NAVEDTRA 14182A Chapter 4 Flashcards
Which two basic classifications are most practical transmitting antennas divided into?
Hertz and Marconi
Which type of antennas operate with one end grounded and are mounted perpendicular to the Earth or to a surface acting as a ground?
Marconi
Hertz antennas are generally used for what frequencies above what?
2 megahertz
Which three parts does a complete antenna system consist of?
Coupling device, feeder, and antenna
What is defined as a conductor or group of conductors used either for radiating electromagnetic energy into space or for collecting it from space?
Antenna
What is the interchangeability of the same antenna for transmitting and receiving known as?
Reciprocity
What cannot be used at medium and low frequencies because the electric lines of force are parallel to and touch the earth?
Horizontal and polarization
Which frequencies is sky-wave transmission used?
High
From approximately 50 to how many megahertz, vertical polarization results in a slightly stronger signal than does horizontal polarization with antennas at the same height?
100
At 30 megahertz, vertical polarization is better for antenna heights below about how many meters?
91
At ordinary antenna mast heights of 12 meters (40 feet), vertical polarization is advantageous for frequencies less than about what?
100 megahertz
Which type of polarization is used by VHF and UHF transmissions?
Horizontal
What is the useful part of the transmitters signal?
Radiated energy
For the antenna in free space, that is, entirely removed from any objects that might affect its operation, the radiation resistance is how many ohms?
73
For most half-wave wire antennas, the radiation resistance is about how many ohms?
65
What is a plot of the radiated energy from an antenna?
Radiation pattern
What is it known as when antenna sources radiate energy equally in all directions?
Isotropic radiation
Which type of radiators emit (radiate) stronger radiation in one direction than in another?
Anisotropic
What is used to indicate directions of minimum radiation?
Null
The electrical length of any antenna wire can be increased or decreased by what way?
Loading
What is the basic unit from which many complex antennas are constructed?
Half-wave antenna
What term is used to indicate an antenna that is be try short compared with the wavelength of the operating frequency?
Doublet
What is the simplest form of a practical antenna?
Doublet
What type of antenna is the shortest antenna that can be used in free space?
1/2 wavelength
What is an ordinary half-wave antenna that has one or more additional conductors connected across its ends?
Folded dipole
What is a parasitic element referred to as when it operates to reinforce energy coming from the driver towards itself?
Director
What is a parasitic element referred to as if it is placed so it causes maximum energy radiation in a direction away from itself and toward the driven element?
Reflector
What designates an array in which the direction of maximum radiation is perpendicular to the plane containing these elements?
Broadside array
What type of array is it when all the elements lie in a straight line with no radiation at the ends of the array?
Collinear
What type of array is it when the principal direction of radiation is along the plane of the array and perpendicular to the elements?
End-fire
What property of an array is the proportion of energy radiated in the principal direction of radiation to the energy radiated in the opposite direction?
Front-to-back ratio
What is an antenna that is a wavelength or longer at the operating frequency?
Long-wire
What is a horizontal, long-wire antenna designed especially for the reception and transmission of low-frequency, vertically polarized ground waves?
Beverage antenna aka wave antenna
What is one of the most popular fixed-station antennas because it is very useful in point to point communications?
Rhombic
What is half-wave radiator with a reflector?
Corner-reflector antenna
What is the heating of an insulated material by placing it in a high frequency electric field?
Dielectric heating
What parts of the human body are highly susceptible to dielectric heating?
Eyes
What is the area of a radiation pattern that is covered by radiation?
Lobe
The optimum gain of a broadside array is obtained when the elements are spaced what distance apart?
.065 wavelength