An antenna is considered part of the electrical circuit of a transmitter or a receiver and has factors including inductance, capacitance, and resistance. Which means the antenna can be expected to display definite voltage and current relationships with respect to a given input. A current through the antenna produces a magnetic field, and a charge on the antenna produces an electric field. These two fields combine to form the inductive field.
The field that exists around every electrically charged object is a force field that can be detected and measured. This force field can cause electric charges to move in the field. When an object is charged electrically, there is either a greater or a smaller concentration of electrons than normal. This results in a difference of potential between a charged object and an uncharged object. An electric field is associated with a difference of potential, or a voltage. This invisible field of force is commonly represented by lines that are drawn to show the paths along which the force acts. The lines representing the electric field are drawn in the direction that a single positive charge would normally move under the influence of that field. A large electric force is shown by a large concentration of lines; a weak force is indicated by a few lines.
An energy wave generated by a transmitter is called a radio wave. The radio wave radiated into space by the transmitting antenna is a very complex form of energy containing both electric and magnetic fields. Because of this combination of fields, radio waves are also referred to as electromagnetic radiation.
The period of a radio wave is simply the amount of time required for the completion of one full cycle. If a sine wave has a frequency of 2 hertz, each cycle has a duration, or period, of one-half second. If the frequency is 10 hertz, the period of each cycle is one-tenth of a second. Since the frequency of a radio wave is the number of cycles that are completed in one second, you should be able to see that as the frequency of a radio wave increases, its period decreases. A wavelength is the space occupied by one full cycle of a radio wave at any given instant. Wavelengths are expressed in meters (1 meter is equal to 3.28 feet). You need to have a good understanding of frequency and wavelength to be able to select the proper antenna.
There are two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna. One way is by ground waves and the other is by sky waves. Ground waves are radio waves that travel near the surface of the Earth and sky waves are radio waves that are reflected back to Earth from the ionosphere.
Natural interference refers to the static that you often hear when listening to a radio and is interference generated by natural phenomena, such as thunderstorms, snowstorms, cosmic sources, and the sun. The energy they release is transmitted to the receiving site in roughly the same manner as radio waves. As a result, when conditions are favorable for the long-distance propagation of radio waves, they are likewise favorable for the transmission of natural interference. This hindrance is very erratic, particularly in the high frequency (HF) band, but generally will decrease as the operating frequency is increased and wider bandwidths are used. There is little natural interference above 30 megahertz.
One of the most notable phenomena on the surface of the sun is the appearance and disappearance of dark, irregularly shaped areas known as sunspots. Their exact nature is not known, but scientists believe these solar flares are caused by violent eruptions on the sun and are characterized by unusually strong magnetic fields. Sunspots are responsible for variations in the ionization level of the ionosphere and can occur unexpectedly with a variable life span. There is however, a regular cycle of sunspot activity that has been documented. This cycle has both a minimum and maximum level of sunspot activity that occurs approximately every 11 years. During periods of maximum sunspot activity, the ionization density of all layers increases. At these times, higher operating frequencies must be used for long-distance communications. The number of sunspots in existence at any one time is subject to change and as some disappear, new ones emerge. As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, the approximate period required for the sun to make one complete rotation. The 27-day sunspot cycle causes variations in the ionization density on a day-to-day basis. Irregular variations in ionospheric conditions have an important effect on radio wave propagation. These variations are irregular and unpredictable so they can drastically affect communications capabilities without any warning.
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