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Electromagnetic waves can come from satellite stations, wireless internet, radio stations, and digital broadcasts. An RF harvesting system captures and converts electromagnetic energy into usable continuous voltage using an antenna and a rectifier circuit as its fundamental building blocks. 
The theme the month is Energy Harvesting , and when the subject of harvesting energy from RF frequencies was mentioned in the theme doc, Project14 | Energy Harvesting: Build Projects that Scavenge Energy from Wasted Energy Sources! , the crystal radio was listed as one of the earliest examples. Like many of you, I have nostalgia for the old radio. I still look back fondly to listening to baseball games on the old AM dial. My radio happened to also double as an alarm clock, it displayed the time in red LED, and plugged into the wall. The thing I remember most about it was listening to Jack Buck and Mike Shannon call St. Louis Cardinals games on KMOX. With a good radio, KMOX's nighttime signal can be heard in most of the Central United States and into Mexico and Canada. I've heard baseball games during station wagon car rides when on family trips, in the country side close to Western Pennsylvania (don't remember where exactly but it was a long way from the Midwest), when I was growing up. There could be a lot of static depending on where you were or what the weather was like outside. As it turns out, that static was simply the radio picking up naturally occurring energy on the RF spectrum. All these invisible radio waves traveling such vast difference. When I found out that anyone can actually create your own crystal radio, one that does not need to be plugged in the wall, so you could presumably listen to Cardinal games without a battery or a power cord, this fascinated me all the more. I dug up some interesting information about these things when I was preparing this months theme and because its a subject I find interesting I thought I'd share with the rest of the community. 
This information is borrowed from the excellent article I found here. To build a crystal radio you need essentially four parts. You need wire (such as magnet wire) to catch the radio waves and allow you to tune the radio. You'll need a crystal/diode. If you want to be totally authentic and use a real crystal you have to be very specific about the type of crystal that you are allowed to use. Galena, a crystal form of lead is a popular option. If a crystal radio uses a real crystal it is called a "cat whisker set" due to the way the wires dangle over the crystal. Using a real crystal can be frustrating because you'll constantly need to hand tune them while listening or you'll lose your signal. You can avoid the headache and use a modern diode such as type 1N34A. A modern diode is essentially cat-whiskers with a fixed and unmovable design. Next, you'll need a high impedance or a ceramic earphone. Modern earphones require power so the one's you have lying around won't work. You'll need an extremely sensitive earphone that will react to tiny vibrations in your circuit. If you're going to do anything other than a basic crystal radio you'll need to use a resistor (if your antenna is longer). To build your crystal radio you'll need to make the coil and turn the coil into a radio. If this peaks your interest then check out the steampunk article here!
The history of technology has always been something that I've found interesting. The history of crystal radios began with a bunch of obscure discoveries in the late 19th century. As you'll recall from the theme doc for the Electromagnetism competition, Project14 | The Force of Electromagnetism: Anything from Solenoids, Motors, to Wireless Transfer! , we discussed how James Clerk Maxwell's mathematical theory of 1873 had predicted that electromagnetic disturbances should propagate through space at the speed of light and should exhibit the wave-like characteristics of light propagation. Unfortunately, he did not live long enough to witness the physical manifestations of many of his mathematical proofs, having passed away in 1879. It wasn't until 1887, till Hertz uncovered many of those mathematical proofs through designing a brilliant set of experiments to test Maxwell's hypothesis. At the time, infrared light was something that had been known about for a long time. What Hertz did is he measured the temperature of each color of light and discovered that the color with the highest temperature was the one that could not be seen as visible light. A year later, inspired by Hershel's discovery, Johann Wilhelm Ritter looked to see what light existed beyond the purple end of the spectrum, and discovered ultraviolet light. Before passing away, James Clerk Maxwell predicted that there should be light with even longer wavelengths than infrared light. In 1887 Heinrich Hertz demonstrated the existence of such waves by producing radio waves in his laboratory. A year later he made the discovery that electromagnetic radiation in the microwave and radio regions of the spectrum display the same basic behavior as visible light—reflection, refraction, diffraction, interference, polarization. Higher-energy (shorter wavelength) light in the electromagnetic spectrum took a bit longer to discover.
