Tag: cell phones

If microwaves are reflected by the door on a microwave oven, how are they able to pass tens of miles from mobile phone transmitters through solid walls and into our mobile phones? — JW, Belfast, Northern Ireland

The door of a microwave oven is carefully designed to reflect microwaves so that they can’t escape from the oven. That mesh that you see in the door isn’t plastic, it’s metal. Metal surfaces reflect microwaves and, even though the mesh has holes in it to allow you to observe the food, it acts as a perfect mirror for the microwaves. Basically, the holes are so much smaller than the 12.2-cm wavelength of the 2.45-GHz microwave that the microwave cannot propagate through the holes. Electric currents flow through the metal mesh as the microwave hits it and those currents re-radiate the microwave in the reflected direction. Since the holes aren’t big enough to disrupt that current flow, the mesh reflects the microwaves as effectively as a solid metal surface would.

As for how your cell phone and the cell tower can communicate for miles despite all the intervening stuff, it’s actually a challenge. The microwaves from your phone and the tower are partly absorbed and partly reflected each time they encounter something in your environment, so they end up bouncing their way through an urban landscape. That’s why cell towers have multiple antennas and extraordinarily sophisticated transmitting and receiving equipment. They are working like crazy to direct their microwaves at your phone as effectively as possible and to receive the microwaves from your phone even though those waves are very weak and arrive in bits and pieces due to all the scattering events they experience during their passage. Indoor cell phone reception is typically pretty poor unless the building has its own internal repeaters or microcells.

There are times when you don’t get any reception because the microwaves from the cell phone and tower are almost completely absorbed or reflected. For example, if you were to stand in a metalized box, the microwaves from your cell phone would be trapped in the box and would not reach the cell tower. Similarly, the microwaves from the cell tower would not reach you. Moreover, the box doesn’t have to be fully metalized; a metal mesh or a transparent conductor is enough to reflect the microwaves. Transparent conductors are materials that conduct relatively low-frequency currents but don’t conduct currents at the higher frequencies associated with visible light. They’re used in electronic displays (e.g., computer monitors and digital watches) and in energy-conserving low-E windows. I haven’t experimented with cell phone reception near low-E windows, but I’m eager to give it a try. I suspect that a room entirely walled by low-E windows will have lousy cell phone reception.

I want to trick my friends into thinking that my cell phone can pop popcorn. Here is my plan: take the magnetron out of my microwave and mount it under a table. Then, put some popcorn kernels on the table right above the magnetron. Finally, place my cell phone near the popcorn and point it at the kernels. Then secretly turn on the magnetron until a couple kernels pop. Will this work and is it safe? — MS, Charlottesville, Virginia

It probably won’t work and it’s definitely not safe. Instead of tricking your friends, you risk cooking them. Here is why I think you’d better leave your plan as a thought experiment only.

Those YouTube videos were complete fakes; they didn’t pop any popcorn while the camera was rolling. To make it appear that the cell phones were popping the corn, the people who produced the videos dropped already prepared popcorn into the frame and then photoshopped away the unpopped kernels. When you watch the video, it looks like the kernels are popping, but they’re really just disappearing via video editing as precooked popcorn is sprinkle onto the set from above.

The reason they had to use video trickery is pretty clear: to pop popcorn with microwaves, those microwaves have to be extremely intense. Each kernel contains only a tiny amount of water and it’s the water that heats up when the kernel is exposed to microwaves. If the microwaves aren’t intense enough, the heat they deposit in the kernel’s water will flow out to the rest of the kernel and into the environment too quickly for the kernel’s water to superheat and then flash to steam.

