The most startling of the ionosphere irregularities is known as a sudden ionospheric disturbance (SID). These disturbances may occur without warning and may prevail for any length of time, from a few minutes to several hours. When SID occurs, long distance propagation of HF radio waves is almost totally "blanked out." The immediate effect is that flight crews listening on normal frequencies are inclined to believe their receivers have gone dead.
For successful communications between any two specified locations at any given time of the day, there is a maximum frequency, a lowest frequency, and an optimum frequency that can be used.
A transmission line is a device designed to guide electrical energy from one point to another. It is used to transfer the output electromagnetic energy of a transmitter to an antenna. This energy will not travel through normal electrical wires without great losses. Although the antenna could be connected directly to the transmitter, in most aircraft this is impractical so generally the antenna is usually remotely located.
The transmission line has a single purpose for both the transmitter and the antenna. This purpose is to transfer the energy output of the transmitter to the antenna with the least possible power loss. How well this is done depends on the special physical and electrical characteristics (impedance) of the transmission line.
A transmission line is electrically long when its physical length is long compared to a quarter-wavelength of the energy it is to carry. The terms "short" and "long" are relative ones; that is, a line that has a physical length of 3 meters (approximately 10 feet) is considered quite short electrically if it transmits a radio frequency of 30 kilohertz. On the other hand, the same transmission line is considered electrically long if it transmits a frequency of 30,000 megahertz.
A transmission line has the properties of inductance, capacitance, and resistance just as most conventional circuits. However, the constants in conventional circuits are more of a constant based on the type of component. A coil of wire is considered an inductor and when a certain amount of inductance is needed in a circuit, a coil of the proper value is inserted. The inductance of the circuit is based on the one component. Two metal plates separated by a small space, can be used to supply the required capacitance for a circuit as a capacitor consists of two conductors separated by an insulator. Similarly, a fixed resistor can be used to supply a certain value of circuit resistance. Transmission lines as well have constants consisting of inductance, capacitance, and resistance combined and the effects of this combination on an electrical energy transmission is referred to as impedance.
The design of the antenna system is very important in a transmitting station. The antenna must be able to radiate efficiently so the power supplied by the transmitter is not wasted. An efficient transmitting antenna must have exact dimensions. The dimensions are determined by the transmitting frequencies. The dimensions of the receiving antenna are not critical for relatively low radio frequencies. However, as the frequency of the signal being received increases, the design and installation of the receiving antenna become more critical. An example of this would be the rabbit ears on a television. If you raise it a few more inches from the ground or give a slight turn in direction, you can change a snowy blur into a clear picture.
Most practical transmitting antennas are divided into two basic classifications, Hertz (half-wave) and Marconi (quarter-wave) antennas. Hertz antennas are generally positioned to radiate either vertically or horizontally. Marconi antennas operate with one end grounded and are mounted perpendicular to the surface acting as a ground. Hertz antennas are often used for frequencies above 2 megahertz. Marconi antennas are used for frequencies below 2 megahertz and may be used at higher frequencies in certain applications.
Aircraft present many challenges to antenna system designers. Even the location has to be matched to the capability. One recent event encountered on a helicopter revealed a situation where the communication radio worked well as long as the aircraft was heading away from the station, however going toward a transmitted signal the crew noticed the range was one-half to two-thirds of the opposite direction. As it turned out the Com antenna was located just behind the main rotor and the signal was effectively blocked with the blades operating. By moving the antenna to the bottom of the craft, the problem disappeared.
Some basics on electromagnetic interference The by-product of producing an ignition spark is the creation of waves of electromagnetic energy within the radio frequency spectrum (above...
Automatic Direction Finders The grandfather of all radio navigation aids By Frank Labue July 1999 Ok, I know. Why are we talking about ADFs when there are VOR/ILS and GPS...
Warning! Radar Operating By Jim Sparks May-June 1998 The primary purpose of weather radar is to detect storms along the flight path and give the pilot a visual indication of rainfall...