extreme. Remember that to see the benefit of high levels of shielding all your I/O will need to be equally well filtered. Regards, Colin.. -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Friday, October 20, 2000 10:31 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]> List-Post: [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]
