What determines the sidelobe level?

For an aperture antenna, the radiation pattern is the Fourier transform of the aperture illumination function. A uniform amplitude distribution across the aperture produces the narrowest possible main beam (highest directivity) but also the highest sidelobes at minus 13.2 dB relative to the main beam. Tapering the illumination so that the edges of the aperture receive less power than the center suppresses the sidelobes at the cost of a wider main beam. A Taylor taper to minus 25 dB sidelobes widens the beam by about 10 percent. A Dolph-Chebyshev taper produces equal-level sidelobes at whatever specified level, optimizing the main beam width for that sidelobe constraint. In phased arrays, this taper is implemented digitally by adjusting the amplitude weights of each element.

What is the difference between E-plane and H-plane patterns?

The radiation pattern of a linearly polarized antenna is typically measured in two orthogonal planes. The E-plane contains the electric field vector and the direction of maximum radiation. The H-plane contains the magnetic field vector and the direction of maximum radiation. For a vertically polarized dipole, the E-plane is the vertical plane passing through the dipole axis, showing the characteristic figure-eight pattern. The H-plane is the horizontal plane perpendicular to the dipole, showing an omnidirectional circle. For a rectangular patch antenna, the E-plane typically has a wider beamwidth than the H-plane because the patch radiates from the two opposite edges separated by the substrate thickness.

How far away must I be to measure a valid pattern?

The far-field boundary (Fraunhofer distance) is defined as R = 2D squared divided by lambda, where D is the largest dimension of the antenna. Closer than this distance, the phase error across the aperture exceeds 22.5 degrees and the measured pattern is distorted. For a 1-meter dish at 10 GHz (lambda = 3 cm): R = 2 times 1 squared divided by 0.03 = 67 meters. For a 5G NR 64-element array at 28 GHz (D = 0.3 m, lambda = 10.7 mm): R = 2 times 0.09 divided by 0.0107 = 16.8 meters. Indoor anechoic chambers use compact range reflectors or near-field scanning with mathematical transformation to simulate far-field distances in much smaller spaces.

Antenna Tech

Antenna Pattern

A 5G base station's 64-element phased array does not radiate equally in all directions. At boresight, the signal is 30 dBi strong. At 15 degrees off axis, the first sidelobe appears at −20 dB relative to the main beam. At 90 degrees, the pattern has dropped by 50 dB. This spatial selectivity is the antenna's radiation pattern: a three-dimensional map of radiated intensity versus direction. The pattern determines where the signal goes and, equally important, where it does not go. In cellular networks, careful pattern shaping through amplitude tapering and null steering concentrates energy toward intended users while minimizing interference toward adjacent cells.

Category: Antenna Tech

Key Regions: Main beam, sidelobes, nulls

Far-field: R = 2D²/λ

Aperture Weighting Controls the Pattern

Taper Function	First SLL	Beam Broadening	Directivity Loss	Sidelobe Behavior
Uniform	−13.2 dB	1.0× (reference)	0 dB	Sinc pattern, slow decay
Cosine	−23 dB	1.36×	0.91 dB	Rapid decay
Hamming	−42 dB	1.50×	1.34 dB	Very low, fast decay
Taylor (−25 dB)	−25 dB	1.10×	0.35 dB	Controlled near-in
Dolph-Chebyshev (−30 dB)	−30 dB	1.15×	0.50 dB	All equal at −30 dB
Blackman-Harris	−92 dB	2.00×	2.95 dB	Extremely low

Far-field distance (Fraunhofer):
R_ff = 2D² / λ
1 m dish at 10 GHz: R = 2(1)²/0.03 = 67 m
0.3 m array at 28 GHz: R = 2(0.09)/0.0107 = 16.8 m

Approximate beamwidth from aperture:
θ_3dB ≈ k × λ/D (radians)
k = 0.886 for uniform, 1.2 for cosine taper

Directivity from beamwidths:
D ≈ 41,253 / (θ_E × θ_H) (degrees)
A patch with θ_E = 65° and θ_H = 80°: D ≈ 41,253 / (65 × 80) = 7.9 = 9.0 dBi. This approximation is accurate to within 1 dB for most practical antenna patterns.

Common Questions

Frequently Asked Questions

What determines sidelobe level?

The pattern is the Fourier transform of the aperture illumination. Uniform: narrowest beam, −13.2 dB SLL. Tapered: wider beam, lower sidelobes. Taylor −25 dB taper: 10% beam broadening, 0.35 dB directivity loss. Phased arrays implement tapers digitally via element weights.

E-plane vs. H-plane?

E-plane: contains the E-field vector. H-plane: contains the H-field vector. A vertical dipole's E-plane (vertical) shows a figure-eight; H-plane (horizontal) shows an omnidirectional circle. Rectangular patches: E-plane is usually wider than H-plane.

How far for a valid measurement?

Fraunhofer distance: R = 2D²/λ. Phase error <22.5° across aperture. Compact ranges and near-field scanners with mathematical transforms simulate far-field conditions in smaller spaces.

Related Terms

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Pattern Analysis

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Enter aperture size, frequency, and taper function. Generate the 2D radiation pattern showing main beam, sidelobes, nulls, and the 3 dB beamwidth for both E and H planes.

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