How Do You Choose Antenna Element Spacing?

For a regularly spaced beam-steering array, start near half the free-space wavelength at the highest operating frequency. Then check the maximum scan angle, mutual coupling, element size and feed layout before choosing the final center-to-center pitch. Half-wavelength spacing is a useful starting point, not a universal minimum or a guarantee of good matching. A narrower scan sector can allow wider spacing; smaller spacing can work when the elements and feed are designed together. [1]

Calculate the wavelength at the top of the band

Use λ_min = c/f_max, where c = 299,792,458 m/s. Here, wavelength means free-space wavelength, and spacing means center-to-center distance along the array. At 10 GHz, λ = 29.979 mm and λ/2 = 14.990 mm. A fixed pitch becomes electrically larger as frequency rises, so using the band-center wavelength can hide a grating-lobe problem at the upper band edge. [2]

For example, a 2–18 GHz array has λ_min = 16.655 mm. A half-wavelength starting pitch is therefore 8.328 mm. That pitch is only 0.0556λ at 2 GHz: clearing the high-frequency grating-lobe check does not establish that the elements will match or radiate efficiently across the full band.

Set the spacing limit for the required scan

For a uniform linear array scanning from broadside through ±θ_max, keep the pitch below d_limit = λ_min / (1 + sin θ_max) to keep grating-lobe maxima outside the visible region, including its boundary. Equality places a grating-lobe maximum at the horizon at the limiting scan angle. Choose a smaller nominal pitch to leave manufacturing and operating margin. This is an array-factor condition; ordinary sidelobes, mismatch and element-pattern limitations still require separate checks. [3]

Values are calculated from the exact speed of light and the stated scan-boundary equation, then rounded to two decimal places. Use unrounded values for tolerance decisions. These boundaries assume periodic spacing and a specified beam direction in one scan plane; they are not complete acceptance criteria for a planar or broadband array.

A worked choice at 10 GHz and ±50°

The calculated boundary is 16.975 mm. A proposed 15.00 mm pitch leaves 1.975 mm of geometric margin. If the adjacent-element pitch tolerance is ±0.50 mm, the maximum pitch is 15.50 mm, still below that boundary. The tolerance corresponds to ±0.0167 in d/λ. This calculation screens the geometry; nonuniform placement errors also change sidelobes and beam pointing and must be assessed in the actual array model.

Do not confuse the steering threshold with the direction of a grating lobe. At 10 GHz, a 15.00 mm uniform pitch reaches the horizon condition only when the beam is steered to about 86.99° from broadside. That is the beam’s steering angle at onset, not a grating-lobe location of 87°. For a different example, d = 25.00 mm and a beam at +30° give sin θ_g = sin 30° − λ/d, placing the first grating lobe at approximately −44.36°. [3]

What a documented hardware design demonstrates

A NASA report published in November 1969 describes a 50-element, lens-compensated sectoral-horn array operating from 15.2 to 16.8 GHz. Its spacing was selected as λ/2 at 16.8 GHz for a ±60° scan requirement. Using the same calculation gives an 8.92 mm pitch and a 9.56 mm scan boundary, leaving about 0.64 mm of geometric margin. This is a published hardware example, not a Dolph Microwave test or a performance guarantee for another element type. [4]

Check coupling rather than assuming a half-wavelength minimum

There is no universal coupling floor at λ/2. An element’s impedance and radiation pattern change when neighboring elements and the array structure are present. The acceptable pitch depends on polarization, element geometry, ground plane, excitation and the required bandwidth. Even a half-wavelength array needs an embedded-element and matching assessment. [1]

Check the multiport scattering parameters across the band, then evaluate the match under the intended beam excitations. For a driven port i, the active reflection coefficient is Γ_active,i = Σ_jS_ija_j/a_i, where a_j are the incident-wave excitations and a_i is nonzero. Consequently, a low nearest-neighbor S₂₁ alone does not prove that the array remains matched while scanning. The dependence on excitation follows from the array’s multiport network description. [5]

Published measurements also show why sub-half-wavelength designs should not be rejected automatically. Coetzee and Yu’s 2008 study used four monopoles in a 2×2 layout at 2.6 GHz, with 15 mm spacing, approximately 0.13λ. The original array had roughly −10 dB mutual coupling. A modal feed network generally suppressed the coupling terms below −20 dB; additional stub networks provided matching. Those results belong to that prototype and feed arrangement, not to every compact array. [5]

Balance aperture, element count and physical fit

Distinguish a fixed aperture from a fixed element count. At fixed element count, increasing pitch enlarges the aperture and can narrow the main beam before grating lobes become limiting. At fixed aperture, larger pitch reduces the number of elements; it does not automatically make the beam narrower or force gain lower. Aperture illumination, efficiency, mismatch and losses determine the realized result. [1]

For a uniform line with N element centers separated by d, the end-center span is L = (N − 1)d. As a layout calculation, a 300 mm available center span with d = 15 mm fits 21 centers. This is different from a 300 mm enclosure: the enclosure must also accommodate the end elements, mounting features and clearances.

