In message <[email protected]>, dated Thu, 24 Jan 2008, "Kunde, Brian" <[email protected]> writes:
>The IEC/EN 61000-4-3:2006 standard says in section 7: > >"NOTE 2: At higher frequencies (e.g., above 1 GHz), tables or supports >made from wood or glass reinforced plastic can be reflective. So, a low >dielectric constant (low permittivity) material, such as rigid >polystyrene, should be used to avoid field perturbations and to reduce >degradation of field uniformity." > >1. Note the words "can" and "should". They are not "will" and "shall". >So I assume this is not an absolute if a practical alternative cannot >be found. The problem is that if the table is reflective, the repeatability of measurements may be degraded, and especially if the effect varies with humidity. > >2. From what I understand, "rigid polystyrene" is Styrofoam, as earlier >mentioned. However, is the white plastic foam sheets, or Instapack, or >other rigid foam materials still considered "polystyrene"? Polystyrene is a specific chemical substance. 'White plastic foam' might be polyurethane or polythene or ..... instead. >Does anyone know if these alternatives are any better than wood in >regards to its permittivity? Dry wood has quite a low relative permittivity (RP), around 2, but the RP of water is around 80, so a little water makes a lot of difference. Dry walnut (which I happen to have figures for) has an RP of 2, whereas with 17% water, the RP is 5. > >3. Is the permittivity of a materials affected by its thickness? Is a >few inches of wood worst than a couple meters of Styrofoam? The permittivity, like other 'ivities', such as 'resistivity', is an inherent property of a material, not related to its dimensions. > >4. The standard mentions "glass reinforced plastic" which I assume is >fiberglass. What about PVC, Delran, or Formica? PVC is not good: its RP is 4, and its loss factor (tan delta) is about 100 times that of polystyrene, even at 10 MHz, and it's very high indeed at higher frequencies. Delrin (note spelling) is polyformaldehyde; depending on grade its RP is between 3 and 4, and its loss factor is 0.005 at 1 MHz, suggesting it may be quite high at UHF. Formica is paper impregnated with phenolic resin and compressed. RP and losses rise with density, so UHF performance is not so good. Figures for 'paper-filled phenolic resin' are RP = 5, loss factor at 1 GHz = 0.08. >To handle the weight of our products our test tables are constructed >from a combination of PVC legs and supports and a wood top with >Formica. > >5. I saw a chart that compared the permittivity of wood, Styrofoam and >several other types of material but for the life of me I can't locate >it. Either it was in one of the trade magazines or someone posted it on >this email group. If anyone has that chart or knows its location, can >you send it to me? If I recall, I thought all the material listed in >the chart was unacceptable except for Styrofoam. Did I read/remember >this correctly? I would say that's pessimistic; high-density polythene, for example, has a lower RP and lower losses at 1 GHz that polystyrene. > >6. Is there a test procedure that can be used to qualify your test >table? For example, run your unified field test with your test table in >the field. If a unified field can be achieved, then your table is OK to >use. > >7. If I wanted to test the permittivity of a material myself, how would >I do this? You make a capacitor with the material as dielectric, and measure its value. You can buy a jig for this, but I've no doubt it's $$$. In principle, it's just two metal discs forming the capacitor plates, but the electric field 'fringes' at the edges of the plates and to get an accurate result, measures have to be taken to control this. See a physic textbook? The capacitance is RP x permittivity of space x material area/material thickness, so you can calculate back from capacitance to RP. Permittivity of space is 8.857 pF/m. > >8. Several people have commented about wood table absorbing moisture >from the air. Is this the problem with wood? Can't wood be sealed to >now absorb moisture? Can wood be used if it is dry? We test in a >chamber and our entire lab has humidity controlled to 30-50%RH. In a >controlled environment, can wood table be used? You would need to try it. The trouble is that since it's repeatability that's mainly affected, you would need to do a lot of testing. It may be simpler and less costly to assume that you need a non-wood table! -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk For very important information, please turn over. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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