Extremely low frequency or ELF refers to the band of radio frequencies from 3 to 30Hz. The purpose of the EXTREMELY LOW-FREQUENCY (elf) communications system is to send short "phonetic letter spelled out" (PLSO) messages from operating authorities to submarines operating at normal mission speeds and depths. Elf has the ability to penetrate ocean depths to several hundred feet with little signal loss. This ability allows submarines to be operated well below the immediate surface and enhances submarine survivability by making detection more difficult.
Application
ELF is used by the US Navy to communicate with submerged submarines Despite the extremely high electrical conductivity of salt water, the water's density shields submarines from most electromagnetic communications. Signals in the ELF frequency range, however, can penetrate much more deeply. The low transmission rate of most ELF communications limits their use as communications channels; generally an ELF signal serves to request that a submarine surface and initiate some other form of contact.
Limitations
One of the difficulties posed when broadcasting in the ELF frequency range is antenna size. In order to transmit internationally using ELF frequencies, an extremely large antenna is required.
This is a one-way communications system from the operating authority to submarines at sea. The large size of elf transmitters and antennas makes elf transmission from submarines impractical.
VERY-LOW-FREQUENCY
Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radionavigation. Because VLF can penetrate water to a depth of 20 metres (66 feet), they are used to communicate with submarines near the surface. (ELF is used for fully submerged vessels.)
VERY-LOW-FREQUENCY (vlf) transmissions provide a highly reliable path for communications in these northern latitudes as well as over and under all oceans and seas of the world.
Application
Vlf is currently used for communications to large numbers of satellites and as a backup to shortwave communications blacked out by nuclear activity.
Secondary applications of the vlf range include worldwide transmission of standard frequency and time signals. Standard frequency and time signals with high accuracy over long distances have become increasingly important in many fields of science. It is essential for tracking space vehicles, worldwide clock synchronization and oscillator calibration, international comparisons of atomic frequency standards, radio navigational aids, astronomy, national standardizing laboratories, and communications systems.
LOW-FREQUENCY
Low Frequency or Longwave radio frequencies are those below 500 kHz, which correspond to wavelengths longer than 600 meters. They have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect nor refract using the ionosphere, so there are fewer phase-caused fadeouts.
The LOW-FREQUENCY (lf) band occupies only a very small part of the radio-frequency spectrum. This small band of frequencies has been used for communications since the advent of radio.
Application
In the past, the fleet broadcast system provided ships at sea with low-frequency communications via cw telegraph transmissions. As technology advanced, the system was converted to single-channel radio teletypewriter transmission. Today If communications is used to provide eight channels of frequency-division multiplex rtty traffic on each transmission of the fleet multichannel broadcast system
Limitations
Low-frequency transmitting installations are characterized by their large physical size and by their high construction and maintenance costs. Another disadvantage is low-frequency signal reception being seriously hampered by atmospheric noise, particularly at low geographical latitudes.
MEDIUM-FREQUENCY
Medium frequency or MF (sometimes called mediumwave) refers to radio frequencies (RF) in the range of 300-3000 kHz. The regular AM broadcast band is found in this range.
The MEDIUM-FREQUENCY (mf) band of the radio-frequency spectrum includes the international distress frequencies (500 kilohertz and approximately 484 kilohertz).
Applications
Mediumwave signals have the properties of following the curvature of the earth (the groundwave) and reflecting or refracting off the ionosphere at night (skywave). This makes this frequency ideal for both local and continent-wide service, depending on the time of day.
Limitations
Only the upper and lower ends of the mf band have naval use because of the commercial broadcast band (AM) extending from 535 to 1,605 kilohertz. Frequencies in the lower portion of the mf band (300 to 500 kilohertz) are used primarily for ground-wave transmission for moderately long distances over water and for moderate to short distances over land. Transmission in the upper mf band is generally limited to short-haul communications (400 miles or less).
SUPER HIGH FREQUENCY
Microwaves, also known as Super High Frequency (SHF) signals, have wavelengths approximately in the range of 30 cm (1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The existence of electromagnetic waves, i.e. microwaves, was predicted by James Clerk Maxwell in 1864 from his famous Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building apparatus to produce radio waves.
Application
The microwave spectrum is defined as electromagnetic energy ranging from approximately 300 MHz to 1000 GHz in frequency. Most common applications are within the 1 to 40 GHz range.
A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.4 GHz for the purpose of cooking food. Microwaves cook food by causing molecules of water and other compounds to vibrate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.
Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.
VERY HIGH FREQUENCY
Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m).
VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication. Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and can't interfere with transmissions thousands of kilometres away) It is also less affected by atmospheric noise and interference from electrical equipment than low frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than higher frequencies. It was also easier to construct efficient transmitters, receivers, and antennas for it in the earlier days of radio. In most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as that operated by taxis and police), marine two-way audio communications, and aircraft radios.
Application
Common uses for VHF are FM radio broadcast at 88-108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems (VOR in particular) and aircraft communications.
EXTREMELY HIGH FREQUENCY
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low (or far) infrared light. This band has a wavelength of one to ten millimeters, giving it the name millimeter band.
Limitations
Radio signals in this band are extremely prone to atmospheric attenuation, making them of very little use over long distances. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength.
Application
This band is commonly used in radio astronomy.
ULTRA HIGH FREQUENCY (UHF)
Ultra high frequency (UHF) radio frequencies are those between 300 MHz and 3.0 GHz, which is higher than those of very high frequency (VHF). UHF and VHF are the most common frequency bands for television.
UHF frequencies have higher attenuation from atmospheric moisture and benefit less from 'bounce', or the reflection of signals off the ionosphere back to earth, when compared to VHF frequencies. The frequencies of 300-3000 MHz are always at least an order of magnitude above the MUF (Maximum Usable Frequency). The MUF for most of the earth is generally between 25-35 MHz. Higher frequencies also benefit less from ground mode transmission.
Limitations
However, the short wavelengths of UHF frequencies allow compact receiving antennas with narrow elements; many people consider them less ugly than VHF-receiving models
HIGH FREQUENCY
High frequency (HF) radio frequencies are between 3 and 30 MHz. This range is often called shortwave.
Since the ionosphere often refracts HF radio waves quite well, this range is extensively used for medium and long range terrestrial radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:
• Sunlight/darkness at site of transmission and reception
• Transmitter/receiver proximity to terminator
• Season
• Sunspot cycle
• Solar activity
• Polar aurora
• Maximum usable frequency
• Lowest usable frequency
• Frequency of operation within the HF range
Applications
HF is often used for continuous wave mores code transmissions.
