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Right option is (c) One half the wavelength of the highest frequency

To explain: 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.

The correct answer is (a) RECTILINEAR antenna

To elaborate: 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.

Right option is (C) Reflector element

To elaborate: The reflector, a parasitic element that is typically about 5 percent LONGER than the half-wave dipole-driven element, is spaced from the driven element by a distance of 0.15λ to 0.25λ. When the signal radiated from the dipole reaches the reflector, it induces a voltage into the reflector and the reflector produces some radiation of its own CREATING a highly FOCUSED BEAM in the direction of the driven element.

Right choice is (c) 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.

Right choice is (b) False

The explanation: The boom does not have to be an INSULATOR. Because there is a VOLTAGE null at the center of a one-half wavelength conductor at the RESONANT frequency, there is no potential difference between the elements and so they can all be connected to a conducting boom with no undesirable effect. In other words, the elements are not “shorted TOGETHER.”

The correct option is (B) PARASITIC elements

The best I can explain: 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.

Right answer is (a) Correcting resonance to a desired frequency

To explain I would say: 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.

Right answer is (c) 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.

The correct CHOICE is (a) To increase CAPACITANCE

To explain: 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 option is (c) 300Ω

Best explanation: A popular variation of the half-wave dipole is the folded dipole. Like the standard dipole, it is one-half WAVELENGTH long. However, it consists of two parallel conductors connected at the ends with one side open at the center for connection to the transmission line. The impedance of this popular ANTENNA is 300 V, making it an excellent match for the widely available 300-V TWIN LEAD.

The correct option is (b) SPHERE

To explain I would SAY: 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.

Right option is (c) Directivity

The EXPLANATION is: 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.

The CORRECT option is (c) Effective radiated power

The explanation is: 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 option is (a) 90°

For explanation I would SAY: 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.

The correct option is (a) DOUGHNUT

The 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.

The CORRECT choice is (a) Conical ANTENNA

The 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.

Right answer is (d) SWR is minimum

To EXPLAIN I would say: 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.

Right OPTION is (b) 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.

The correct answer is (d) HERTZ antenna

The explanation is: 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 is (C) DUPLEXER

The explanation is: 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.

The correct option is (C) 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 option is (c) Circular polarized wave can follow the curvature of earth

Easiest explanation: In circular polarized wave the POLARIZATION ANGLE of the electric FIELD and the earth is CONTINUOUSLY changing. This does not affect the transmission direction and hence circular polarized waves cannot bend with the curvature of earth, like any other EM wave.

The correct choice is (c) 90°

Easy explanation: Polarization refers to the ORIENTATION of MAGNETIC and electric fields with respect to the earth. If an electric field is parallel to the earth, the ELECTROMAGNETIC WAVE is said to be horizontally polarized; if the electric field is perpendicular to the earth, the wave is vertically polarized.

Correct choice is (b) FRAUNHOFER zone

Best explanation: The far FIELD is also CALLED the Fraunhofer zone. It is named after the BAVARIAN physicist JOSEPH Ritter von Fraunhofer.

Correct CHOICE is (c) 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 option is (B) False

Easy explanation: The near field describes the region directly around the antenna where the electric and magnetic FIELDS are DISTINCT. These fields are not the radio WAVE, but they do indeed contain any information transmitted. The near field is also referred to as the FRESNEL zone.

Right answer is (C) 90°

The best I can explain: The radiation from a transmission line can be GREATLY improved by bending the transmission line conductors so that they are at a right angle to the transmission line. The magnetic fields no longer cancel and, in fact, aid one another. The electric FIELD spreads out from conductor to conductor. The result is an ANTENNA.

The CORRECT ANSWER is (d) A parallel wire transmission, when left open, does not radiate

For explanation I would say: 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.

The CORRECT ANSWER is (d) Nothing happens

To explain I would say: 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.

The correct option is (b) Potential difference

For explanation I would SAY: 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.