Ham radio has been around since the beginning of electronic communication through space almost a century. One can only imagine the changes that will take place in the coming 100 years. Wireline telegraphy gave way to the invisible radio waves in 1901. The entire subject of "radio" can be pretty confusing. You have probably heard dozens of radio terms but weren't sure just what some of them meant. Let's try to simplify and explain just how those mysterious radio waves really work.
Early radio pioneers found that by varying the characteristics of a radio wave frequency, amplitude, or phase these waves could be made to communicate information of many types, including audio, video, and data. Radio waves that carry information are called radio signals, and the process of encoding intelligence onto a radio wave so that it can be transmitted over the air is called "modulation."
The most common modulation techniques are amplitude modulation (AM) and frequency modulation (FM.) The Amateur Service utilizes FM modulation for most of its voice contacts, although a variation of AM (called single sideband) is used at the lower frequency levels.
In the process of modulation, the information or message to be transmitted a human voice, digitized data, or a television signal is impressed (modulated) onto a "carrier" radio wave that is then transmitted over the air. When a radio signal is received, the information is converted back into its original form ("demodulated") by a receiver and output as sound, images or data.
Radio waves are distinguished from each other by their "frequency" or their wavelength. The length of a single cycle can be many miles long to under a half an inch in length! Frequency represents the number of cycles a radio wave completes in 1 second, and is the most common description of a radio communication signal.
The international unit of frequency measurement is the "hertz" (Hz), which represents 1 cycle per second. Multiples of the hertz are indicated by the prefixes: "kilo" for one thousand, "mega" for one million, and "giga" for one billion. Thus a million hertz a million cycles per second is expressed as one megahertz (abbreviated MHz).
Radio signals can be distributed great distances through space and are identified by their wavelength. The "radio spectrum" are those frequencies which are higher than those which may be heard by the human ear. The radio spectrum is generally considered to extend from 30 kHz to 300 GHz. While we may think that all of the radio spectrum is used up, such is really not the case! The technology has just not been developed to make use of the higher microwaves. Actually, less than 1% of the radio spectrum is being effectively used today.
Signals with long wavelengths have lower frequencies, while those at higher frequencies have shorter wavelengths. The radio spectrum is divided into "bands" that correspond to various groups of radio frequencies. These bands are identified in many ways: by their frequencies, wavelengths, descriptive acronyms and uses. You can thus refer to the same band in a number of ways.
Several types of descriptive names have been attached to various portions of the spectrum. One method denotes relative position in the spectrum: very low frequency (VLF), high frequency (HF), very high frequency (VHF), and superhigh frequency (SHF). It is also common to refer to radio bands by their length in meters - such as the 11-meter CB band, the 19-meter international broadcast band or the 10-meter Amateur band.
Another method derives from usage developed in World War II to keep secret the actual frequencies employed by radar and other electronic devices: such as the L-band, S-band, and C-band. The frequency used by police radar and the detectors purchased by consumers, are usually referred to in this manner. And X, K and Ka band radar detectors sound more exotic than those at 5, 10 or 20 gigahertz!
The ITU classifies frequencies according to band numbers Band 1, Band 2, etc. Frequency bands are also known by the services which use them - the AM broadcast band, the ham band, the business band, the police band, and so on. Thus the 420-450 MHz ham band is also correctly labeled as a UHF band, L-band or the 70- centimeter band.
"Short wave radio", an expression first used in the 1920's, is now a meaningless term. It simply meant those wavelengths which were higher than those in use which at that time was around 3 megahertz. Thus the "high frequency" (HF) short waves began at 3 MHz. Today, even shorter "microwave" (another irrelevant term) frequencies up in the gigahertz range have great radio communications value. The usable wavelengths just keep getting shorter. Many Amateurs consider short wave radio to mean those high frequencies (essentially in the 3 to 30 MHz range) which are refracted back to earth from the upper atmosphere.
Since radio waves do not respect international boundaries, the various nations of the world meet periodically to determine how the radio spectrum will be used. These meetings, called World Administrative Radio Conferences or WARC's, have been taking place every few years for more than 125 years. They are conducted by the ITU (International Telecommunication Union) a United Nations agency based in Geneva, Switzerland. The ITU divides the world into three geographical areas each with their own regional frequency allocations. North and South America are located in ITU Region 2.
Besides determining international radio frequency band usage, the ITU develops regulations to encourage the most efficient and interference-free use of the spectrum. It is the ITU that requires Amateur radio operators to have manual telegraphy proficiency when their operation takes place below 30 MHz. This will undoubtedly be eliminated at the next General WARC since telegraphy proficiency is in the process of being abolished for radio officers sailing the high seas.
