Cell Phones and the Electromagnetic Spectrum

The Hidden World of the Electromagnetic (EM) Spectrum

Wireless Phones and the EM Spectrum

Effective utilization of the EM spectrum is key to all wireless communications. It's why the government has designated every part of the usable spectrum for a specific purpose. Without an imposed order, the spectrum would become all but useless. It's analogous to allowing fans to bring bull horns to a sporting event. With large numbers of individuals amplifying their voices, communication would become impossible.

In the early days, talking wirelessly over a great distance required a high powered transmitter. When you used the transmitter it would interfere with all other communications on the same frequency within the transmitter's range (see yellow representation at right).

If the transmitter were adjusted to reach a single listener only 10 miles away it would interfere with transmissions on the same frequency within a 10 mile radius, wiping out communications in a 314 square-mile area. Having a two-way conversation as normally done on a telephone made the situation even worse. It required two frequencies.

In theory there's an infinite number of frequencies available for communication, but in reality, to prevent interference, the frequencies have to be separated by a significant amount. This limited the number of simultaneous conversations to a relatively low number.

First generation personal communication devices greatly reduced this problem with a grid of radio towers (see blue dots at left). Cell phone "A" would communicate with a nearby tower by using a weak signal (represented as red circles). The tower would transmit the conversation via the telephone system (represented as a solid red line) to a distant tower that would then transmit to the receiving cell phone "B", again using a very weak signal. If each signal had a range of say one mile, they would only wipe out other communications on the same frequency in a total area of only 6.28 square miles--giving a dramatic improvement in the number of possible simultaneous conversations. What's more, the conversations could be carried for thousands of miles with no additional interference problems.

First generation analog phones, which converted voice sounds directly into electrical signals, were eventually replaced by digital phones. These first convert or digitize voice sounds into binary numbers and then transmit them. These binary numbers or digital signals can be compressed allowing numerous communications to be simultaneously transmitted using the same frequency--again a big improvement in the number of possible conversations.

The next step was to make better use of temporarily unused parts of the allotted spectrum with various innovative systems such as spread spectrum. Here the signal is spread across the a segment of spectrum in what appears to be a random manner. In reality, however, the signal is not truly randomized and can be easily recovered using a specific decoding key. What's next for wireless communication? Click here to find out

cell phone grid

Cell Phone (shown in red) vs. Conventional Broadcasts (shown in yellow)

Human Wireless Communication

Much of the EM spectrum with wavelengths longer than visible light is used for human communications. For example, infrared is used for TV remote controls, microwaves for cell phones, radio waves for (you guessed it) radio, etc.

Waves in this section of the spectrum are not considered ionizing and at low levels of intensity they're not considered harmful. However, at high levels such as those found in microwave ovens, they can cause burns. The extremely high levels of infrared given off by nuclear bomb blasts can actually vaporize objects including people.

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Ionizing Radiation

EM waves are often referred to as EM radiation because waves radiate outward from their source. While we typically think of all radiation as harmful, most is not. The key dividing line is roughly at visible light. At shorter wavelengths (ultraviolet to gamma rays), EM radiation is called ionizing radiation because even the lowest levels of it can ionize by removing electrons from otherwise neutrally charged atoms. This removal creates sites for harmful chemical reactions resulting in conditions such as sunburn or defects in DNA molecules

The body can defend against ionizing radiation, if the level is not too high and the exposure not too long. For example, sun tanning helps defend against skin damage from ultraviolet (UV) light. However, even with a suntan, long term exposure to UV will eventually damage the skin and give it a leathery appearance.

EM Particles--Photons

While EM radiation is often modeled as a wave, it can also be modeled as particles called photons. Often the 2 models are combined. For example, the energy each photon has is directly related to the EM radiation's wavelength. The photon's energy increases as wavelength decreases.

A single photon of ionizing EM radiation has enough energy to remove a single electron from its atom. The brighter or more intense the EM radiation is, the more photons it contains and the more electrons it can remove.

Relative Size: bacteria
Frequency: 300,000 GHz
Energy per Photon: 2.0 x 10	-19	J

Relative Size: virus
Frequency: 3,000,000 MHz
Energy per Photon: 2.0 x 10	-18	J

Relative Size: football field
Frequency: 0.003 GHz
Energy per Photon: 2.0 x 10	-27	J

Relative Size: blueberry
Frequency: 30 GHz
Energy per Photon: 2.0 x 10	-23	J

Relative Size: wheel dia of an 18-wheeler
Frequency: 0.3 GHz
Energy per Photon: 2.0 x 10	-25	J

Relative Size: water molecule
Frequency: 3,000,000,000 GHz
Energy per Photon: 2.0 x 10	-15	J

Relative Size: proteins
Frequency: 30,000,000 GHz
Energy per Photon: 2.0 x 10	-17	J

Relative Size: human hair
Frequency: 3,000 GHz
Energy per Photon: 2.0 x 10	-10	J

Untitled Document

Center for Research in Wireless Communications, 301 Fluor Daniel Engineering Innovation Building
Holcombe Department of Electrical and Computer Engineering, Box 340915, Clemson, SC 29634-0915 -- 864.656.3946 (voice & fax)
Wilson.Pearson@ces.clemson.edu

Maintained by Tom Rogers. Last Updated: July 13, 2007

Area Code: 864, Information: 656-3311

<-- Wave Length = 7x10^-7meters

Why does a red object look red?
White light is a mixture of all the wave lengths in the visible part of the EM spectrum. Shine white light on a red object, say an apple, and the apple will reflect most of the red waves while absorbing most of the others. Some of the reflected red light travels to your eyes so that the image of the apple looks red. Shine a blue light on a red apple and it will look black.

<-- Wave Length = 4x10^-7meters