There has been some very interesting work going on at University of Utah - I was not aware that they were doing so much work in this area. They reported what appears to be the first active cloaking method for various types of waves including microwaves and a low cost radio tomography method to image through walls.
In recent years, scientists devised and tested various cloaking schemes. Experiments so far have been limited to certain wavelengths such as microwaves and infrared light, and every method tried so far has limitations. "We have shown that it is numerically possible to cloak objects of any shape that lie outside the cloaking devices, not just from single-frequency waves, but from actual pulses generated by a multi-frequency source," says Graeme Milton, senior author of the research and a distinguished professor of mathematics at the University of Utah.
Most previous research used interior cloaking, where the cloaking device envelops the cloaked object. Milton says the new method "is the first active, exterior cloaking" technique: cloaking devices emit signals and sit outside the cloaked object. Compared with passive cloaking by metamaterials, the new method - which involves generating waves to protect or cloak an object from other waves - can cloak from a broader band of wavelengths. The problem with metamaterials is that their behavior depends strongly on the frequency you are trying to cloak from.
This cloaking method utilizes destructive wave forms so the drawback is that you have to know the characteristics of the incoming waves before the arrive so the exact opposite wave can be transmitted as it arrives.
Here is a link to the full announcement which includes a video.
University of Utah engineers have shown that a low cost wireless network of radio transmitters can track people moving behind solid walls. The system could help police, firefighters and others nab intruders, and rescue hostages, fire victims and elderly people who fall in their homes or even be used in retail marketing and border control.
Their method uses radio tomographic imaging (RTI), which can "see," locate and track moving people or objects in an area surrounded by inexpensive radio transceivers that send and receive signals. They used commercially available ZigBee radios which are low cost and easily available. Several studies were done by placing transmitters around an area and measuring the variations in signal strength as people moved around to demonstrate the concept.
RTI measures radio signal strengths on numerous paths as the radio waves pass through a person or other target unlike radar that measures the reflected signal so it is more like CT or X-ray imaging. The researchers developed math formulas and used them in a computer program to convert attenuated signals - which occur when someone creates "shadows" by walking through the radio signals - into a blob-like, bird's-eye-view image of that person walking. So they were able to get rough images through a wall of people and objects moving around the room.
Here is a link to the full announcement about this subject which also has a video link. Great stuff - if you find any interesting research such as these projects going on, please let us know about them.