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What are sidelobe level and front-to-back ratio?

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Sidelobe level (SLL) is the peak of the highest sidelobe relative to the main-beam maximum, in dB; front-to-back ratio (F/B) is the ratio of the maximum directivity of the main beam to the directivity in a specified rearward region, usually the direction 180° from the peak.[1] A uniformly illuminated line source has a first sidelobe at −13.26 dB.[2]

Why do sidelobes matter?

Sidelobes radiate interference into other systems, collect interference and ground noise, cause false targets in radar and reduce gain. Regulatory masks such as those for satellite earth stations and point-to-point links limit sidelobe envelopes.[3][4]

How are sidelobes reduced?

Tapering the aperture or array amplitude toward the edges lowers sidelobes at the cost of wider beamwidth and lower aperture efficiency. Dolph–Chebyshev weighting gives equal-ripple sidelobes at a chosen level; Taylor weighting gives a practical near-in sidelobe control with decaying far-out sidelobes.[5][6] See array tapering.

What front-to-back ratios are typical?

A two-element Yagi typically has F/B of roughly 10–20 dB depending on tuning, longer optimized Yagis 20–30 dB, patches on finite ground planes around 15–25 dB, and dishes often 30–40 dB or more.[7][2]

Frequently asked questions

What is the first sidelobe of a uniform circular aperture?

About −17.6 dB.

Do sidelobes reduce gain?

Yes; power in sidelobes is power not in the main beam.

Is F/B the same as front-to-rear?

Front-to-rear usually averages or takes the worst case over a rear sector; F/B uses one direction.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
  2. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  3. “Radio Regulations”. International Telecommunication Union. 2024 edition.
  4. “Introduction to Radar Systems, 3rd ed.”. M. I. Skolnik, McGraw-Hill. 2001.
  5. “A Current Distribution for Broadside Arrays Which Optimizes the Relationship Between Beam Width and Side-Lobe Level”. C. L. Dolph, Proceedings of the IRE, vol. 34, no. 6, pp. 335-348. June 1946.
  6. “Design of Line-Source Antennas for Narrow Beamwidth and Low Side Lobes”. T. T. Taylor, IRE Transactions on Antennas and Propagation, vol. 3, no. 1, pp. 16-28. January 1955.
  7. “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
Categories: Fundamentals