The directivity of an antenna array can be increased by adding more antenna elements, as a larger number of elements:

Antenna Directivity Explained

An antenna array consists of multiple individual antenna elements working together. The primary goal of an antenna array is often to achieve a highly directive radiation pattern, meaning it can concentrate radiated power in a specific direction or receive signals preferentially from a particular direction. Directivity (often denoted as $D$) is a measure of an antenna's ability to concentrate radiated power in a given direction. A higher directivity indicates a narrower main beam and less power radiated in undesired directions.

The directivity of an antenna array is a crucial performance metric. It quantifies how much power density is radiated in a specific direction compared to the average power density over all directions from an isotropic antenna (which radiates uniformly in all directions) with the same total input power.

Elements and Effective Area

When more antenna elements are added to an antenna array, the overall physical size of the array increases. This increase in physical size directly leads to an increase in the effective area ($A_e$) of the antenna. The effective area is a concept used to quantify how much power an antenna can capture from an incident electromagnetic wave (in reception) or how effectively it can radiate power in a specific direction (in transmission).

- The relationship between directivity ($D$) and effective area ($A_e$) for an antenna is given by the formula: $D=\frac{4\pi A_e}{{\lambda }^2}$ where $\lambda$ is the wavelength of the electromagnetic wave.
- From this formula, it is clear that directivity is directly proportional to the effective area. Therefore, an increase in the effective area of the antenna array directly results in an increase in its directivity.
- By adding more antenna elements, the array forms a larger "aperture" or "receiving/transmitting surface." This larger effective aperture allows for more precise control over the phase and amplitude of the electromagnetic waves, enabling the array to focus energy more tightly into a desired direction, thereby boosting the antenna array's directivity.

Why Other Options are Not Correct

Let's examine why the other options do not primarily explain how adding more antenna elements increases directivity:

- Improves the radiation efficiency: Radiation efficiency relates to how much of the input power is actually radiated versus being lost as heat. While a well-designed array aims for high efficiency, adding more elements itself doesn't inherently or primarily improve the efficiency of each element or the overall array. Directivity and efficiency are distinct parameters.
- Results in a better impedance matching: Impedance matching refers to ensuring maximum power transfer from the source to the antenna (or vice versa) by matching their impedances. While proper impedance matching is crucial for overall antenna performance, adding more elements to an array does not automatically guarantee or primarily result in better impedance matching for the array system. In fact, adding elements might complicate the matching network.
- Allows more power to be transmitted by the antenna: The maximum power an antenna can transmit is primarily limited by the power handling capabilities of its components (e.g., amplifiers, feed lines) and regulatory limits, not solely by the number of elements. While a larger array might be designed to handle more power, the fundamental reason for increased directivity with more elements is related to the effective area, not just the raw power transmission capability.

Therefore, the most accurate explanation for why adding more antenna elements increases the directivity of an antenna array is that it increases the effective area of the antenna. This allows the antenna array to focus its radiated power more effectively in a particular direction.