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 angular distribution of the transmitted power around the antenna is generally known as :
Radiation pattern
The standard term is the radiation pattern. It is defined as a graphical representation of the antenna's radiation properties as a function of direction — that is, how the radiated power is distributed over the angular coordinates θ and φ at a fixed large distance. That is exactly what the question describes, so the answer is option 3.
Why option 2 is the near miss. "Power pattern" is a genuine term, but it is a subdivision of the radiation pattern rather than the general name. A radiation pattern may be plotted in several forms:
| Form | Quantity plotted |
|---|---|
| Field pattern | \(|E(\theta,\phi)|\) |
| Power pattern | \(|E(\theta,\phi)|^{2}\), usually in dB |
| Phase pattern | the phase of the field |
The question asks for what the distribution is "generally known as", and the general, umbrella term is radiation pattern. Note also that a 3 dB drop in the power pattern corresponds to a 0.707 drop in the field pattern — the same angular width, read on different scales.
Why the other two fail. "Angular pattern" is not standard terminology at all. An antenna array is a physical arrangement of several radiating elements, not a description of the radiated power — though it is one way of shaping a pattern.
The features a radiation pattern displays.
The main lobe contains the direction of maximum radiation; side lobes are the smaller maxima either side of it; the back lobe points opposite the main beam; and nulls are the directions of zero radiation.
The numbers read off it are the half-power beamwidth (HPBW), the beamwidth between first nulls (BWFN), the side-lobe level and the front-to-back ratio.
Where the pattern must be measured. Only in the far field, beyond
\(R \gt \dfrac{2D^{2}}{\lambda}\)
because closer in the relative phases of the contributions from different parts of the aperture are still changing with distance and the pattern has not settled into its final shape.
Reciprocity, which the passage itself notes. Because an antenna is bidirectional, its receiving pattern is identical to its transmitting pattern — so a single measurement serves both roles.
Hence, the angular distribution of transmitted power is the radiation pattern.
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