1.1 Radio Waves

Radio waves are created when electrically charged particles accelerate with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. Other emissions that fall outside of the RF spectrum include X-rays, gamma rays, and infrared and ultraviolet light. When a radio wave passes a copper wire or another electrically sensitive device, it produces a moving electric charge, or voltage, which can be transformed into an audio or data signal.

Radio waves can be depicted mathematically as a sinusoidal curve, as shown in Figure 1-1.

Figure 1-1. 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's CPU operates at 2 GHz, the internal clock of your CPU generates signals roughly at two billion cycles per second.

Note that frequency is inversely proportional to the wavelength: the longer the wavelength, the lower the frequency; the shorter the wavelength, the higher the frequency. 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 .

1.1.1 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 1-1 shows some of the bands defined by the FCC (see http://www.fcc.gov/oet/spectrum/table/fcctable.pdf).

Table 1-1. 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 higher	Extremely High Frequency (EHF)

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

• 1 kilohertz (kHz) = 1,000 Hz

• 1 megahertz (MHz) = 1,000 kHz

• 1 gigahertz (GHz) = 1,000 MHz

Wireless networks use a variety of radio frequencies. Table 1-2 shows some common wireless network protocols and the corresponding radio frequencies.

Table 1-2. Frequencies used by various wireless networks

Frequency range	Wireless network
2.45 GHz	Bluetooth
2.4 to 2.483 GHz	802.11, 802.11b, 802.11g
5.180 GHz to 5.805 GHz	802.11a
1.2276 and 1.57542 GHz	GPS

1.1.2 Radio Wave Behavior

Radio waves, similar to light waves, exhibit certain characteristics when coming into contact with objects.

Reflection occurs when a radio wave hits an object that is larger than the wavelength of the radio wave (see Figure 1-2). The radio wave is then reflected off the surface.

Figure 1-2. Reflection of a radio wave

Refraction occurs when a radio wave hits an object of a higher density than its current medium (see Figure 1-3). The radio wave now travels at a different angle ”for example, waves propagating through clouds.

Figure 1-3. Refraction of a radio wave

Scattering occurs when a radio wave hits an object of irregular shape, usually an object with a rough surface area (see Figure 1-4), and the radio wave bounces off in multiple directions.

Figure 1-4. Scattering of a radio wave

Absorption occurs when a radio wave hits an object but is not reflected, refracted, or scattered . Rather, the radio wave is absorbed by the object and is then lost (see Figure 1-5).

Figure 1-5. 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. Generally, transmissions made at higher frequencies are more subject to radio absorption (by the obstacles) and larger signal loss. Larger frequencies have smaller wavelengths; 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.

Diffraction occurs when objects block a radio wave's path . In this case, the radio wave breaks up and bends around the corners of the object (see Figure 1-6). This property allows radio waves to operate without a visual line of sight.

Figure 1-6. Diffraction of radio waves