General WARC's are held about every twenty years to consider all frequency allocations in radio spectrum, while specialized WARC's cover only certain radio services and bands. A specialized WARC, which occurred earlier this year, discussed international broadcasting needs and innovative new digital radio and television services.
The allocation of radio frequencies is further refined by the various national governments involved. For example, the ITU nations agree that the 20 meter ham band will extend from 14.000 to 14.350 MHz. But it is Part 97, Title 47 of the Communications Act of 1934 as amended and administered by the Federal Communications Commission which determines the various operator responsibilities, restrictions and technical standards that apply to its various segments.
The physical properties of radio waves determine how far radio signals can travel. The Amateur Service is allocated various bands of frequencies each of which have different distribution or "propagation" characteristics. Several factors affect the transmission of radio signals. All radio signals are "attenuated" or reduced as they pass through rain or any kind of water in the air such as clouds, snow or sleet. The higher the frequency, the greater the attenuation or signal loss. This makes radiocommunication, especially over long distances, extremely difficult above 10 GHz.
Radio waves may travel from the transmitting to the receiving antenna by three kinds of paths. "Ground waves" travel along the surface of the earth, "space waves" travel through the air and "sky waves" are returned to Earth from the upper atmosphere. The route that a radio wave takes is primarily dependent upon its frequency.
Radio waves are also bent and/or reflected as they pass through the various layers of electrified ("ionized") particles of the upper atmosphere. At night some of these layers disappear or merge which greatly affects communications capability. This bending is called "refraction." If atmospheric conditions are right, radio waves are also bounced or "reflected" in different directions from the "ionosphere" the top layer of the Earth's atmosphere.
Ionospheric reflection an extreme form of refraction enables some radio signals to travel thousands of miles and accounts for the long-distance communication that is possible in the high frequency band. The area between the transmitter and where the sky wave returns to Earth is known as the "Skip Zone." The ionosphere varies in height from 50 to 200 miles above the Earth. Lower radio frequencies are more easily bent and bounced around.
Above certain frequencies, atmospheric conditions are such that there is little radio signal refraction and reflection. The point at which this occurs is called the "Maximum Usable Frequency" ("MUF") and is generally in the range of 10 to 15 MHz. It can be as high as the 6-meter ham band (50 MHz) or as low as the 80-meter ham band (3.5 MHz), depending on time of day, season, atmospheric conditions, and a curiosity known as the "Sunspot Cycle."
Storms on the surface of the sun and their associated magnetic disturbances have a marked influence on radio communications. The intensity of these solar storms which appear as dark spots on the sun historically run in cycles of approximately 11 years. High sunspot activity results in longer periods of profuse ionization with accompanying increased sky wave propagation at the higher HF frequencies. This is always good news for Amateur DX operators! They can carry on international communications for a longer period of time in the 15, 10 and often the 6 meter band.
Below the "MUF", radio signals can be used for long-distance communication by reflecting the signal off the ionosphere. Above the "MUF", the signal travels straight through the atmosphere and into space. These "uplinked" signals are frequently captured by an orbiting satellite and retransmitted ("downlinked") by an on-board radio "transponder" back to Earth.
At higher frequencies above the MUF, radio signals travel in a straight "line of sight" from the transmitter to receiver. The distance of a line-of-sight signal can travel is usually limited to the horizon. The higher the antenna, the farther the signal is likely to travel (since the horizon appears further away). Line-of-sight transmission requires that there be no obstacles between the transmitter and receiver. A building or mountain will block the signal. Communications in the popular 2-meter ham band propagate by line-of-sight.
Since VHF/UHF signals have a limited range, their frequency may be "re-used" again by a distant station without interference. One of the basic functions of spectrum management is to locate radio stations operating on the same frequency at specified distances from each other to reduce interference. This is handled by volunteer frequency coordinators in the Amateur Service.
Atmospheric conditions can greatly affect line-of-sight radiocommunications. Differences in atmospheric temperature or the amount of water in the air can cause radio signals to travel far beyond the usual line-of-sight distance. This condition is called "ducting." At such times, signals travel for many miles beyond the horizon just as though the Earth were flat.
So there you have it! A capsule version of the characteristics of the remarkable radio wave and how many stations can operate on different wave lengths in the same geographical area at the same time without interfering with each other. The radio spectrum is an incredibly valuable natural resource. Just think of all of its applications microwave cooking, public safety communication, international broadcasting, time signals, communications satellites and more! The Amateur Service is very fortunate indeed to have such wide access to its various bands.
73, Fred, W5YI
Transmitted: 95-02-16 09:39:41 EST