Crystal came to be used in radios in the early 20th century when it was discovered that certain metallic minerals such as galena could be used to detect radio signals. Before that, the coherer was used as a primitive form of a radio signal detector and it was used in the first wave of radio receivers at the beginning of the 20th century. Its use in radios was made possible by the 1890 findings of a French physicist, Edouard Branly and its subsequent adaptation by other physicists and inventors over the course of the next 10 years. A coherer was a device that consisted of a tube or capsule that contained two electrodes that were spaced a small distance apart, with loose metal filings in the space between. When a radio frequency signal is applied to the device, the metal particles cling together or "cohere", thus reducing the initial high resistance of the device, and allowing a much greater direct current to flow through it. In the receiver, the current activates a bell, or a Morse paper tape recorder, making a record of the received signal. The metal filings remain conductive after the signal (pulse) has ended so that the coherer is "decohered" by tapping it with a clapper actuated by an electromagnet, each time a signal was received, thus restoring the coherer to its original state.
Coherers were widespread until around 1907, but fell out of use due to their lack of sensitivity to weak signals, and in favor of more sensitive electrolytic and crystal detectors. A discovery by an Indian physicist named Jagadish Chandra Bose, four years after Edouard Branly's findings, helped pave the way for the use of crystals to be used to detect radio signals instead. In 1894, Bose became the first to use a crystal as a radio wave detector when he used galena detectors to receive microwaves. In 1901, he filed for a U.S. patent mentioning the use galena detectors in "A Device for Detecting Electrical Disturbances." It was granted in 1904. A patent for a silicon crystal detector was filed by Greenleaf Whittier Pickard on August 30th, 1906 and granted on November 20, 1906. A crystal detector typically consists of a crystal, a thin wire or metal probe that contacts the crystal, and a stand or enclosure to hold everything in place. While the most common crystal is a small piece of galena; pyrite is also frequently used because it is more easily adjusted and a stable mineral, sufficient for urban signals. The benefit of using crystals to detect RF is that they can demodulate amplitude modulated signals. This technology helped bring radiotelephones and voice broadcasts to a public audience. This simple and inexpensive technology allowed the radio broadcasting industry to grow and develop while it was still in its infancy, in the 1920s and the 1930s.
In 1922, what is now known as NIST (National Institute of Standards and Technology) was referred to as the US Bureau of Standards. They published an article called "Construction and Operation of a Simple Homemade Radio Receiving Outfit" that showed how anyone with simple tools could build their own radio and use it tune into weather, crop prices, time, news and the opera. That same year, NBS followed suit with a more selective two-circuit version, Construction and Operation of a Two-Circuit Radio Receiving Equipment With Crystal Detector, and their method is still popular with enthusiasts today. (You can download an attachment of this below). Here's a video that show you how to build a crystal radio:
Here's a video that shows you how a crystal radio works:
In addition to mineral crystals, the oxide coatings of many metal surfaces act as semiconductors (detectors) capable of rectification. For this reason, you can improvise for a crystal radio using detectors made from rusty nails, corroded pennies, and many other common objects for your Energy Harvesting project. A really fascinating description on fox hole radios can be found here. The author muses over a Heath Kit for making a crystal radio which excited his interest in electronics. It them gives a pretty interesting explanation of fox hole radios. Fox hole radios were made by GI's during WWI, WWII and later conflicts, to listen to news, music, news from home, and the all important sports scores. Although the enemy propaganda was laughable, those stations attracted GI’s with their popular music pumped out by powerful radio transmitters. As was the case with crystal radios, these Energy Harvesting devices required no batteries and were powered by the signal from a distant radio station and collected a the receiver antenna. They were built and used while they were waiting, in a fox hole.
 | nbs121.pdf |