Introduction to HF and Lower Frequencies


Learning Objectives



The Navy today uses a large number of radio frequencies for communications, targeting, search and rescue, navigation and other uses. The radio spectrum is becoming a crowded place. It is necessary for the Navy to use a wide variety of frequencies to accomplish its mission. In this module we will examine the lower radio frequencies. The environmental factors affecting these frequencies primarily deal with what is going in the upper reaches of the atmosphere, a region called the ionosphere. The characteristics of the ionosphere and how it affects electromagnetic radiation will be discussed later. First you will get some basic background information on lower radio frequency radiation and how the Navy uses it.


What are we talking about?

The lower frequency electromagnetic radiation that is the topic of this module is in the following bands:

Full Name
Frequency Range
ELF Extremely low frequency
30 Hz - 300 Hz
10,000 Km - 1,000 Km
VF or ULF Voice frequency or
Ultra Low frequency
300 Hz - 3 kHz
1,000 Km - 100 Km
VLF Very low frequency
3 kHz - 30 kHz
100 Km - 10 Km
LF Low frequency
30 kHz - 300 kHz
10 Km - 1 Km
MF Medium frequency
300 kHz - 3 MHz
1 Km - 100 m
HF High frequency
3 MHz - 30 MHz
100 m - 10 m

View schematic of EM Spectrum and Military uses.

Of the above bands, the most commonly used by the Navy is HF (High Frequency). This band will be the primary focus of this module. The term "High Frequency" is somewhat confusing because HF radio waves are actually at a lower frequency than most radio and radar systems. Probably for this reason, you will usually see these radio transmissions referred to as simply "HF" without the word "High Frequency" spelled out. The reason for this confusing nomenclature is because when the EM spectrum bands were first named in the early part of the 20th century, most of the radio communications were centered around the MF (Medium Frequency) band and in relation to this, HF radiation was relatively high frequency. Now with extensive use of Very High Frequency (VHF) and higher, HF is a relatively low frequency. You have probably heard the term "short wave radio". This is an older, but still commonly used term that refers to the same radio band as HF.


Why use HF and lower frequency radiation? (Advantages)

A general rule (and there are exceptions) for EM radiation is that is affected by features that have sizes the same or larger size than the wavelength of the radiation. As you can see from the chart, the radio bands above have relatively large wavelengths that are not as affected by objects such as building, trees, weather events or other features in the troposphere as the higher frequency radiation. Another general rule for radiation is that the lower the frequency, i.e. the longer the wavelength, the further a signal can penetrate through solid objects or liquids such as sea water. This type of radiation can also travel further distances around the Earth than other types of radiation, for reasons we will discuss later. Therefore HF and lower frequency radiation is the best to use for direct long distance communications (when satellites or long cables cannot be used).


Why not always use HF and lower frequency radiation? (Disadvantages)

The three main problems with this type radiation for communication are:

  1. The transmitting antennas must be very large, ideally the size of the wavelength, which complicates HF and lower transmissions from aircraft or MF and lower from surface ships or submarines. Receiving antennas can be smaller but are more efficient if they are large also.

  2. The amount of information that can be sent is limited. The amount of information that can be sent (often referred to as "bandwidth") is proportional to the frequency. Therefore these wavelengths, particularly those in the VLF or lower range are not useful when large amounts of information or data needs to be transmitted. Also there is less frequency "room" in the EM spectrum for these low frequencies than in the higher frequency bands.

  3. HF frequencies must be changed. HF communications often rely on the skywave transmission mode. The actual frequency used within the HF band for skywaves depends on the properties of the ionosphere, which are highly variable. Therefore the use of HF frequencies requires the operator to change frequencies at least twice a day and sometimes more often. This is less of a problem for lower frequencies that do not rely on the skywave transmission mode.


The Commander Naval Telecommunications Command (COMNAVTELCOM) controls the specific frequencies used by the Navy and Marine Corps. This section provides a general overview of the Navy uses in the lower frequencies.

