Figure 1. Main-beam and sidelobe regions used to define antenna beam efficiency.
What Is Antenna Beam Efficiency?
Antenna beam efficiency, also called main-beam efficiency, is the fraction of an antenna's total radiated power that falls inside a defined main-beam region. It answers a practical design question: how much of the available radiation is concentrated toward the intended angular sector instead of being distributed into sidelobes and the back lobe?
For a receiving antenna, reciprocity allows the same idea to be viewed as the fraction of received power collected through the main beam when the surrounding field and polarization conditions are defined consistently. Beam efficiency is therefore especially useful in radar, radiometry, satellite links, point-to-point microwave systems and antenna measurement, where energy arriving from or leaving toward unwanted directions can reduce accuracy or increase interference.
In Figure 1, the main lobe is centered on the z-axis. If the cone angle θ1 marks the first null or another explicitly specified main-beam boundary, beam efficiency is calculated by integrating the power pattern inside that cone and dividing it by the power integrated over the complete radiation pattern.
Antenna Beam Efficiency Formula
Using the normalized power pattern Pn(θ,φ), the general beam-efficiency expression is shown below:
The numerator represents the power contained inside the selected main-beam solid angle, ΩMB. The denominator represents the total radiated power integrated over 4π steradians. The resulting value, ηB, is dimensionless and is normally reported as a percentage.
- Pn(θ,φ) is the normalized power pattern. If measured electric-field data are used, the corresponding power term is proportional to |E|2.
- ΩMB is the specified main-beam region, often bounded by the first nulls or by a defined cone angle.
- θ and φ are the elevation and azimuth coordinates used to describe the full radiation pattern.
The main-beam boundary must always be stated with the result. Two laboratories can obtain different numerical values from the same pattern if one integrates to the first null while the other uses a fixed angular cone. For an approximately rotationally symmetric pencil beam, the integration can be simplified as shown in the next expression.
How Is Antenna Beam Efficiency Measured?
Beam efficiency is obtained from radiation-pattern data rather than from a single boresight reading. A calibrated far-field range, compact antenna test range, or near-field system with a validated far-field transformation may be used. For phased arrays, the measurement should be repeated for each required beam state, frequency and polarization because scanning and amplitude or phase errors can redistribute power into sidelobes.
- Measure the radiation pattern over enough angular coverage to include the main beam, sidelobes and back lobe.
- Convert the measured field to a power pattern and apply the same normalization to every angular sample.
- Define the main-beam boundary, such as the first-null contour or a specified cone angle.
- Integrate the power inside that boundary and divide it by the power integrated over the complete pattern.
- Report the test conditions, including frequency, polarization, beam state, angular resolution and the boundary definition.
A pair of two-dimensional E-plane and H-plane cuts can show beamwidth and sidelobe behavior, but it does not normally provide a complete three-dimensional beam-efficiency value unless rotational symmetry has been justified. Full-sphere or sufficiently complete pattern data give a more defensible result. Learn more about RF MISO's antenna testing capabilities.
Beam Efficiency vs. Other Antenna Efficiency Terms
Beam Efficiency vs. Radiation Efficiency
Radiation efficiency compares radiated power with the power accepted by the antenna and therefore reflects conductor and dielectric losses. Beam efficiency describes where the radiated power goes. An antenna can have low internal loss but still have modest beam efficiency if substantial power appears in sidelobes or the back lobe.
Beam Efficiency vs. Directivity and Gain
Directivity describes the peak concentration of the radiation pattern relative to an isotropic radiator. Gain combines directivity with radiation efficiency, while realized gain also accounts for impedance mismatch. Beam efficiency is different because it integrates power across an angular region instead of evaluating only the peak direction.
Beam Efficiency vs. Beamwidth
Half-power beamwidth is the angular width between the -3 dB points of the main lobe. A narrow beam often accompanies high directivity, but it does not by itself prove high beam efficiency. A pattern can have a narrow peak and still send significant energy into sidelobes. Beamwidth and sidelobe level must therefore be reviewed together with the integrated beam-efficiency value.
What Controls Antenna Beam Efficiency?
