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The correct option is (a) SCALING FACTOR

The EXPLANATION is: Scaling factor is the ratio of the lengths and spacing’s of the dipoles. It is also called as periodicity factor. If rndenotes the LENGTH of the nth dipole then the scaling factor is given by

Scaling factor = \(\frac{r_n}{r_{n+1}}\)

Correct choice is (a) Loop antenna

To explain I would say: The radiation PATTERN of a helical antenna is a COMBINATION of SMALL DIPOLE and loop antenna. The helical antennas behave like a loop antenna when spacing between turns becomes zero. It becomes a linear dipole when the diameter is zero that means itch angle is zero. PARABOLIC is a reflector antenna. Helical antenna is a wire antenna.

The correct answer is (a) 0 < α < 90

Easiest EXPLANATION: The radiation pattern of the helical ANTENNA is a combination of the small DIPOLE and loop antenna. Pitch angle is the TANGENT of RATIO between the spacing and the circumference of the antenna. α=0, then helical antenna becomes loop antenna. α=90, the helical antenna becomes linear antenna.

The CORRECT OPTION is (b) False

To explain: LPDA is a broadband antenna while Yagi-Uda is a narrow BAND antenna. To increase the more number of channels for TV reception we prefer LPDA. It is also USED for LONG distance communication.

Correct option is (b) 60π

Best explanation: ONE of the PROPERTIES of the self-complementary antennas is CONSTANT impedance. Its impedance is half of the intrinsic impedance at all FREQUENCIES. Impedance of self-complementary antenna = \(\FRAC{Z_o}{2} = 60π.\)

Right option is (d) Parabolic antenna

Best explanation: Parabolic antenna is a REFLECTOR antenna. Spiral, log periodic,helical antenna are examples of FREQUENCY independent ANTENNAS. A true frequency independent antenna must have a fixed physical size and operates on instantaneous basics over a wide bandwidth at a CONSTANT impedance, pattern.

The correct answer is (a) 16

Explanation: The relation between FREQUENCY ratio and the scaling factor is given by

\(\FRAC{f_{MAX}}{f_{min}} = \frac{1}{τ^{N-1}} \)

\(\frac{f_{max}}{f_{min}} =\frac{1}{τ^{N-1}} =\frac{1}{(0.5)^{5-1}}=2^4=16. \)

The correct ANSWER is (a) VHF and UHF

The best EXPLANATION: Log periodic ANTENNA uses AROUND 30MHz to 3GHZ frequency range.

MF – Medium Frequency – 300-3000 KHz

HF – High Frequency – 3-30 MHz

VHF – Very high frequency – 30-300 MHz

UHF – Ultra high frequency – 300MHz to 3GHz

So it uses VHF and UHF.

The correct answer is (a) Frequency independent antennas

Easiest explanation: Rumsey’s principle is APPLICABLE to the frequency independent antennas where the antenna size is SPECIFIED in terms of ANGLE and THUS making the impedance and pattern independent of frequency.

The correct CHOICE is (b) Normal MODE

To explain: The circumference of the HELICAL ANTENNA C= πD varies with the wavelength in different modes as follows

In Normal mode: πD << λ

Axial mode:C ≈ λ

Conical mode: C ≈n λ

The correct option is (d) 7 dB to 12 dB

The EXPLANATION: For best performance of the LPDA ANTENNA, the gain LIES between 7 to 12dB. It is a broadband antenna. LPDA is a wideband antenna and provides gain and directivity combined over a wide band of frequencies.

The CORRECT CHOICE is (b) πD ≈ λ

For EXPLANATION I would say: In Normal MODE: πD << λ

Axial mode:C ≈ λ

Conical mode: C ≈n λ

D=0 then there is no loop and is no longer helical.

Correct option is (a) \(\FRAC{L_N}{L_{N+1}} = \frac{S_N}{S_{N+1}} = τ\)

To ELABORATE: The ratio of adjacent spacing of dipoles and its LENGTHS in the LPDA are equal. The electrical properties of the log periodic antenna are repeated PERIODICALLY in terms of logarithmic frequency. The relation between the antenna dipole adjacent lengths, spacing and the SCALING factor is given as \(\frac{L_N}{L_{N+1}} = \frac{S_N}{S_{N+1}} = τ.\)

Right choice is (d) It is a function of LOGARITHMIC of frequency

Easiest explanation: Impedance is function of logarithmic of frequency. So it is called LOG PERIODIC ANTENNA. Both the impedance and STANDING wave ratio are functions of logarithmic of frequency.

Right OPTION is (a) Dimensions of antenna are SPECIFIED in terms of angles

To elaborate: Rumsey’s principle states that the impedance & pattern of antenna is frequency independent if antennasize is specified in terms of ANGLE. The Rumsey’s principle is applicable for the frequency independent antennas.The frequency independent antennas are fixed in SIZE and operate over wide bandwidth with CONSTANT impedance, polarization and pattern.

