Radio waves surround you! Signals are transmitted and received by millions of stations. AM and FM radio stations, television, mobile business radios, fire and police radio systems, amateur radio stations, cordless and cellular telephones, pagers, nursery room monitors, garage door openers, and of course your personal Citizens Band Radio Station! Radio waves travel at the speed of light. The invisible radio wave consists of an electrostatic field and a magnetic field traveling at right angles in the direction of their transmission.
To help you to visualize the behavior of invisible radio waves imagine the effect on the surface of the water when you toss a small stone into the air and it lands in the water. Tiny ripples radiate in all directions. If you increase the size of the stone the size of the ripples will also increase and the distance between the peak of each ripple will increase also.
If you could count the number of times the peak of each wave went past a specific point over one second, you would know the waves' frequency and wavelength. In radio waves, we measure frequency by the number of wave cycles per second. The more waves that pass a specific point in one second, the higher the frequency. And the distance that the wave travels in one cycle is the wavelength.
The unit for measuring frequency is hertz (Hz). It is named after Heinrich Hertz, a German physicist who discovered electromagnetic waves. The frequencies that human can hear are audio frequencies and range from about 20 Hz to about 18,000 Hz or 18 kHz. A dog can hear up to about 20 kHz or possibly even higher. The radio frequency spectrum starts at around 10 kHz and ranges up to 300,000 Megahertz (MHz). A frequency of 10 kHz means that the wave cycles 10,000 times per second. Similarly, a frequency of 300 million hertz (300 MHz) means that the wave cycles 300 million times per second.
In mobile radio communications we use the ranges of frequencies in the medium frequency (MF), high frequency (HF), and very high frequency (VHF) range.
Guess where the FCC assigned the short-range citizens band (CB) service. They allocated 23 channels - ultimately expanded to 40 channels - at 27 MHz. Is that below the magic 30 MHz dividing line? Sure is, and the planned short-range unlicensed CB radio service instantly turned into an undisciplined sky-wave free-for-all! Although intended to fulfill the need for personal short-range communications, regular high-frequency sky-wave skip signals blasted in from the ionosphere, causing short-range channel chaos.
During periods of reduced ionospheric- activity, the short-range capabilities of CB can be realized. But when the skip is active, a CB radio transmitter with only a few watts of output power can put strong signals into receivers thousands of miles away. This is commonly referred to as ?DXing?. DXing has taken on a very active part in the CB world and continues to promote the use of Citizens Band use and operation.
Many well-established CB radio operators are strongly dedicated to the ?art? of DXing. Although skip is mainly unpredictable, DXers spend many long hours studying and charting the activity of skip signals. Some have even come very close to an established pattern that reveals some level of reliability. However, the pattern of skip is solely chance. Most DXers are satisfied to make distant contacts wherever and whenever the ionospheric skip permits.
MOBILE RADIO WAVE RANGE
Different radio frequencies travel over the surface of the earth and into space in many different ways. Sometimes the bounce, sometimes they are refracted (bent) by the ionosphere. Some radio frequencies easily penetrate the thickness of a forest, yet other radio frequencies can be blocked by a single leaf.
Every radio transmission travels out from the antenna over two paths: the ground wave and the sky-wave. Ground waves are the real work horse for vehicle mobile communications up to 100 miles. Ground waves normally are vertically polarized signals that travel out in all directions from an omni-directional antenna and are usually strong enough to be detected up to 50 to 100 miles away.
In rural areas, land mobile radio systems may use frequencies in the VHF and UHF spectrum to maximize the potential of ground wave propagation. VHF signals propagate best in areas of rolling hills and over water. UHF signals become quite reflective, and are best propagated in the downtown area of cities with huge skyscrapers. When VHF and UHF signals require a range of more than 100 miles, repeater relay stations and satellites may have to be used to get these normally "line-of-sight" signals to go a lot further.
The second component of a transmitted radio signal is the sky-wave. In VHF and UHF mobile radio bands, sky-waves normally travel out into space and keep right on going. UHF and microwave sky-waves have traveled millions of miles to earth from space probes sent out into deep space.
Sky-waves on medium frequencies and high frequencies take a remarkable skip off of the ionosphere that circles our planet at altitudes of 50 to 250 miles. 'When a medium or high frequency radio wave enters the ionosphere, the radio wave is refracted back to earth (see drawing). Just as a prism refracts sunlight into different color bands over a range from blue too red, radio waves refracted by the ionosphere may skip back to earth over a range of 200 to 3000 miles away from the transmitter. If the hop back to earth is strong enough and lands in the very conductive ocean, that medium frequency or high frequency wave takes a double hop - sometimes a triple hop - and literally skips around the world.
Ham radio operators regularly take advantage of this sky-wave excitement on their specific frequencies in the medium and high frequency spectrum. Same thing with ocean voyaging mariners - they chose their bands carefully to bounce their signal back to shore to stay in touch with their mobile marine station hundreds and thousands of miles away. On a hot summer afternoon, 30 MHz is the usual dividing line between sky-wave communications that come back to earth and sky-waves that simply keep on going into outer space. For frequencies below 30 MHz, the sky waves bounce back to earth. Above 30 MHz, skip usually is not a problem.
HOW IMPORTANT IS POWER OUTPUT?
The output power of a mobile transmitter, which might be as low as I watt or as high as 1000 watts, has very little to do with the maximum distance that the signal may travel. Although signal strength varies inversely with the distance, a major increase in power output will have little or no effect in establishing communications to a distant point that couldn't hear the ground wave or sky-wave at the original power setting.
Out on the oceans, mariners use 25-watt radios for short-range communications on the VHF band. Up to about 100 miles, the sea water has little effect on VHF waves, but on the worldwide lower frequencies, the high conductivity of sea water causes the signal strength of the ground wave to vary inversely with the distance. When operating on these frequencies, mariners require more power and rely almost entirely on sky-wave propagation to get their signals received hundreds and thousands of miles away.