1. Sidelobe and Back-Lobe Levels
Power outside the selected main-beam region directly reduces beam efficiency. High sidelobes may result from aperture discontinuities, array spacing, amplitude errors or an illumination profile that has not been optimized for the required pattern. Back radiation and scattering from nearby structures can have a similar effect.
2. Aperture Illumination and Amplitude Taper
Aperture antennas and arrays use an amplitude distribution across the radiating aperture. A nearly uniform distribution can produce a narrow main beam and high directivity, but it may also produce comparatively high sidelobes. Applying an amplitude taper can suppress sidelobes and place more of the total pattern power in the desired region, although excessive taper can broaden the main beam and reduce peak gain or aperture efficiency. The best setting is therefore an application-specific trade-off.
3. Feed Spillover and Structural Blockage
In a reflector antenna, energy that misses the reflector is spillover and does not contribute to the desired main beam. Feed supports, a subreflector or nearby hardware can also block or scatter part of the aperture field. Correct feed selection, focal positioning and mechanical alignment help control these losses and unwanted lobes.
4. Phase, Amplitude and Surface Errors
Phase imbalance between array elements, unequal feed loss, manufacturing tolerance, reflector surface error and radome distortion can move energy away from the intended beam. These effects usually become more important as frequency increases because a fixed dimensional error represents a larger fraction of a wavelength.
5. Polarization Purity
Some beam-efficiency definitions include only power in the desired polarization. Cross-polarized radiation inside or outside the main beam can then reduce the reported value. Polarization should be specified consistently in simulation, measurement and system requirements.
Beam Efficiency, Beamwidth and Bandwidth Are Different
Beam efficiency is an integrated angular power ratio. Beamwidth describes the angular width of the main lobe at a stated level, commonly the half-power or -3 dB points. Antenna bandwidth describes the frequency range over which selected requirements are satisfied. These parameters influence one another, but they are not interchangeable.
A design may maintain acceptable VSWR across a wide frequency range while its beam direction, sidelobe level or beam efficiency changes substantially near the band edges. Pattern performance should therefore be verified at several frequencies rather than inferred from impedance bandwidth alone. For a detailed treatment of frequency-range definitions, see Antenna Bandwidth.
How Can Beam Efficiency Be Improved?
- Choose an aperture taper that balances sidelobe suppression, main-beam width and gain.
- Optimize reflector-feed position, edge illumination and spillover.
- Use suitable element spacing and accurate amplitude or phase control in arrays to avoid grating lobes.
- Reduce blockage, cable scattering and unwanted interaction with the mounting structure or radome.
- Control machining tolerances, surface accuracy and alignment at the highest operating frequency.
- Verify co-polar and cross-polar patterns across the full operating band and every required beam state.
Where Does High Beam Efficiency Matter Most?
High beam efficiency is particularly valuable when power or received energy must be confined to a well-defined direction. Typical examples include target-tracking and imaging radar, satellite communication links, radio astronomy and radiometry, point-to-point microwave links, reflector-feed systems, calibration antennas and electronically steered arrays.
There is no universal beam-efficiency percentage that is correct for every antenna. A useful requirement must state the operating frequency, polarization, beam state and angular boundary, together with acceptable sidelobe and back-lobe behavior. This makes supplier data, simulation and measured results directly comparable.
What Should Buyers Check in an Antenna Datasheet?
When beam control is important, do not evaluate an antenna from peak gain alone. Ask the manufacturer which pattern data and test conditions are available for the exact model and frequency range.
- Operating frequency range and the frequencies used for pattern measurements
- Gain, half-power beamwidth and first sidelobe level versus frequency
- Co-polar and cross-polar radiation patterns in the required planes or over a spherical scan
- The angular boundary and polarization used to calculate beam efficiency, when that value is specified
- VSWR, connector or waveguide interface, power handling and environmental requirements
- Mechanical tolerances, mounting arrangement and any radome or reflector-feed configuration
RF MISO Microwave Antenna Solutions
RF MISO supplies microwave and millimeter-wave antennas for measurement, radar, communication and system integration. Relevant product families include standard gain horn antennas, reflector antennas and array antennas. Model-specific specifications and available test data should be reviewed against the required frequency, polarization, gain and pattern performance.
For antenna selection or a customized requirement, contact RF MISO at info@rf-miso.com.
Post time: Jul-12-2024
