More sophisticated smart phones can perform similar functions of a portable computer. Cell phones use radio waves to communicate. Radio waves transport digitized voice or data in the form of oscillating electric and magnetic fields, called the electromagnetic field EMF.
The rate of oscillation is called frequency. Radio waves carry the information and travel in air at the speed of light. Cell phones transmit radio waves in all directions. The waves can be absorbed and reflected by surrounding objects before they reach the nearest cell tower.
For example, when the phone is placed next to your head during a call, a significant portion over half in many cases of the emitted energy is absorbed into your head and body. Cell phones contain at least one radio antenna in order to transmit or receive radio signals. An antenna converts an electric signal to the radio wave transmitter and vice versa receiver.
Some cell phones use one antenna as the transmitter and receiver while others, such as the iPhone 5, have multiple transmitting or receiving antennas. An antenna is a metallic element such as copper engineered to be a specific size and shape for transmitting and receiving specific frequencies of radio waves. While older generation cell phones have external or extractable antennas, modern cell phones contain more compact antennas inside the device thanks to advanced antenna technologies.
Many modern smart phones also contain more than one type of antenna. As mentioned earlier, a cell phone is a two-way wireless communication device and needs both the inbound signal reception and the outbound signal transmission to work.
The connectivity between a cell phone and its cellular network depends on both signals and is affected by many factors, such as the distance between the phone and the nearest cell tower, the number of impediments between them and the wireless technology e. You know the hand crank on those old-fashioned telephones? It was used to generate the ring-signal AC wave and sound the bell at the other end! Not only is a telephone a simple device, but the connection between you and the phone company is even simpler.
In fact, you can easily create your own intercom system using two telephones, a 9-volt battery or some other simple power supply and a ohm resistor that you can get for a dollar at Radio Shack. You can wire it up like this:. Your connection to the phone company consists of two copper wires. Usually they are red and green. The green wire is common, and the red wire supplies your phone with 6 to 12 volts DC at about 30 milliamps. If you think about a simple carbon granule microphone, all it is doing is modulating that current letting more or less current through depending on how the sound waves compress and relax the granules , and the speaker at the other end "plays" that modulated signal.
That's all there is to it! The easiest way to wire up a private intercom like this is to go to a hardware or discount store and buy a foot phone cord. Cut it, strip the wires and hook in the battery and resistor as shown. Most cheap phone cords contain only two wires, but if the one you buy happens to have four, then use the center two.
When two people pick up the phones together, they can talk to each other just fine. This sort of arrangement will work at distances of up to several miles apart.
The only thing your little intercom cannot do is ring the phone to tell the person at the other end to pick up. The "ring" signal is a volt AC wave at 20 hertz Hz.
If you go back to the days of the manual switchboard, it is easy to understand how the larger phone system works. In the days of the manual switchboard, there was a pair of copper wires running from every house to a central office in the middle of town. The switchboard operator sat in front of a board with one jack for every pair of wires entering the office. Above each jack was a small light. A large battery supplied current through a resistor to each wire pair in the same way you saw in the previous section.
When someone picked up the handset on his or her telephone, the hook switch would complete the circuit and let current flow through wires between the house and the office. This would light the light bulb above that person's jack on the switchboard. The operator would then send a ring signal to the receiving party and wait for the party to pick up the phone.
Once the receiving party picked up, the operator would connect the two people together in exactly the same way the simple intercom is connected! It is that simple!
In a modern phone system, the operator has been replaced by an electronic switch. When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know that the switch and your phone are working. For more information on tones, see How Guitars Work. The dial tone sound is simply a combination of hertz tone and a hertz tone, and it sounds like this. Click here to hear a dial tone.
You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones:. Click here to listen to a touch-tone number.
If the number is busy, you hear a busy signal that is made up of a hertz and a hertz tone, with a cycle of one-half second on and one-half second off, like this:. Click here to listen a busy signal. In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3, hertz. All of the frequencies in your voice below hertz and above 3, hertz are eliminated.
That's why someone's voice on a phone has a distinctive sound. Compare these two voices:. Call up someone you know and play the 1,hertz sound file on your computer. The person will be able to hear the tone clearly. The person will also be able to hear the 2, and 3,hertz tones. However, the person will have trouble hearing the 4,hertz tone, and will not hear the 5, or 6,hertz tones at all!
That's because the phone company clips them off completely. For lots more information on telephones, telephone networks and related technologies, check out the links below. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots.
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Telephone Design " ". A switch to connect and disconnect the phone from the network - This switch is generally called the hook switch. It connects when you lift the handset. A speaker - This is generally a little cent, 8-ohm speaker of some sort. A microphone - In the past, telephone microphones have been as simple as carbon granules compressed between two thin metal plates.
Sound waves from your voice compress and decompress the granules, changing the resistance of the granules and modulating the current flowing through the microphone.
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