In retrospect the Portland Symposium was okay. The Oregon Convention Center was big enough for the boat show or the auto show and you could easily get lost in the cavernous empty rooms that we weren't using. But the lecture rooms were too small, so there was standing room only and it was out in the hall. The food on sale outside the door of the exhibit hall was adequate even though a bit pricy, but the diners and such across the street were very nice. There was more than enough room for the exhibits, even including dancing girls and magic shows. One exhibit and I'm not kidding was dancing girls in grass skirts yet. In case anyone missed the point, the next Symposium is going to be in Hawaii.



Portland was okay too, although the City of Roses has a few thorns. The free trolley system was cool. And the hotel rooms had adequate hot water, but I for one have seen Portland and I won't be back. I didn't appreciate the bums--yes, bums--panhandling on most corners and sleeping on the sidewalks when you came back from a trendy restaurant where you ate crawfish etouffee.



So it's back to the factory or the lab and back to work pushing buttons and turning knobs. (Does any of the EMI measuring equipment still have knobs?) But the old lab won't be the same. No more wooden tables. That's right, you are going to have to get rid of that wooden table you made that cost you maybe $50 and has served you for the last 20 years. Someone in Europe has gone into the styrofoam business, so for a little greasing of the palms they have had it written into the CISPR specification, and maybe all the other specks, that you have to use a 40cm block--probably a meter by a meter. Nice size to have to ship to your lab and guess who pays the shipping. Of course, the styrofoam block will be gouged with deep furrows the first time you drag an EUT a few centimeters across its surface to get it to just the right distance from the antenna and all. But that's okay. You really don't want a fixture that does the job well and lasts 20 years when you can buy a new block of styrofoam every month. I'll bet you can't dump it into the landfill, so let's find the geniuses who thought this up and ship the blocks to them.



Other than the speck writers and Uncle Antoine being in the styrofoam business, is there any other reason for using styrofoam instead of wood? As far as I can tell there isn't. The idea is that the bottom of a PC is about 1/4 of a meter square and at 40cm from the floor forms a parallel plate capacitance of about 1pf. If you put a dielectric in this capacitor, say a wooden table--which last time I looked was mostly air--you change that value a bit. Not as much as you would think since most of the field in this geometry is what the know-it-alls call fringing field and most of it isn't even in the wood. Add the capacitance due to the cabling and the power cord probably lying on the floor where the distance between the wires and the ground plane is maybe two-tenths of a centimeter through a dielectric material which increases this capacitance. The capacitance between the wires and the floor is in parallel with the capacitor in which the table is the dielectric. The net result? The table has almost no effect on the EMI currents nor on the resonance of the common mode circuit which is probably more important anyway since any harmonic at this resonant frequency will see only minuscule real resistance, say a few milliohms.



If the dielectric sandwiched between the PC board and the ground plane floor of the chamber or OATS is so darned important, why not get rid of it altogether? Let's hang the EUT 40cm from the floor with nylon rope, tie it around the PC like a Christmas package, then run it from the wrapping up to a boom. Why fool with styrofoam? This plan has the added benefit of making at least half the chamber owners rework their present semi-anechoic chambers or discard them and buy new ones. You see, I have this uncle in the chamber business and...