For a half wave dipole antenna A. The average value of current is 0.64 (unit). B. The ratio of electric field intensity just near to the antenna surface and the potential developed on the antenna surface after reception of signal, is equal to the effective height C. Effective aperture will remain same if antenna gain will be improved by some technique. D. The effective height will be 0.64 l, where l is the physical length of antenna. E. If length of antenna becomes l = 0.1λ, then its current distribution become triangular. Choose the correct answer from the options given below :
A, D and E only
Background. A half-wave dipole (l = λ/2) carries an approximately sinusoidal current: maximum at the centre feed point, falling to zero at the two open ends.
A — "The average value of current is 0.64." TRUE. Averaging a half-sine over its length gives
\(I_{avg}=\dfrac{2}{\pi}I_{max}=0.637\,I_{max}\approx 0.64\)
in normalised units. It is the same 2/π that appears as the average of a rectified sine wave.
D — "Effective height = 0.64 l." TRUE. The effective height (or effective length) is the length a uniform-current antenna would need in order to radiate the same field as the actual tapered-current antenna. Because the actual current averages 2/π of its peak,
\(h_{eff}=\dfrac{2}{\pi}\,l = 0.64\,l\)
For a half-wave dipole this works out to \(h_{eff}=0.64\times\lambda/2 = \lambda/\pi\).
E — "If l = 0.1λ the current distribution becomes triangular." TRUE. On a very short (Hertzian) dipole only a tiny portion of the sine curve is used, and near its peak a sine is almost linear. The distribution therefore degenerates into a straight taper from maximum at the feed to zero at each end — a triangular distribution — for which the effective height is \(h_{eff}=l/2\).
B — the garbled "ratio of electric field intensity near the surface to the potential developed … equals the effective height." FALSE. Effective height is properly defined either as \(h_{eff}=\dfrac{V_{oc}}{E}\) (open-circuit induced voltage per unit incident field, i.e. potential ÷ field, not field ÷ potential) or by the current-distribution integral above. As written the ratio is inverted, so the statement is not accepted.
C — "Effective aperture will remain the same if the gain is improved." FALSE. Gain and effective aperture are locked together at a fixed wavelength:
\(A_e=\dfrac{\lambda^{2}}{4\pi}G\)
so raising G necessarily raises Ae. (For a half-wave dipole G = 1.64, giving \(A_e = 0.13\lambda^{2}\).)
Hence, the correct answer is A, D and E only.
Similar Questions
The expression given below is :
\(P_{r}=P_{t}\dfrac{A_{et}\cdot A_{er}}{r^{2}\lambda^{2}}\ \left(\text{W}\right)\)
If the effective area of an antenna becomes \(\frac{2A}{3}\) from its initial value of 'A', while keeping its operating frequency same. Then, the antenna gain becomes \(\left(\frac{2x+4}{15}\right)\) times of its initial value. The value of x will be:
Match List I with List II
LIST I (Type of Aperture Antenna) LIST II (Beam widtd half power points) A. Uniformly illuminated linear Array I. \(\frac{58}{D_\lambda}\) B. Uniformly illuminated circular aperture II. \(\frac{56}{a_{E\lambda}}\) C. Optimum E-plane rectangular horn III. \(\frac{67}{a_{E\lambda}}\) D. Optimum H-plane rectangular horn IV. \(\frac{51}{L_\lambda}\) Choose the correct answer from the options given below:
Match the following lists in terms of radiation resistances of various antennas :
List – I List – II a. Short vertical monopole i. \(31200\left(\dfrac{\text{Area of Loop}}{\lambda^{2}}\right)^{2}\) b. Small loop antenna ii. \(80\pi^{2}\left(\dfrac{L}{\lambda}\right)^{2}\) c. Dipole antenna iii. 73 ohms d. Radiation resistance of half wave dipole iv. \(400\left(\dfrac{\text{Physical height}}{\lambda}\right)^{2}\) Choose the correct answer from the codes given below:
Following statements are given :
(a) Beam width between first nulls for a broadside long array is given by \(\dfrac{2\lambda}{nd}\).
(b) Beam width between first nulls for an end fire long array is given by \(2\sqrt{\dfrac{2\lambda}{nd}}\).
(c) Beam width between first nulls for a broadside long array is given by \(\dfrac{\lambda}{nd}\).
(d) Beam width between first nulls for an end fire long array is given by \(\dfrac{\lambda}{nd}\).
Which of the above statements are correct ?
The most basic antenna element is :
Antennas are used for receiving and transmitting the electromagnetic signals. Their size depends upon the operating frequency / wavelength. Higher is the frequency, lower is the size of antenna. They work on Maxwell equations for field theory. They are of various types for different applications like TV transmission, AM transmission, FM transmission and satellite transmission. The waves travel in free space.
An antenna is a key component of a wireless link which efficiently couples electromagnetic energy from the transmitter to free space and from free space to the receiver. An antenna is generally a bidirectional device, i.e, the power through the antenna can flow in both the directions, hence it works as a transmitting as well as a receiving antenna. An antenna acts as an interface between the radiated electromagnetic waves and the guided waves. It can be thought of as a mode transformer which transforms a guided wave field distribution into a radiated-wave field distribution.
Read the following passage and answer the questions that follow :
Antennas have become increasingly importance to the society and at present, they are indispensable. They are being used every places. They are available in vast varieties. They are operating at various frequencies which are depending on different application. They operate on the principle of Maxwell's equation. They have different types of radiation patterns. There are several atmospheric losses in the way of propagation of waves. Due to which signal fades down, when it travels from transmitter to receiver antennas.
Based on the above para, answer the following questions :
Important Questions from Antennas
A device that makes possible the use of the same antenna for transmission and reception both
Match column A with column B.
Column A
Column B
1.
Point electromagnetic source
P.
Highly directional
2.
Dish antenna
Q.
End fire
3.
Yagi-Uda antenna
R.
Isotropic
Broadside arrays have
A. Number of dipoles of unequal size
B. Number of dipoles equally spaced
C. Collinear dipoles
D. Dipoles in phase
E. Dipoles are 90 out of phase
Choose the correct answer from the options given below:
Consider the following statements:
(a) Fiber optic cable is much lighter than copper cable
(b) Fiber optic cable is not affected by power surges or electromagnetic interference
(c) Optical transmission is inherently bidirectional.
Which of the statements is (are) correct?What is the value of directivity of an isotropic antenna?