ELF - This is the only band that can penetrate hundreds of meters below the surface of the ocean. The US Navy used to transmit ELF messages using a huge antenna in Wisconsin and Michigan created by several miles of cable on towers in conjunction with the underlying bedrock. This band was used to send short coded "phonetic letter spelled out" (PLSO) messages to deeply submerged submarines that were trailing long antenna wires. The communication was only one way, therefore it was used primarily for prearranged signals or to direct the submarine to come closer to the surface for faster communications. Environmental factors do not have a strong influence on changing the signal and therefore it was quite reliable. However, for several reasons, including large power requirements and possible effects on the environment and people, the US Navy's ELF communication system was discontinued in 2004. The Russian Navy reportedly still operates an ELF transmitter, ZEVS, located northwest of Murmansk on the Kola Peninsula in northern Russia.

ULF - This band can also penetrate through the earth and sea water. It is used for communications in mines. It is rarely used for military purposes.

VLF - This band can penetrate several meters below seawater and can transmit much more information than ELF, therefore it is useful for submarine communications when the submarine cannot surface, but can come close to the surface. It can be affected by salinity gradients in the ocean, but these usually do not present problems for near-surface submarines. There are natural sources of VLF radiation, but in general, like ELF, it is not strongly influenced by changes in environmental conditions therefore it is useful for reliable global communications. The transmission antennas need to be large, therefore it is primarily used for one-way communications from shore-based command centers to surface ships and submarines. It can also be used to broadcast to several satellites at once, which can in turn relay messages to the surface. The Navy's VLF systems serve as a back-up for global communication use during hostilities when nuclear explosions may disrupt higher frequencies or satellites are destroyed by enemy actions. VLF is also used for aircraft and vessel navigation beacons and for transmitting standard frequencies and time signals.

LF - The Navy uses this band primarily as an alternative to other bands that are often crowded. It is particularly useful in polar regions where ionospheric disturbances can affect higher frequency transmissions. A primary use for LF systems are for airport and vessel navigation. For example, the LORAN navigation system uses 100 kHz. It is also useful for long range communications. The Navy now uses eight LF channels for the Fleet Multichannel Broadcast System using radio teletypewriters. A drawback to LF is that large antennas must be used and many frequencies are susceptible to atmospheric noise, particularly in tropical regions.

MF - Commercial AM radio stations use the central part of this band, therefore the military uses are restricted to the high and low ends. This band is used for communications, international distress frequencies and for search and rescue. This is a useful band for moderate distance transmissions because the coverage usually extends further than VHF and UHF (discussed next module), but is not as affected by ionospheric disturbances as HF. The reception distance over ocean is considerable greater than over land, making it particularly useful for Naval use. Like all the lower frequencies, transmission in this band are usually ground waves, which limits the transmission distance, but makes them less susceptible to ionospheric disturbances than HF.

HF - The Navy makes extensive use of this band for communications. It is also used for long range ("over-the-horizon") radar. Due to the skywave transmission mode, HF radiation can travel great distances, sometimes to the other side of the earth. Due to its versatility and large coverage area, this is a very crowded band and the military can only use a few frequency regions scattered throughout this band. The most efficient transmissions require fairly large antennas, therefore it is most useful when at least one of the stations is on shore. The antenna size limits its use on aircraft. It cannot be used for satellite communications since it is reflected by the ionosphere. Many of the former uses of HF by the Navy are now being taken over by satellite communication systems. However, we expect that the Navy will continue to use HF for quite some time in the future. The primary drawback to HF use is that it is highly susceptible to changes in the ionosphere and therefore several frequencies must be available for use.



The rest of this module will examine how the ionosphere affects HF transmission and how to predict these effects.


Study Questions

  1. What do ELF, VF, VLF, LF, MF and HF stand for?
  2. What part of the atmosphere has the most influence on the transmission of these frequencies?
  3. Why is the term High Frequency (HF) somewhat of a misnomer (wrong name)?
  4. What are the three main limitations to the use of LF and lower frequencies?
  5. What is the primary advantage to ELF over other frequency bands?
  6. Which of the bands discussed above is the most used by the Navy?