1.2 Understanding Radio Waves

Most of the wireless technologies mentioned in the last section make use of radio waves. Wi-Fi, GPRS, GPS, and Bluetooth all utilize radio waves to transmit signals.

1.2.1 Radio Wave Basics

Put simply, 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 as shown in Figure 1-5.

Figure 1-5. A sine wave representing a radio wave

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. You should be familiar with this unit of measurement: if your new computer operates at 2 GHz, the internal clock of your CPU generates signals at roughly two billion cycles per second.

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.

1.2.2 Modulating Radio Waves

The sine wave carries data. To receive the transmission (such as audio or video), a radio wave receiver needs to tune itself to the same frequency as the transmitter. The receiver examines the amplitude or the frequency of the received electromagnetic wave in order to get at the transmitted data.

In the next section, I discuss three ways to carry data using radio waves.

1.2.2.1 Pulse Modulation

Pulse Modulation (PM) works by switching the radio signals ON and OFF (see Figure 1-6). This is similar to sending information using Morse code. The atomic clock set up by the National Institute of Standards and Technology in Fort Collins, Colorado uses PM to synchronize remote clocks and watches.

Figure 1-6. Pulse Modulation (PM)

1.2.2.2 Amplitude Modulation

Amplitude Modulation (AM), as the name implies, works by varying the amplitude of the sine waves (see Figure 1-7). Different amplitudes represent different values. The most famous example use of AM is in your radio.

Figure 1-7. Amplitude Modulation (AM)

1.2.2.3 Frequency Modulation

Frequency Modulation (FM) varies the frequency (the wavelength) of the sine waves (see Figure 1-8). The frequency of the sine waves changes slightly to represent different values. FM is commonly used in radios as well as popular household items such as televisions and cordless phones. Your mobile phone also uses FM.

Figure 1-8. Frequency Modulation (FM)

1.2.3 Radio Frequency Spectrum

To regulate the use of the various radio frequencies, the Federal Communications Commission (FCC) in the United States determines the allocation of frequencies for various uses. Table 1-2 shows some of the bands defined by the FCC (see http://www.fcc.gov/oet/spectrum/table/fcctable.pdf).

Table 1-2. Range of frequencies defined for the various bands
Frequency	Band
10 kHz to 30 kHz	Very Low Frequency (VLF)
30 kHz to 300 kHz	Low Frequency (LF)
300 kHz to 3 MHz	Medium Frequency (MF)
3 MHz to 30 MHz	High Frequency (HF)
30 MHz to 328.6 MHz	Very High Frequency (VHF)
328.6 MHz to 2.9 GHz	Ultra High Frequency (UHF)
2.9 GHz to 30 GHz	Super High Frequency (SHF)
30 GHz and above	Extremely High Frequency (EHF)

Table 1-3 shows some example of radio devices and their frequency ranges.

Table 1-3. Some common radio devices and their frequency ranges
Frequency range	Device
535 kHz to 1.705 MHz	AM radio
5.95 MHz to 26.1 MHz	Short wave radio
54 to 88 MHz	Television stations (channels 2 through 6)
88 MHz to 108 MHz	FM radio
174 to 216 MHz	Television stations (channels 7 through 13)
~ 900 MHz, ~ 2.4 GHz, ~ 5 GHz	Cordless phones
1.2276 and 1.57542 GHz	Global Positioning Systems (GPS)

You can get a more detailed frequency allocation chart from http://www.ntia.doc.gov/osmhome/allochrt.pdf. The following is a conversion list that should help you understand this chart:

1 kiloHertz (kHz) = 1000 Hz
1 MegaHertz (MHz) = 1000 kHz
1 GigaHertz (GHz) = 1000 MHz

1.2.4 Radio Wave Behavior

Radio waves, like light waves, exhibit certain characteristics when coming into contact with objects. Here are some of the possible behaviors.

1.2.4.1 Reflection

Reflection occurs when a radio wave hits an object that is larger than the wavelength of the radio wave (see Figure 1-9). The radio wave is then reflected off the surface.

Figure 1-9. Reflection of a radio wave

1.2.4.2 Refraction

Refraction occurs when a radio wave hits an object of a higher density than its current medium (see Figure 1-10). The radio wave now travels at a different angle. An example would be radio waves propagating through clouds.

Figure 1-10. Refraction of a radio wave

1.2.4.3 Scattering

Scattering occurs when a radio wave hits an object of irregular shape, usually an object with a rough surface area (see Figure 1-11), and the radio wave bounces off in multiple directions.

Figure 1-11. Scattering of a radio wave

1.2.4.4 Absorption

Absorption occurs when a radio wave hits an object that does not cause it to be reflected, refracted, or scattered, so it is absorbed by the object (see Figure 1-12). The radio wave is then lost.

Figure 1-12. Absorption of a radio wave

Radio Interference and Absorption

Radio waves are subject to interference caused by objects and obstacles in the air. Such obstacles can be concrete walls, metal cabinets, or even raindrops. In general, transmissions made at higher frequencies are more subject to radio absorption (by the obstacles) and larger signal loss. Larger frequencies have smaller wavelengths, and hence signals with smaller wavelengths tend to be absorbed by the obstacles that they collide with. This causes high frequency devices to have a shorter operating range.

For devices that transmit data at high frequencies, much more power is needed in order for them to cover the same range as compared to lower frequency transmitting devices.

1.2.4.5 Diffraction

Sometimes a radio wave will be blocked by objects standing in its path. In this case, the radio wave is broken up and bends around the corners of the object (see Figure 1-13). It is this property that allows radio waves to operate without a visual line of sight.