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.
The most basic antenna element is :
Hertzian dipole
The Hertzian dipole is the elementary building block of antenna theory — option 1.
What it is. An infinitesimally short current element of length \(dl\ll\lambda\) carrying a uniform current I along its whole length. It is an idealisation, not a buildable antenna: no real element can carry uniform current right to its open ends, where the current must fall to zero.
Why "most basic" belongs to it. Because the current is uniform, its fields can be integrated in closed form:
\(E_\theta=\dfrac{j\eta_0 I\,dl\,\sin\theta}{2\lambda r}e^{-jkr}\)
and every other antenna is then obtained by summing Hertzian dipoles along its length with the appropriate current distribution. The half-wave dipole, for instance, is the integral of elementary dipoles carrying \(I(z)=I_0\cos kz\). So the Hertzian dipole plays the same role in antenna theory that the impulse plays in signals: the elementary response from which all others are built by superposition.
Why the other three are not "basic".
| Antenna | Current distribution | Directivity | Rrad |
|---|---|---|---|
| Hertzian dipole | uniform, \(dl\ll\lambda\) | 1.5 | \(80\pi^{2}(dl/\lambda)^{2}\) |
| Short dipole | triangular, tapering to zero | 1.5 | \(20\pi^{2}(l/\lambda)^{2}\) |
| Half-wave dipole | sinusoidal | 1.64 | 73 Ω |
| Monopole | half a dipole plus its ground image | 3.28 | 36.5 Ω |
The short dipole is more realistic but its triangular current distribution is already an integration of Hertzian elements — note that its radiation resistance is exactly a quarter of the Hertzian value for the same length, because the average current is half as large. The half-wave dipole is the most basic practical antenna and the usual gain reference, and the monopole is derived from it by image theory.
The distinction the question is testing is between "most basic" in the theoretical sense — the elementary radiator — and "most basic" in the practical sense of what one would actually build. The passage's framing of an antenna as a mode transformer is a theoretical one, and the theoretical primitive is the Hertzian dipole.
Its pattern. The \(\sin\theta\) dependence gives a doughnut: maximum broadside, nulls along the axis, and a half-power beamwidth of 90°. That is the pattern every simple wire antenna approaches as it is made shorter.
Hence, the most basic antenna element is the Hertzian dipole.
Similar Questions
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:
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 :
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:
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:
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 expression given below is :
\(P_{r}=P_{t}\dfrac{A_{et}\cdot A_{er}}{r^{2}\lambda^{2}}\ \left(\text{W}\right)\)
Important Questions from Antennas
Which of the following antennas is the standard reference antenna for the directiveness?
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?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:
To match the impedance of a 'ground penetrating radar antenna' to the ground, impedance of ground is given by the expression, (if ϵ r= 14, μ r= 1, σ = 10 −2 ℧/m, operating frequency = 200 MHz)
For an isotropic antenna P n(θ, φ) = 1, D = 1, for all θ and φ. The beam area for the isotropic antenna is given by:
