M11L3 radio communication

• Increasing frequency of FM wave

  Wavelength decreases
• AM carrier wave frequencies range from 540,000 to 1,600,000 Hz
• FM carrier frequencies range from 88 million to 108 million Hz
• FM signals tend to be more clear than AM signals
• Broadcasting radio waves


  1. Electric signal causes electrons in antenna to vibrate

  2. Vibrating electrons create electromagnetic waves that travel outward
• Moving away from broadcasting antenna


  Signal strength becomes weaker
• Radios in New York City do not pick up FM radio stations broadcast in Los Angeles
• Bad weather, surrounding mountains, and artificial structures can interfere with radio transmissions
• Receiving radio waves


  1. Electrons in metal antenna vibrate

  2. Vibrating electrons produce changing electric current with information

  3. Current makes speakers vibrate to create sound waves
• Amplitude-Shift Keying (ASK)


  Digital modulation method resembling traditional AM
• Frequency-Shift Keying (FSK)


  Digital modulation method resembling traditional FM
• Television stations in the US use multiple amplitude modulations to encode data on the carrier wave
• Cell phones and cordless phones are transceivers
• Cordless phones must remain close to the base unit
• Cordless phones can pick up nearby conversations on the same frequency
• Cordless phones have a channel button to switch to another frequency
• Pagers are small, portable radio receivers used in hospitals to avoid interfering with medical equipment
• Pager operation


  1. Caller leaves message at central terminal

  2. Message converted to electronic signal and transmitted with pager ID

  3. Pager receives all messages on its assigned frequency and responds to its ID
• Restaurants use pagers to notify customers their table is ready
• Communications satellite

  Receives signal, amplifies it, and transmits it to a particular region on Earth
• Satellites receive signals at one frequency and broadcast at a different frequency to avoid interference
• Satellite telephone systems allow calls from remote locations by relaying signals through satellites
• Satellite links can have delays due to the large distance signals travel
• Satellite television uses microwaves rather than radio waves, which travel more easily through the atmosphere
• Ground receivers for satellite TV are dish-shaped to focus the microwaves onto an antenna
• GPS satellites are owned and operated by the US Department of Defense but the signals can be used by anyone
• GPS is used for navigation by airplanes, ships, cars, cell phones, and pet collars