Which of the following antennas is the standard reference antenna for the directiveness?
Isotropic antenna
Understanding Antenna Directiveness and Reference Antennas
Antenna directiveness, often referred to as directivity, is a fundamental characteristic that describes how well an antenna concentrates radiated power in a particular direction compared to an antenna that radiates equally in all directions. A high directivity means the antenna is very directional, focusing power in a narrow beam. A low directivity means the antenna radiates more broadly.
To quantify the directiveness of any real antenna, we need a standard benchmark or reference. This reference antenna allows us to compare the performance of different antennas consistently.
What is the Standard Reference Antenna for Directiveness?
The standard reference antenna for directiveness is the theoretical Isotropic antenna.
- An Isotropic antenna is a hypothetical antenna that radiates electromagnetic waves equally in all directions in three-dimensional space.
- It does not exist in reality, but it serves as a perfect theoretical model for comparison.
- Because it radiates uniformly in all directions, its directivity is the minimum possible value, which is 1 (or 0 dB).
- Measuring the directivity of a real antenna involves comparing its radiation intensity in the direction of maximum radiation to the radiation intensity of an isotropic antenna radiating the same total power.
Why Other Options Are Not the Primary Reference for Directiveness
- Half-wave doublet (Half-wave dipole): This is a very common and practical antenna. It is often used as a reference antenna for gain measurements (comparing an antenna's gain to that of a half-wave dipole) because it is a realizable standard. However, for directiveness specifically, the theoretical isotropic antenna is the fundamental reference because it represents uniform radiation (directivity of 1). The directivity of a half-wave dipole in free space is approximately 1.64 (or 2.15 dB).
- Rhombic antenna: This is a type of directional antenna used primarily in HF and shortwave bands. It has significant directivity and gain, but it is not used as a standard reference antenna.
- Elementary doublet (Hertzian dipole): This is another theoretical antenna, an infinitesimally short dipole. It radiates in a 'doughnut' pattern. While theoretical, its directivity is about 1.5 (or 1.76 dB), slightly less directional than a half-wave dipole but more directional than an isotropic antenna. It is not the standard reference for directiveness.
In summary, the isotropic antenna, despite being theoretical, is the universally accepted standard reference for defining and measuring antenna directiveness due to its perfect omnidirectional radiation pattern and baseline directivity of 1.
Comparison of Antenna Directivities (Approximate)
| Antenna Type | Directivity (dimensionless) | Directivity (dB) | Use as Reference for Directiveness |
|---|---|---|---|
| Isotropic Antenna | 1 | 0 | Standard Reference |
| Elementary Doublet (Hertzian Dipole) | 1.5 | 1.76 | No |
| Half-wave Doublet (Half-wave Dipole) | 1.64 | 2.15 | No (Common reference for gain relative to a dipole) |
Revision Table: Key Antenna Concepts
| Term | Definition | Related Concept |
|---|---|---|
| Directivity | Measure of how much an antenna concentrates power in a specific direction compared to an isotropic source. | Antenna Gain (Directivity accounting for efficiency) |
| Isotropic Antenna | Theoretical antenna radiating uniformly in all directions. | Standard Reference Antenna for Directivity |
| Half-wave Dipole | A practical antenna widely used, often a reference for gain measurements. | A realizable antenna with specific directivity |
Additional Information on Antenna Directiveness and Gain
While directivity is a measure of the antenna's ability to concentrate power purely based on its radiation pattern, Antenna Gain is a more practical measure. Antenna gain takes into account the antenna's electrical efficiency in addition to its directivity. Gain is defined as directivity multiplied by efficiency ($\text{Gain} = \text{Directivity} \times \text{Efficiency}$). Like directivity, gain is often expressed relative to a reference antenna, usually the isotropic antenna (dBi) or sometimes the half-wave dipole (dBd).
Understanding directivity and gain is crucial for selecting the right antenna for a specific application, ensuring power is directed towards the intended receiver and minimizing wasted energy.
Important Questions from Antennas
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:
A device that makes possible the use of the same antenna for transmission and reception both
