FM - Frequency modulation

FM, which stands for Frequency Modulation, is a widely used modulation type in amateur radio, particularly for voice communications in the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands. FM is known for its high audio quality and resistance to interference, making it ideal for local communications.


1. The basic principle of FM

  • In frequency modulation, the frequency of the carrier signal is varied according to the modulation signal (e.g. speech), while the amplitude of the carrier signal remains constant.
  • The frequency changes in proportion to the amplitude of the modulation signal: as the amplitude of the speech signal increases, the carrier frequency increases, and as the amplitude decreases, the carrier frequency decreases.
  • This frequency variation carries the information of the modulation signal without affecting the amplitude of the carrier signal.


2. use in amateur radio

  • VHF and UHF bands: FM is mainly used in the VHF and UHF bands, particularly the 2 metre (144-148 MHz) and 70 centimetre (430-440 MHz) bands. These frequency ranges are ideal for local communications, for example between mobile stations or via relay stations.
  • Relay mode: FM is the most common operating mode for relay stations, which are often used to extend the range of handheld radios and mobile devices. Relay stations receive the signal on one frequency and retransmit it on another, usually higher, frequency.
  • Emergency communications: Because of its reliability and clear audio quality, FM is often used in emergency radio networks and disaster relief operations.


3. Advantages of FM

  • High audio quality: FM provides excellent audio quality because it is less susceptible to amplitude interference (such as noise and static).
  • Resistance to interference: FM is less susceptible to electromagnetic interference and static, making it ideal for use in urban areas and for mobile communications.
  • Constant signal strength: The constant amplitude of the FM signal means that amplifiers and receivers are simpler to design and do not require automatic gain control (AGC).


4. Disadvantages of FM

  • Wider bandwidth: FM signals require a wider bandwidth than SSB or AM. A typical FM signal in the amateur radio band can have a bandwidth of about 12.5 kHz (narrowband FM, NBFM) to 25 kHz (wideband FM, WBFM).
  • Range: While FM on VHF and UHF provides excellent audio quality, its range is limited in terms of over-the-horizon distances. FM is less suitable for communicating over long distances, as is possible with shortwave SSB signals.
  • Multipath effects: In urban areas or mountainous terrain, FM signals can be distorted by reflections and multipath effects, resulting in slight audio distortion.


5. technical details

  • Deviation: The "deviation" describes how much the frequency of the carrier signal is varied. In practice, this is controlled by narrowband and wideband FM techniques.
  • FM deviation: The deviation of the frequency from its mean value is directly proportional to the amplitude of the modulation signal. Higher deviations (larger sweeps) result in better audio quality, but require more bandwidth.
  • CTCSS and DCS: FM signals in amateur radio often use Continuous Tone-Coded Squelch System (CTCSS) or Digital Coded Squelch (DCS) to identify specific signals and avoid interference from other users. This is particularly useful when using relay stations.

 

6. FM operation in ham radio

  • Simplex operation: Communication takes place directly between two stations on a single frequency without the use of a relay.
  • Duplex operation: One frequency is used for transmitting and another for receiving, often via a relay. This allows continuous communication without the need for a station to switch.
  • FM repeaters: Commonly used on VHF and UHF, these repeaters significantly increase the range of FM signals. They are particularly useful in urban and densely populated areas.


7. History and development

  • FM was developed by Edwin Armstrong in the 1930s and quickly became the standard for high quality voice communication.
  • In amateur radio, FM became increasingly popular after the Second World War, especially with the availability of mass-market VHF and UHF equipment.


In summary, FM is an extremely important modulation in amateur radio, especially for local communications on the VHF and UHF bands. Its high audio quality, robustness against interference and the wide availability of FM relay stations make it the preferred choice for many radio amateurs, especially in urban areas and for mobile applications.