Correct choice is (a) True

To explain: The helical antenna RADIATION pattern is a COMBINATION of small dipole and loop antenna.If the diameter of the helical antenna is zero (PITCH angle 90) then it is linear antenna and when SPACING LENGTH becomes zero (pitch angle 0) then it acts like a loop antenna.

Correct choice is (a) Normal mode

Best explanation: In normal mode of radiation, it provides NARROW bandwidth and LOW radiation efficiency. In AXIAL mode of radiation, the radiation is in end-fire DIRECTION ALONG the helical axis. The radiation pattern of the antenna is broad, directional along axis and produces minor lobes at oblique angles.

Correct choice is (a) 90

To explain I would say: C is the circumference and L is the LENGTH, s is the spacing between turns.

When pitch angle α=90, the separation between turns becomes parallel to length (TENDING to INFINITY pitch angle) HENCE it acts as a LINEAR antenna.

The correct OPTION is (a) Zo/2

Explanation: The self COMPLEMENTARY ANTENNAS have constant impedance which is half of their intrinsic impedance at all FREQUENCIES. These have a METAL area congruent to open area. Impedance of self-complementary antenna = Zo/2.

The correct option is (a) True

The explanation is: Rumsey’s principle is the working principle of the FREQUENCY INDEPENDENT ANTENNAS. According to it the impedance & pattern of antenna is frequency independent if antenna size is specified in TERMS of angle. The antennas have fixed size and operate over a wide bandwidth having constant impedance and POLARIZATION.

The correct choice is (a) Inactive transmission LINE region

For explanation: In LPDA, the length of dipole \(L < \frac{λ}{2}\)COMES under inactive transmission region. These region dipoles offer capacitive impedance. So the current leads the base voltage in the inactive transmission line region. Active region \(L=\frac{λ}{2} \)offers Resistive Impedance and Inactive stop region \(L > \frac{λ}{2} \)offers INDUCTIVE impedance.

Right option is (a) Angles

The explanation: If the ANTENNAS are defined only in terms of angles, then they are frequency INDEPENDENT antennas. Its impedance and radiation pattern are independent of the frequency. EXAMPLE of frequency independent antennas are log periodic antennas, spiral antennas.

Correct option is (b) The active REGION doesn’t CHANGE with the change in frequency in Log Periodic ANTENNA

To explain: In Log period antenna, the active region changes with the change in frequency. There are three regions of operation in the LPDA.Impedance is function of logarithmic of frequency in LP so it is CALLED log periodic antenna.

Right answer is (B) False

Explanation: In normal mode of RADIATION, radiation is normal to the direction of axis and is directed in the BROADSIDE. The dimensions of the antenna are small compared to the wavelength. In AXIAL mode, it is end-fire directed.

Right ANSWER is (a) 1.11

To explain I WOULD say: The axial ratio of helical ANTENNA is given by \(AR = \FRAC{2N+1}{2N} \)

\(AR = \frac{2N+1}{2N} = \frac{7}{6} = 1.11\)

Correct answer is (a) The impedance & pattern of antenna is frequency independent if antennasize is specified in terms of ANGLE

The explanation: The frequency independent ANTENNAS are fixed in size and operate over WIDE bandwidth with constant impedance, polarization and pattern. Rumsey’s principle STATES that the impedance & pattern of antenna is frequency independent if antennasize is specified in terms of angle.

The correct choice is (b) \(L > \frac{λ}{2} \)

The BEST explanation: BASED on the LENGTHS of the dipoles in LPDA, operation region is classified into three types:

 Inactive transmission line region, Active region and Inactive STOP region. The region below \(L < \frac{λ}{2} \) belongs to Inactive transmission region. The region above \(L > \frac{λ}{2} \) is Inactive stop region.

Right option is (a) \(σ=tan^{-1}(\frac{1-τ}{4σ}) \)

Easiest EXPLANATION: In LPDA, the ratio of successive spacing of elements is EQUAL to the ratio of adjacent dipole lengths. The spacing factor is \(σ=\frac{d_n}{2L_n} \,and\, d_n =\) spacing BETWWEN adjacent elements‘n’ and ‘n+1’ and Ln is the length of n^th dipole. The relation between the spacing factor σ and the SCALING factor τ is \(σ=tan^{-1}(\frac{1-τ}{4σ}). \)

Correct choice is (a) Capacitive

For explanation: Depending on the length of the dipoles, in different regions they OFFER different resistances.

In INACTIVE TRANSMISSION line region \(L < \frac{λ}{2}\)offers Capacitive Impedance

Active region \(L=\frac{λ}{2} \)offers Resistive Impedance

Inactive stop region \(L > \frac{λ}{2} \)offers Inductive impedance.

