Topic 7. Detecting electromagnetic radiation

General principle: For radio signals, the size of the receiving antenna is about the same as the size of the wavelength it is designed to receive.

For electromagnetic waves used for communications (radio, TV, cell phones, etc.) , it is common to use an antenna that is one-quarter of the wavelength of the wave to be generated. Unlike a transmitter, the size of the antenna is often driven by other considerations – the physical size of a cell phone, for example.

In addition to tuning the receiver based on the length of the antenna, there are usually additional frequency-selective elements in a receiver.

Many modern receivers use digital tuning, in which the frequency-selection is made using software.

The radio frequency portion of the spectrum extends from very low frequencies on the order of kHz to frequencies of 20 GHz (or even higher in some applications). The corresponding wavelengths range from hundreds of meters for the low frequencies to several centimeters for the highest frequencies so that it is feasible to construct antennas that are comparable in size to the wavelength of the desired signal.

The wavelengths of optical signals are too short to design antennas based on these considerations. Optical detectors tend to be based on the interaction between the electromagnetic radiation and atoms or molecules, which often respond only to specific wavelengths or to a specific range of wavelengths. The details of the detection mechanism depend on the detailed properties of the material, and understanding these mechanisms requires complex calculations using quantum mechanics.

Since a black body absorbs all of the energy that is incident on it and uses the energy to raise its internal temperature, the temperature of a black body can be used as a relatively crude indicator of the size of a received signal. By definition (a black body absorbs everything equally well), this sort of detector cannot provide any information on the frequency of the incident radiation. A very sensitive detector based on this principle is called a bolometer, which is basically a black body combined with a very sensitive thermometer.

Another common detection method uses the photoelectric effect, in which an incident light wave transfer enough energy to an electron in the receiving material to cause the electron to escape from the material. The energy with which the electron is ejected from the material is related to the wavelength of the incident radiation. This electron can be captured by a nearby electrode, and it can be detected in this way. Variations on this idea are used in television cameras, to turn street lights on at dusk, etc. As with other interactions between light and matter, a detailed understanding of the photoelectric effect requires quantum mechanics.

In addition to discrimination based on the frequency or wavelength of a signal, many receivers include complex pattern-matching algorithms. These algorithms are gradually growing more complex as the spectrum becomes more crowded and more applications share the same frequency allocation. Pattern-matching algorithms are common in cell phones, pagers, and optical processing. They are especially important in real-world optical applications, such as distinguishing between red T-shirts and red traffic lights.

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