Paolo,
I looked in the "Shielding Design Guide" by Tecknit and found the same
formula that you have below.
The formula is claimed to be relavant for threat frequencies that are less
than one tenth of the cutoff frequency(fc). This may be of some importance,
because their claimed cutoff frequency for 1/8" honeycomb is 47GHz. If your
application runs up to 40GHz, you will be over the fc/10 limit for the
formula's usefulness. I would still use the formula as a guide, but beware!
Another concern that I have is from looking through graphs of shielding
effectiveness in the Instrument Specialties catalog for honeycomb. They
seem to be concerned with the specific metal and plating used to make the
contact around the edge of the honeycomb vent. I would recommend taking
care to make sure you get a good seal. After all, what good is a honeycomb
with 90dB of attenuation if it has a leaky seal around the edge? At 40Mhz,
it would be easy for the seal around the honeycomb to become a leakage
factor.
I liked Derek's (another response) idea of putting a bend in the cooling
airflow path with some ferrite absorber. I've never thought of it, but it
sounds like it may offer some improvement (if you think you need it). I've
never seen experimental proof of this, but maybe someone else has.
By the way, page 150 of the Instrument Specialties Catalog and Design Guide
has a graph for determining the required vent size for a given amount of
airflow and static pressure. I assume that you have already determined your
vent size, but if you haven't, this graph may help.
Chris Maxwell, Design Engineer
GN Nettest Optical Division
6 Rhoads Drive, Building 4
Utica, NY 13502
PH: 315-797-4449
FAX: 315-797-8024
EMAIL: chr...@gnlp.com
> -----Original Message-----
> From: jim_bac...@mail.monarch.com [SMTP:jim_bac...@mail.monarch.com]
> Sent: Friday, October 20, 2000 10:31 AM
> To: Paolo Roncone; emc-p...@majordomo.ieee.org
> Subject: Re:Holes, waveguides and honeycombs
>
>
> forwarding for pronc...@cisco.com
>
> ____________________Reply Separator____________________
> Subject: Holes, waveguides and honeycombs
> Author: Paolo Roncone <pronc...@cisco.com>
> 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
>
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