Even when you put popcorn kernels in a closed microwave oven, it takes a minute or two for the kernels to accumulate enough thermal energy to pop. In that closed microwave oven, the microwaves bounce around inside the metal cooking chamber and their intensity increases dramatically. It’s like sending the beam from a laser pointer into a totally mirrored room—the light energy in that room will build up until it is extremely bright in there. In the closed cooking chamber of the oven, the microwave energy also builds up until the microwave intensity is enough to pop the corn. How intense? Well, a typical microwave oven produces 700 watts of microwave power. Since the cooking chamber is nearly empty when you’re popping popcorn, the cooking chamber accumulates a circulating power of very roughly 50,000 watts.

Although that power is spread out over the cross section of the oven, the microwaves are still seriously intense — thousands of watts per square inch. To put that in perspective, a cell phone transmits a maximum of 2 watts and that power is spread out over at least 5 square inches so the intensity is less than 1 watt per square inch. When I saw those videos in Summer 2008, I realized that there was no way cell phones were ever going to pop popcorn. They certainly wouldn’t do it while they are ringing, because that’s when they are primarily receiving microwaves, not when they’re transmitting them. It’s when you’re talking that your cell phone is regularly producing microwaves. It was all obviously just fun and games.

So what about your disassembled microwave oven? Since there is no metal box to trap the microwaves and accumulate energy, they’ll only have one shot at popping the corn kernels. The microwaves will emerge from the magnetron’s waveguide at high intensity, but they’ll spread out quickly once there is nothing to guide them. You could probably pop kernel right at the mouth of the magnetron but not a few inches away. Unless you use microwave optics to focus those microwaves, they’ll have spread too much by the time they get through the table and reach the kernels of popcorn and the kernels will probably never pop.

If that were the whole story, the worst that would happen with your experiment would be that it wouldn’t cook popcorn. But there is a real hazard here. Sending about 700 watts of microwaves into the room isn’t exactly safe. It’s something like having a red hot coal emitting 700 watts of infrared light, except that you won’t see anything with your eyes and this microwave “light” is coherent (i.e., laser-like) so it can focus really tightly. You’d hate to have some metal structure in the room or even inside the walls of the room focus the microwaves onto you. You absorb microwave much better than the corn kernels and you’ll “pop” long before they do. Actually, your eyes are particularly sensitive to microwave heating and you might not notice the damage until too late. Without instruments to observe the pattern of microwaves in the room when the magnetron is on, I wouldn’t want to be in the room.

Can or should a microwave be disposed with the normal trash, what if any are the environmental impacts of the magnetron or other parts sitting in a landfill? — DNR

I figure that some day, we’ll turn to our landfills as resources for precious elements like copper and gold. That assumes, of course, that we survive global warming. In the meantime, we’ll just keep throwing stuff out.

Despite the scary title “microwave radiation,” a microwave oven is basically just another household electronic device. It is an extremely close relative of a convention cathode-ray-tube television set. If you’re OK with putting CRT televisions and computer monitors in the landfill, you should have no problems with putting microwave ovens there, too. Even when the microwave oven is on, all it has inside it is microwave radiation and that’s just not a big deal. The instant you turn it off, it doesn’t even have those microwaves in it. It’s just boring inert electronic parts and they’ll sit in the landfill for generations, rusting and decaying like every other abandoned electronic gadget. I’d rather see it go to a recycling center and have its precious materials returned to the resource bin, but as landfill junk goes, it’s not all that bad. Given that toxic chemicals are the primary concern with landfills, microwave ovens are probably rather innocuous. They have no radioactive contents and although the high-voltage capacitor might have oil in it, that oil can no longer be the toxic PCBs that were common a few decades ago. Even when that oil leaks into the environment, it’s probably not going to do much.

So there you have it, microwave ovens go to their graves no more loudly or dangerously than old televisions or computers or cell phones.

In fact, I might start calling cell phones “microwave phones” because that’s exactly what they are. They communicate with the base unit by way of microwave radiation. Given the number of people who have cell phones semi-permanently installed in their ears, concerns about microwave radiation should probably be redirect from microwave ovens to “microwave phones.” Think about it next time your six-year-old talks for an hour with her best friend on that “microwave phone.”