Correct Answer : Does not change
Explanation : The magnetic field produced by a current carrying conductor is only influenced by the current and not the voltage. Since the current is maintained constant, there is no change in the magnetic field strength or radius.
Correct Answer : Potential difference
Explanation : The most important thing for an electric field to exist is a potential difference between two conductors. It is not that only metal conductors can produce an electric field, potential difference between fluids also produces electric fields.
Correct Answer : Nothing happens
Explanation : The permeability of a material only affects the magnetic field and not the electric field. Electric field depends upon permittivity of the material between the two conductors.
Correct Answer : A parallel wire transmission, when left open, does not radiate
Explanation : If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end of the line and radiate into space. This radiation, however, is inefficient and unsuitable for reliable transmission or reception.
Correct Answer : Duplexer
Explanation : An antenna can transmit and receive at the same time as long as some means is provided for keeping the transmitter energy out of the front end of the receiver. A device called a duplexer is used for this purpose.
Correct Answer : Signal strength reduces
Explanation : A vertical or horizontal antenna can receive circular polarized signals, but the signal strength is reduced. When circular polarization is used at both transmitter and receiver, both must use either left- or right-hand polarization if the signal is to be received.
Correct Answer : 10 wavelengths
Explanation : The far field that is approximately 10 wavelengths from the antenna is the radio wave with the composite electric and magnetic fields. For example, at 2.4 GHz, one wavelength is 984/2400 = 0.41 feet. The far field is 10 times that, or 4.1 ft or beyond.
Correct Answer : Impedance of space
Explanation : The ratio of the electric field strength of a radiated wave to the magnetic field strength is a constant. It is called the impedance of space, or the wave impedance.
Correct Answer : Hertz antenna
Explanation : One of the most widely used antenna types is the half-wave dipole antenna. This antenna is also formally known as the Hertz antenna after Heinrich Hertz, who first demonstrated the existence of electromagnetic waves.
Correct Answer : Radiation resistance
Explanation : The transmission line is connected at the center. The dipole has an impedance of 73 V at its center, which is the radiation resistance. At the resonant frequency, the antenna appears to be a pure resistance of 73 V.
Correct Answer : SWR is minimum
Explanation : When the radiation resistance of the antenna matches the characteristic impedance of the transmission line, the SWR is minimum and maximum power reaches the antenna. This allows maximum power to be transmitted.
Correct Answer : Conical antenna
Explanation : A common way to increase bandwidth in the antenna is to use a version of the dipole antenna known as the conical antenna. The overall length of the antenna is 0.73λ or 0.73(984)/f = 718.32/f. This is longer than the traditional one-half wavelength of a dipole antenna, but the physical shape changes the necessary dimensions for resonance.
Correct Answer : Doughnut
Explanation : The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna. Typically that radiation is concentrated in a pattern that has a recognizable geometric shape. The radiation pattern of a half-wave dipole has the shape of a doughnut.
Correct Answer : 90°
Explanation : The beam width is measured between the points on the radiation curve that are 3 dB down from the maximum amplitude of the curve. The maximum amplitude of the pattern occurs at 0° and 180°. The 3-dB down points are 70.7 percent of the maximum. The angle formed with two lines extending from the center of the curve to these 3-dB points is the beam width. The beam width is 90°. The smaller the beam width angle, the more directional the antenna.
Correct Answer : Marconi antenna
Explanation : The same effect as dipole antenna can be achieved with a one-quarter wavelength antenna or Marconi antenna. A vertical dipole with the doughnut-shaped radiation pattern, in which one-half of the pattern is below the surface of the earth. This is called a vertical radiation pattern.
Correct Answer : Sphere
Explanation : An isotropic radiator is a theoretical point source of electromagnetic energy. The E and H fields radiate out in all directions from the point source, and at any given distance from the point source, the fields form a sphere.
Correct Answer : Effective radiated power
Explanation : The power radiated by an antenna with directivity and therefore gain is called the effective radiated power (ERP). The ERP is calculated by multiplying the transmitter power fed to the antenna Pt by the power gain Ap of the antenna.
Correct Answer : Directivity
Explanation : The measure of an antenna’s directivity is beam width, the angle of the radiation pattern over which a transmitter’s energy is directed or received. Beam width is measured on an antenna’s radiation pattern.
Correct Answer : Correcting resonance to a desired frequency
Explanation : When a vertical antenna is made less than one-quarter wavelength, the practical effect is a decreased inductance. The antenna no longer resonates at the desired operating frequency, but at a higher frequency. To compensate for this, a series inductor, called a loading coil, is connected in series with the antenna coil. The loading coil brings the antenna back into resonance at the desired frequency.
Correct Answer : To increase capacitance
Explanation : A top hat is a structure that increases the capacitance to surrounding items, bringing the antenna back into resonance. Obviously, such an arrangement is too top-heavy and inconvenient for portable and mobile antennas. However, it is sometimes used in larger fixed antennas at lower frequencies.
Correct Answer : Parasitic elements
Explanation : A parasitic array consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line. These extra conductors are referred to as parasitic elements, and the antenna itself is referred to as the driven element.
Correct Answer : One half the wavelength of the highest frequency
Explanation : The lengths of the driven elements vary from long to short and are related logarithmically. The longest element has a length of one-half wavelength at the lowest frequency to be covered, and the shortest element is one-half wavelength at the higher frequency. The spacing is also variable.
Correct Answer : Rectilinear antenna
Explanation : Each element in a driven array antenna receives RF energy from the transmission line, and different arrangements of the elements produce different degrees of directivity and gain. The three basic types of driven arrays are the collinear, the broadside, and the end-i re. A fourth type is the wide-bandwidth log-periodic antenna.
Correct Answer : Lightning protection
Explanation : The centers of the parasitic elements are neutral electrically; these elements can be connected directly to the boom. For the best lightning protection, the boom can then be connected to a metal mast and electrical ground.
