I've tried considering the various angles to this question, but I'm asking you just from an electronics-theory point of view, is it it true that these high-freq Microwave frequencies (or even higher ones beyond, like X-Ray, Ultra-violet, etc) cannot be effectively shielded? Now that I think about it, if this is the case, why is it dentists usually put that heavy lead apron around patients then scurry out of (supposed) range before snapping X-ray photos? Furthermore, why are such clunky aprons not properly grounded before being used? ;) ..... but I digress. Care to take a whack at the Microwave/shielding question? Thanks!

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Well, since you asked.

1. Lead aprons do not have to be grounded because they are not conducting the energy away. They are blocking/absorbing it. The reason the techs leave the room is that for every X-ray dose you are exposed to each year, they are exposed to hundreds or thousands. The biological effects of that much X-ray exposure are accumulative over time, even with lead underware.

2. Yes, microwaves can be shielded against. That's what the perforated metal screen in the door of your microwave oven does. It's a trade off, visibility vs. radiation. Visible light has a much shorter wavelngth than does the radiation in an oven, so a hole big enough to let out some light lets out much less at the lower freq of the oven. Ovens run at 2.4GHz, the same freq as most Cell Phone s, many cordless phones, and wireless LAN systems (802.11b/g). They all center on this band because it is one of the few frequency bands where unlicensed transmitters are allowed by the FCC. A microwave oven is a 600 watt radio transmitter where most of the energy is contained inside the box, a Cell Phone is 3 watts with most of the energy going outside the box, both way above what is allowed in other frequency bands. For example, you can buy or build a wireless microphone for the AM or FM broadcast bands, but it can not radiate more than 0.1 watts legally.

There are two basic schelding schemes, absorbtion and conduction. 99 feet of concrete blocks might effectively shield Cell Phone tower transmissions, but not very practically. Extra heavy aluminum foil will do the same thing if ***properly*** grounded. A completely surrounding conducting surface, grounded, is called a Faraday Cage, after Michael Faraday. When you go to a screen or perf metal cage, you trade off some shielding effectiveness for the transmission of something beneficial, such as light or air. Think of squeezing toothpaste through holes of different sizes. With constant paste pressure, the smaller holes let less paste through over time. Smaller hole equals greater attenuation, but no matter how small the holes are, some paste always gets through. For RF, the rule of thumb is that a hole diameter of 1/20th of the wavelength of the frequency of interest attenuates the radiation by 20dB (90%). At the microwave oven/cell phone freq of 2.4GHz, one wavelength is about 4.9". The holes in the oven door are about 1/8" diameter, a ratio of about 40 to 1, so the attenuation is on the order of 95%. Real shielding design takes into account the thickness of the perf metal, the thickness of the webs between the holes, and of course the characteristics of the metal alloy being used, but the basic physics are pretty straightforward.

There are other shielding techniques, such as waveguide-at-cutoff, which let through much more air with larger hole sizes, yet have much higher attenuation. Different physics, very hairy math. When I need to use it, I give my specs to a vendor and let them do the work.

So... yes, microwaves can be shielded against, especially at the relatively low frequencies in cell phone networks. No matter which technique you use, the better the shield, the less convenient it will be.

By the way, a few years ago Siemens announced that they could detect the overflight of a stealth aircraft with their cell phone towers, much the same way we detect submerged submarines. Cool.

ak