Radio Waves and Radio Transmission
- [Cornell University]
- Radio Waves
Radio waves are used for wireless transmission of sound messages, or information, for communication, as well as for maritime and aircraft navigation. The information is imposed on the electromagnetic carrier wave as amplitude modulation (AM) or as frequency modulation (FM) or in digital form (pulse modulation). Transmission therefore involves not a single-frequency electromagnetic wave but rather a frequency band whose width is proportional to the information density. The width is about 10,000 Hz for telephone, 20,000 Hz for high-fidelity sound, and five megahertz (MHz = one million hertz) for high-definition television. This width and the decrease in efficiency of generating electromagnetic waves with decreasing frequency sets a lower frequency limit for radio waves near 10,000 Hz.
- Wireless Transmission and Radio Waves
All wireless transmission is accomplished through waves. In the case of Wi-Fi, the waves are produced on the electromagnetic (EM) spectrum. The EM spectrum consists of many different types of waves that make up the spectrum. The smallest type of waves is the gamma-ray, followed by X-rays. Further down the spectrum is the visible spectrum of light. Further, still are micro-waves. Finally, the largest waves found at the other end of the spectrum are radio waves. Radio waves are the waves upon which data are transmitted.
All types of waves can be measured by their amplitude and frequency. The amplitude of a wave measures how tall a wave is from its midpoint to its top or bottom. The frequency of a wave measures how fast a wave is traveling. This also translates into how compressed the wave is or how many crests and troughs can be found per unit of distance. Radio waves can be naturally generated through electric pulses. Both the amplitude and the frequency of a wave can be modulated.
In short, the transmission of data wirelessly is made possible by the manipulation of radio waves. These waves are generated naturally by generating pulses of electricity. These radio waves can then be modified by their amplitude or frequency in order to transmit sound or data. This process can also be improved in order to increase the amount of data transmitted as well as the speed by which the data can be transmitted. The fundamentals of this process are relatively simple but can quickly become complex as the process is explored in greater detail.
- Examples of Applications
Radio waves transmit music, conversations, pictures and data invisibly through the air, often over millions of miles -- it happens every day in thousands of different ways! Even though radio waves are invisible and completely undetectable to humans, they have totally changed society. Whether we are talking about a cell phone, a baby monitor, a cordless phone or any one of the thousands of other wireless technologies, all of them use radio waves to communicate.
Here are just a few of the everyday technologies that depend on radio waves: AM and FM radio broadcasts, Wireless networks, Cell phones, GPS receivers, Ham radios, Satellite communications, Police radios, Wireless clocks. The list goes on and on...
Even things like radar and microwave ovens depend on radio waves. Things like communication and navigation satellites would be impossible without radio waves, as would modern aviation -- an airplane depends on a dozen different radio systems.
A radio wave is an electromagnetic wave. It can propagate through a vacuum, air, liquid, or even solid objects. It can be depicted mathematically as a sinusoidal curve. The distance covered by a complete sine wave (a cycle) is known as the wavelength . The height of the wave is called the amplitude . The number of cycles made in a second is known as the frequency. Frequency is measured in Hertz (Hz), also known as cycles per second. So, a 1 Hz signal makes a full cycle once per second. If your new computer operates at 2 GHz, the internal clock of your CPU generates signals at roughly two billion cycles per second.
A short wavelength means that the frequency will be higher because one cycle can pass in a shorter amount of time. Similarly, a longer wavelength has a lower frequency because each cycle takes longer to complete.
Note that frequency is inversely proportional to the wavelength - the longer the wavelength, the lower the frequency; the higher the frequency, the lower the wavelength. The wavelength of a 1 Hz signal is about 30 billion centimeters, which is the distance that light travels in one second. A 1 MHz signal has a wavelength of 300 meters.
- Radio Frequency (RF) and RF Filters
Frequencies are found everywhere in nature, so you need to identify their unique ranges to filter out the ones you don't want to hear and isolate the ones you do.
Filters work by reducing - or ideally, eliminating - frequencies that we are not interested in. With a camera, you can filter out ultraviolet (UV) rays to improve image quality.
In the cellular spectrum, the large number of available frequencies is divided into channels so that conversations can take place on one channel without being disturbed by conversations happening on other channels at the same time. However, this only works if you can isolate a channel from all other frequencies in that spectrum.
Radio frequency (RF) filtering isolates and uses specific frequencies within a channel without having to deal with all other channels that exist at the same time.
There are four ways to filter these frequencies:
- Filter out the high frequencies and pass through only low ones.
- Filter out the low frequencies and pass through only high ones.
- Isolate some range of frequencies, eliminating all frequencies above and below that range – this range is referred to as a “band,” and such a filter is a “band-pass” filter.
- Eliminate only one range of frequencies, keeping all others intact – this is referred to as a “band-stop” filter.
Therefore, RF filters are critical to our modern cellular data systems. Each channel is a frequency band, and some modern cell phones may have as many as 60 bandpass filters, each isolating a channel.
[More to come ...]