Correct ANSWER is (a) \(L = \frac{λ}{2} \)

To explain I would say: Based on the lengths of the dipoles in LPDA, operation region is CLASSIFIED into three types:

Inactive TRANSMISSION line region \(L < \frac{λ}{2} \)

Active region \(L=\frac{λ}{2} \)

Inactive stop region \(L > \frac{λ}{2}. \)

The correct choice is (a) True

To elaborate: In normal mode, the DIMENSIONS of the antenna are small COMPARED to the WAVELENGTH. So the current magnitude and phase is constant overall the length. The far field PATTERN is independent of the number of TURNS. It considers as a single loop.

The correct option is (a) The dimensions of ANTENNA must be smaller compared to the wavelength

Easiest EXPLANATION: In normal mode, the radiation is normal to the AXIS of the antenna hence it is broadside directed. The dimensions of the antenna are SMALL compared to the wavelength in normal mode. In AXIAL mode, it is approximately equal to the wavelength. In conical mode, it is greater than wavelength.

Correct option is (a) VHF & UHF

Explanation: LOG periodic antenna uses VHF and UHF band.

MF and UHF – 300 kHz to 3GHz

MF and HF – 300 kHz to 30MHz

HF and UHF – 3MHZ to 3GHz

LPDA operates at 30MHz to 3GHz FREQUENCY range.

The correct answer is (a) True

To explain: The Reflective region is also known as the Inactive stop region. The Inactive stop region\(L > \frac{λ}{2}\) offers Inductive impedance. This region is called reflective region because any small INCIDENT wave gets REFLECTED EASILY by LARGE inductive impedance and thus offers inductive impedance.

The correct ANSWER is (c) 30 MHZ to 3 GHz

Easy explanation: Log periodic antenna USES around 30MHz to 3GHz frequency range.

MF – MEDIUM Frequency – 300-3000 KHz

HF – HIGH Frequency – 3-30 MHz

VHF – Very high frequency – 30-300 MHz

UHF – Ultra high frequency – 300MHz to 3GHz

So it uses VHF and UHF.

Right option is (a) BROADSIDE

Best EXPLANATION: The normal MODE of radiation radiates in the direction normal to the axis of the ANTENNA. So radiation is directed in broadside. In Axial mode, it is directed along the end-fire (along the axis of antenna).

Correct answer is (c) Conical MODE

The best I can explain: In AXIAL mode of RADIATION, the DIMENSIONS of antenna are approximately in the order of the wavelengths. In normal mode, dimensions of antenna are SMALLER compared to wavelength. In conical mode, the circumference of the antenna is greater than the wavelength.

Right answer is (a) Normal mode

The BEST I can EXPLAIN: In normal mode, the radiation is perpendicular to axis. It is also known as perpendicular mode. In this the ANTENNA dimensions are smaller compared to the wavelength.Axial mode is also known as BEAM mode or end-fire mode. In Axial mode of radiation, the radiation is in end-fire DIRECTION along the helical axis.

Correct choice is (a) it has narrow BANDWIDTH and dependency on small dimensions of the antenna compared to wavelength

To elaborate: The normal MODE of RADIATION depends on the size of the antenna. Its critical dependency on dimensions of antenna which has to be less than the wavelength is difficult and so it causes for narrow bandwidth and LOW radiation efficiency.

The correct OPTION is (a) Wire TYPE

The explanation: Helical antenna BELONGS to wire type antenna. It is in the form of the helix and OPERATES in the VHF and UHF. It is mainly a circular POLARIZED antenna.

Correct option is (a) 26.56

To explain: From the FIGURE, \(α=TAN^{-1}⁡\frac{S}{C}=tan^{-1}\frac{2}{4}=tan^{-1}⁡\frac{1}{2}=26.56\)

Correct CHOICE is (a) < 1

To elaborate: The PERIODICITY factor also known as the scaling factor is the RATIO of the adjacent lengths of the dipole \(\frac{L_N}{L_{N+1}} = τ \,and\, L_N < L_{N+1}\) where L is the length of the dipole.

Right option is (c) INDUCTIVE

The best explanation: The varying length of the dipoles in LPDA forms different regions and they offer different RESISTANCES. The INACTIVE transmission line region at \(L < \frac{λ}{2}\)offers Capacitive Impedance, Active region at \(L=\frac{λ}{2} \)offers Resistive Impedance and Inactive stop region \(L > \frac{λ}{2} \)offers Inductive impedance.

The correct choice is (a) Radiation is broad, directional along axis and produces MINOR lobes

For explanation: In Axial MODE of radiation, the radiation is in end-fire direction along the HELICAL axis. The radiation pattern of the antenna is broad, directional along axis and produces minor lobes at oblique ANGLES. The dimensions of antenna are approximately in the ORDER of the wavelengths.