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annular duct. A duct, or passage, that surrounds an object. An annular fan-discharge duct surrounds the
gas generator of a turbofan engine.
annular orifice. A ring-shaped orifice, normally one that surrounds another orifice.
annulus. A ring or groove cut around the outside of a circular body or shaft. An annulus gets its name from
its resemblance to the circular shape of the annual rings (the growth rings) which form in the trunk of a
tree.
annunciator panel. A panel of warning lights in plain view of the pilot of an airplane or the operator of
other types of equipment. The lights are identified by the name of the system they represent and are usually
covered with a colored lens to show the meaning of the condition they announce. Red lights are used to
indicate a dangerous condition, amber lights show that some system is armed, or active, and green lights
show that a condition is safe.
By having all the lights together in an annunciator panel, the pilot or operator can determine the
overall operating condition of the equipment at a glance.
anode. The electrode in a vacuum tube or semiconductor diode to which the electrons travel after they
leave the cathode. The anode in a vacuum tube is called the plate.
The anode of a semiconductor diode is the end made of P-type material. It is not marked — the
cathode has the mark. In the diode symbol, the anode is shown by the arrowhead.
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anode current. Current that flows in the anode of an electrical device. In an electron tube, anode current is
commonly called plate current.
anode of a chemical cell. The area in a chemical cell from which electrons have migrated. When electrons
leave an area, it becomes less negative, or takes on a positive charge. Electrons then try to move from an
area having an excess of electrons, the cathode of the cell, to this anodic area.
anodic area. An area inside a piece of metal that has lost some of its electrons.
This leaves a net positive charge which attracts negative ions from the electrolyte. Positive
metallic ions and negative ions from the electrolyte unite to form the salts of corrosion.
anodizing. A method of preventing corrosion of aluminum alloy parts. A hard oxide film is formed on the
surface of the metal by an electrolytic process. This film prevents oxygen reaching the surface of the metal.
The aluminum alloy is the anode of the electrolytic cell, and chromic acid is used as the
electrolyte.
anomalous propagation (radar meteorology). The greater-than-normal bending of a radar beam which
causes echoes to be received from ground targets at greater distances than normal ground clutter.
anoxia. A severe case of hypoxia, or the serious effect on the human body caused by a lack of oxygen.
A-N radio range (radio navigation aid). The low-frequency radio range which was one of the first
practical radio navigational aids.
Two antennas, arranged so they are at approximately right angles to each other, radiate
low-frequency signals in the form of two figure eights. One antenna transmits the Morse code letter A (• –),
and the other transmits the letter N (– •).
A pilot flying an airplane in the area where the two signals overlap hears a solid tone. As the
airplane moves to one side or the other, the letter A or N predominates, and this indicates to the pilot the
side to which he has moved.
The area of overlapping signals, usually about three degrees wide, is called the “beam.”
antenna. A conductor used with a radio transmitter or receiver to radiate or receive electromagnetic
energy.
antenna coupler (radio equipment). A special transformer used to connect an antenna to a radio receiver
or transmitter.
The antenna coupler allows the maximum amount of energy to be transferred between the antenna
and the receiver and between the transmitter and the antenna.
antenna current. The amount of radio-frequency current flowing in a radio antenna.
antenna duplexer. A device that allows two transmitters to use a single antenna without either transmitter
interfering with the other.
antenna lens (microwave antenna component). A device installed near a microwave antenna that focuses
the radiated energy in much the same way a glass lens focuses light waves.
antenna matching (radio installation). The process of matching the impedance of a radio antenna with
the impedance of the transmission line carrying the signal from the transmitter to the antenna.
For the maximum amount of energy to be transferred, the impedance of the transmission line must
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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be the same as the impedance of the antenna. Antenna matching is done by the selection of proper
　
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