From "On the Air Magazine", Vol. 2, No.1, September 1998
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While the vast majority of radio communication and broadcasting now takes place in the HF , VHF and UHF portions of the radio spectrum, the frequency range between 10 kHz and 500 kHz was once the dominant location of such activity. This was due to the ability of these frequencies to provide reliable long distance communications under adverse conditions. Even during periods of intense solar flares when all other frequencies were unusable, a VLF signal not only got through, but their propagation characteristics improved! In this page we will outline the specific characteristics for each range of frequencies, and how these affect the way radio communication is practised in those bands.
Up to the beginning of the space age in the 1960's, many important commercial and military transmitting stations operating in the VLF frequencies still carried a large amount of routine "traffic" due to this ability to deliver a message. The expanded use of communications satellites has brought about a decline in the use of the LF/VLF range of frequencies, but there are still used in many transmissions that people can monitor. In this page an outline of who is using low frequencies today will be provided, and these will be grouped into the three main bands of the low frequency spectrum. A further identification will be made of which government, military or private organisations are using which particular frequencies, and why they have chosen to use the frequencies that they are.
We will first present a summary of the propagation characteristics found in the LF/VLF frequencies in order to show how greatly they differ from those in the HF range.
 | Propagation characteristics of the LF/VLF/ELF frequencies |
The "long wave" spectrums of frequencies are divided into three main grouping by convention. These are;
LF (Low Frequencies) 535 to 150 kHz
VLF (Very Low Frequencies) 150 kHz to 3 kHz
ELF (Extremely Low Frequencies) 3 kHz to 300 cycles (approx.)
The actual division points between the groups based on frequency is relative (see figure 1). What is more important than dividing up frequencies into such groups is understanding the propagation characteristics that are found at a particular frequency, particularly how this affects the ability to communicate with local and distant stations. What those characteristics are will be the main focus of the following sections.
| LF - Low Frequencies |
These frequencies range from the bottom of the Broadcast Band to roughly 100 kHz. With in that range of frequencies there is a further division based upon the change in propagation characteristics that occurs, as the frequencies become lower. In the frequencies that are found between 535 kHz to 300kHz, one tends to find much the same propagation characteristics as the low end of the Broadcast Band. Here you will find the frequencies can only use ground wave propagation during the daytime, and these cover short distances of roughly 200 miles. At night much greater distances can be covered when conditions in the ionosphere enables skywave propagation. This range of frequencies is used primarily by utility stations and navigational beacons, which many low frequency listeners still track down and log; finding these signals to be a great DX challenge due to their low power operation.
Below 300 kHz to roughly 100 kHz the frequency propagation characteristics begin to become different than those found in the higher frequencies. When transmitted, these radio waves travel in wave fronts that follow a "trough" formed between the earth’s ionosphere and its surface. Interestingly enough there is little or no sky-wave reflection when these waves interact with the ionosphere. This is desirable for reliable long distance communications as it eliminates "flutter" and other forms of interference associated with "skip" phenomena. Signals at these frequencies are also less effected by solar flares due to the fact that these radio waves are not as dependent upon the ionosphere as they are in the HF portion of the spectrum. These low frequency waves also "hug" the contour of the earth’s crust and ocean’s surface very closely, which eliminates signal loss on the "lea" side of hills and mountains, as well as in the depths of valleys. These later characteristics have made this portion of the radio spectrum popular with European and Asian broadcasters who have audiences in mountainous areas, as well as those who are at sea in coastal areas, where short wave and medium wave cannot deliver dependable signals.
| VLF - Very Low Frequencies |
The spectrum below 100 kHz offers many interesting monitoring possibilities due to its unique characteristics. Here propagation is even more "unusual" than those of the LF range are. The frequency range between 30 kHz to 9 kHz (the lowest limit of "regulated" radio spectrum) exhibits the toughing and wave front characteristics described before, but to a greater extreme, with absolutely no ionsopheric effects taking place. In addition to open air propagation, these frequencies have an additional characteristic of being able to travel long distances through solid rock. This has been used to great advantage by cave explorers and mining crews to keep in contact with different team members underground. The waves can also penetrate under the surface of the ocean and seas, which normally blocks radio waves at the higher frequencies. This has allowed the navy to be able to transmit radio messages to submarines working at depth and at speed. At one time these frequencies were used by commercial interests to transmit and receive the majority of "wireless" message traffic in a large network of stations, but today only a handful of military and government stations remain on the air on a regular basis.
| ELF - Extremely Low Frequencies |
A great deal of attention is paid by the scientific and experimenter communities to the frequencies below 9 kHz. Here you can monitor, and actually hear a wide range of "natural radio" phenomena. Events such as "whistlers", which are believed to be the sound of the energy pulse of a lightning bolt following the earth’s magnetic field, can be heard with special radio receivers. These radios (which are surprisingly easy to build) are capable of tuning down to 300 cycles and below, where these signals are heard. There are many other sounds from known and unknown sources that are being investigated as well. Many who assist the scientific community in such studies do not even listen to the sounds produced, but monitor such things as the changes to background noise using pen traces on paper tape or computer sampling of variations in measured voltages. Through this activity further knowledge is being developed about the radio propagation in this region, which still contains many mysteries.