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The simplest definition of radio transmission is the sending of something (usually information) with the use of radio waves. Radio waves are one type of electromagnetic wave. Other types of electromagnetic waves are visible light, infrared, ultraviolet, microwaves, x-rays, and gamma rays.
Common examples of radio transmissions are the signals coming from over-the-air radio and television stations, cellular phones, or walkie-talkies. In fact, the vast majority of wireless communications devices rely on radio transmissions.
It is necessary to have two elements to make a radio transmission. The first is a
transmitter, which is a device that generates an alternating electrical current of a very high frequency. The transmitter is connected to the second necessary element, an
antenna, which changes this electrical current into electromagnetic waves.
Different radio signals have different frequencies, which is why it is possible to tune in just one transmission. The tuning dial on a radio or the channel selector on a television changes the frequency that the device will pick up. Other devices, such as a cellular phone, do not have a tuning knob, but they are pre-tuned at the factory to pick up just one frequency or just a few frequencies. This is why your phone is able to pick up just the signal of a cellular tower, without being overwhelmed by the radio transmissions being sent on millions of other frequencies.
Different kinds of radio transmissions have different frequencies. For example, radio stations use frequencies of 540 - 1700 kHz for AM signals and 88 - 108 MHz for FM signals. A Hertz (abbreviated Hz) means one cycle per second. So an AM radio station with a frequency of 540 kHz is sending out radio transmissions that are vibrating at 540,000 times per second. An FM station with a frequency of 88 would be sending out transmissions that are vibrating 88,000,000 times per second.
Most cellular phones use frequencies of either 800 or 1900 MHz. Computers with a WiFi connection use either 2.4 or 5 GHz. GHz stands for "GigaHertz" and means one billion cycles per second. So your WiFi connection is sending out radio transmissions vibrating billions of times per second.
Of course, just sending out radio waves doesn't accomplish anything. A radio transmission needs to have some kind of purpose, and that purpose is almost always sending some kind of information. In the case of a radio or TV station, the information is contained in the sound or picture. For a cellular phone, the information is the sound of the telephone call. For a computer connected to WiFi, the information is the data that you are sending. The process of adding this information to the radio wave is called modulation. The process of getting the information after the signal is received is called demodulation or detection.
Every radio receiver contains a part called the detector which performs the function of extracting the information from the radio signal. The picture shown here is one of the earliest types of detector, known as the "cat's whisker." It is made up of a thin piece of wire which contacts a piece of mineral (usually galena). When a radio signal goes in, this simple detector is able to remove the sound that has been modulated onto the radio wave.
Most modern radio receivers use a part called a
diode, which is the modern equivalent of the cat's whisker.
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