In the radiation pattern of an antenna, the main lobe represents the antenna's main beam, through which the maximum and most concentrated energy is radiated.
Beamwidth is the angular width of the aperture through which the majority of the power is radiated. The two main parameters used to characterize beamwidth are the Half‑Power Beamwidth (HPBW) and the First Null Beamwidth (FNBW).
Half-Power Beamwidth (HPBW)
According to the standard definition, the angular separation over which the radiation pattern amplitude drops by 50% (i.e., -3 dB) from the peak of the main lobe is called the half‑power beamwidth.
In other words, the beamwidth is the region over which the antenna radiates the majority of its power, which corresponds to the region near the peak power. The half‑power beamwidth is the angular range within which the relative power in the antenna's effective radiation field exceeds 50% of the peak power.
Geometric Interpretation of HPBW
On the radiation pattern, draw a line from the origin to each side of the main lobe at the half‑power points. The angle between these two vectors is the half‑power beamwidth (HPBW). The following figure helps illustrate this concept.
The figure shows the main lobe of the antenna and the half‑power points on the main lobe.
Mathematical Expression
An approximate formula for the half‑power beamwidth is:
where:
•λ is the operating wavelength,
•D is the antenna aperture dimension (typically diameter or side length).
The unit of half‑power beamwidth (HPBW) is the radian or the degree.
First Null Beamwidth (FNBW)
According to the standard definition, the angular separation between the first nulls adjacent to the main lobe is called the first null beamwidth.
In simple terms, FNBW is the angular span between the first pattern nulls on either side of the main beam.
Geometric Interpretation of FNBW
From the origin of the radiation pattern, draw lines tangent to the main beam on each side. The angle between these two tangent lines is the first null beamwidth (FNBW). The following figure helps illustrate this concept more clearly.
The figure above shows the half‑power beamwidth (HPBW) and the first null beamwidth (FNBW) on a radiation pattern, with the main lobe and side lobes indicated.
Mathematical Expression
The relationship between the first null beamwidth (FNBW) and the half‑power beamwidth (HPBW) can be approximated as:
Substituting HPBW ≈ 70λ/D, we obtain:
where λ is the wavelength and D is the antenna aperture dimension.
Unit
The unit of First Null Beamwidth (FNBW) is the radian (rad) or the degree (°).
Effective Length and Effective Area
Among antenna parameters, effective length and effective area are also important metrics that help evaluate antenna performance.
Effective Length
The effective length of an antenna is used to characterize its polarization efficiency.
Definition: Effective length is the ratio of the open‑circuit voltage amplitude at the receiving antenna terminals to the amplitude of the incident electric field strength in the same polarization direction as the antenna.When an incident wave reaches the antenna input, it possesses a certain electric field strength whose amplitude depends on the antenna’s polarization. This polarization should match the voltage amplitude at the receiver terminals for optimal signal reception.
Mathematical Expression
The mathematical expression for effective length is:
where:
•le is the effective length of the antenna,
•Voc is the open‑circuit voltage amplitude at the receiving antenna terminals,
•Ei is the amplitude of the incident electric field strength in the same polarization direction as the antenna.
Effective Area
Definition: Effective area is the portion of a receiving antenna’s area that absorbs energy from the incident wavefront and converts it into an electrical signal; it is generally smaller than the antenna’s physical aperture area.
During reception, the entire physical area of the antenna is exposed to the incident electromagnetic wavefront, but only a part of it effectively captures the signal. This part is termed the effective area.
The reason only a fraction of the wavefront energy is utilized is that some of the incident wave is scattered by the antenna, while another part may be dissipated as heat. Therefore, under ideal conditions without losses, the area that, when multiplied by the incident power density, gives the maximum obtainable power from the antenna is called the effective area.
The effective area is commonly denoted by Aeff.
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Post time: Apr-30-2026
