At the beginning of the 20th century, Guglielmo Marconi transmitted the first transatlantic radio signals from England to Newfoundland. Since then, radio communications have continually improved. Radio communication and principles are often taken for granted in these modern times; it is so commonplace!
Televisions, cordless phones, garage-door openers, Radio-Controlled model airplanes and cars, pagers, cellular phones, and security systems are all examples of commonplace radio communication technology.
The Radio Frequency Spectrum
Listed below are the frequency ranges for various “bands” of the frequency spectrum:
- VLF = Very Low Frequency
- LF = Low Frequency
- MF = Medium Frequency
- HF = High Frequency
- VHF = Very High Frequency
- UHF = Ultra High Frequency
- SHF = Super High Frequency
- EHF = Extremely High Frequency
Band ITU Frequency Waveform Name Band Range Name ---- ---- -------------- --------------- VLF 4 3 - 30 KHz Myriametric LF 5 30 - 300 KHz Kilometric MF 6 300 - 3000 KHz Hectometric HF 7 3 - 30 MHz Decametric VHF 8 30 - 300 MHz Metric UHF 9 300 - 3000 MHz Decimetric SHF 10 3 - 30 GHz Centimetric EHF 11 30 - 300 GHz Millimetric (IR) 12 300 - 3000 GHz Decimillimetric
Other Frequencies and Waveforms
Frequency Range Description ------------------------------- ----------------------------- 3x10E11 (300 GHz) - 4.3x10E14 Infrared Light 4.3x10E14 - 1x10E15 Visible Light (Red to Violet) 1x10E15 - 6x10E16 Ultraviolet Light 6x10E16 - 3x10E19 X-Rays 3x10E19 - 5x10E20 Gamma Rays 5x10E20 - 8x10E21 Cosmic Rays
Radio Wave Propagation
Radio waves are often categorized into 2 general categories of propagation:
- Ground Wave
- Sky Wave
Ground Wave Propagation
Ground Waves travel along the surface of the earth. There are three types of Ground Waves; the Surface Wave, the Direct Wave, and the Ground-Reflected Wave.
VLF (3 – 30 KHz) and LF (30-300 KHz) signals typically use Surface Wave propagation, for spanning thousands of miles in distance. Such systems include submarine and maritime communications, as well as the LORAN-C navigational system (100 KHz).
Signals above 30 MHz (including the FM broadcast band and Microwave Radio frequencies) generally use Direct Wave propagation. This includes the VHF, UHF, SHF, and EHF frequency bands. This type of propagation is also known as “Line Of Sight” transmission, since a direct wave will not (usually!) traverse the horizon. Line Of Sight transmission is usually used for communications purposes. In typical systems, the transmitter and receiver must be within 25-30 miles of each other.
Ground-Reflected waves are usually detrimental to communications, since a REFLECTED wave causes a 180-degree phase shift. This phase shift, coupled with a valid Surface or Direct wave, results in cancellation and sub-optimal transmission quality. Often, the use of a beam antenna can eliminate degradation from ground relections.
Sky Wave Propagation
Sky Waves travel up into the earth’s atmosphere, where they either continue or are REFRACTED by the ionized air particles in the upper atmosphere. This depends upon the strength of the signal, the angle at which the signal hits the ionized layer, and the frequency of the signal. Also, signals that reflect off the ionosphere may travel further (skip distance) at night.
The Ionosphere consists of the “D” (30-60 miles), “E” (60-100 miles), F1 (90-160 miles), and the F2 (160-250 miles) layers. At night, the “E” layer disappears and the F1 and F2 layers combine to form the “F” layer (somewhere between 90 and 250 miles). Since the “E” layer disappears at night, the lower frequency Sky Waves (MF) travel further up into the atmosphere, where they are REFRACTED by the “F” layer. That’s why at night, your radio often picks up many more AM broadcast stations!
Frequencies in the range of 300 KHz to 30 MHz (Medium Frequency and High Frequency bands) usually utilize sky wave transmission. Generally, these frequencies are not used for communications, since the transmission media (the Ionosphere) is highly variable. Additionally, their lower frequencies results in lower bandwidth; thus, they can support fewer channels.
Another propagation method worthy of note is “Scatter” propagation. When Direct Waves in the UHF/SHF range are transmitted, they can be affected by the dense characteristics of the Troposphere or Stratosphere. As such, these waves may be REFRACTED in a forward, side, or even backwards direction. Powerful transmitters and sensitive receivers are needed, but these wideband signals can traverse distances of 200 – 400 miles!
The most common form of Scatter propagation is known as Forward Propagation by Tropospheric Scatter (FPTS). It is often useful for military tactical communications, where wideband signals can be transmitted over long distances without the benefit of Microwave Relay stations or wideband satellite facilities. Over-The-Horizon RADAR systems use Scatter propagation techniques.
However, long-haul Tropospheric Scatter propagation requires high transmit power and very large receiving antenna arrays. Also, signal strength losses are great and the signal is affected by many kinds of noise sources. Because the received signal strength is so low, Tropscatter systems are susceptible to jamming at the Receiver site.