Check the element footprint in the direction of each lattice axis, including connectors, feed routing and the radome. A dipole’s length does not automatically consume the pitch along a perpendicular axis. Likewise, a horn is not necessarily compact enough for a wide-scan lattice; its aperture and flange envelope must fit the proposed cell.

The feed architecture also matters. A corporate divider can supply many elements from one input, while independently controlled channels may feed elements or subarrays. Element count is therefore not automatically the number of RF chains or phase shifters. Define that mapping when specifying antenna feeder networks.

Use catalogue information without assuming the missing geometry

Dolph Microwave’s published waveguide array antenna specifications, checked on October 24, 2026, list 9.30–9.50 GHz models, lengths of 1500, 1800 and 2200 mm, and a scan-range entry of ±25°. The page explicitly distinguishes these catalogue entries from a description of the electronic steering architecture.

Here is how to use those figures in a preliminary review. If the requirement were a uniform linear electronically steered array operating to 9.50 GHz and ±25° from broadside, the free-space calculation would give λ_min = 31.557 mm and d_limit = 22.182 mm. These are conditional design calculations, not the slot pitch or element count of the listed product.

Before translating the catalogue into a layout, obtain the slot-center drawing, identify the scan plane and steering mechanism, and confirm whether the listed length is an enclosure length or an active aperture. Ask how the channels connect to slots or subarrays. Those details determine whether the simple spacing equation applies to the proposed configuration. The public catalogue alone does not establish them.

Adapt the spacing method to the array type

Planar arrays

A two-dimensional array needs a grating-lobe check over the complete azimuth and elevation scan region. A rectangular or triangular lattice cannot be approved from one nearest-neighbor distance alone. Evaluate its lattice vectors and all required beam directions; a triangular layout does not provide an unconditional scan-angle allowance. Varney’s steering-limit examples illustrate how the accessible field of view depends on the two-dimensional geometry. [1]

Slotted waveguide arrays

Use both the feed-wave propagation and the free-space radiation geometry. Guide wavelength affects the phase progression between slots; free-space wavelength still governs the directions in which their radiated fields combine. Replacing every λ in the grating-lobe equation with guide wavelength is not a valid shortcut. NASA’s scanning-array study treats the radiation geometry and waveguide arrangement together. [3]

A NASA JPL design report describes iterating lattice spacing, aperture distribution and waveguide dimensions, followed by full-wave analysis and tolerance simulations. It also identifies off-principal-plane butterfly lobes associated with alternating slot offsets. Check the complete pattern, not only the two principal cuts. [6]

Log-periodic dipole arrays

An LPDA has progressively changing element lengths and spacings rather than a single uniform pitch. In the South Dakota School of Mines and Technology design convention, adjacent full dipole lengths satisfy L_n+1 = τL_n, and their separation is d_n = 2σL_n. Choose τ and σ for the required directivity and bandwidth, then check the active-region allowance and actual element sequence. [7]

For an illustrative geometry with τ = 0.85, σ = 0.15 and a longest dipole of 749.48 mm, the first separation is 224.84 mm. If nine elements are used, the ninth length is 749.48 × 0.85⁸ = 204.23 mm, and the eight gaps total about 1090.51 mm. This demonstrates the geometric progression; it does not by itself certify operating bandwidth or gain. Do not force the shortest element to a half-wavelength band-edge value while retaining an incompatible element count.

MIMO and diversity antennas

Two antennas can be beamformed, so the port count does not remove the steering question. For a system used only for MIMO or diversity, judge spacing through efficiency, matching, coupling and radiation-pattern correlation in the intended propagation environment. Half-wavelength separation does not guarantee uncorrelated channels. The compact modal-feed example shows one way to address close spacing, with performance tied to the particular array and feed network. [5]

Validate the candidate pitch before releasing the drawing

• Define the operating envelope. Record the full frequency band, polarization, scan region, available aperture and required gain, sidelobes and matching.
• Screen the geometry. Use the shortest wavelength and actual scan directions. Leave margin below the grating-lobe boundary and include the maximum manufactured pitch.
• Model the installed array. Include the feed, ground plane, neighboring elements and radome. Evaluate active matching and embedded patterns across the required excitations.
• Check tolerances and losses. Include placement and feed-phase errors, amplitude imbalance, temperature-dependent dimensions and the intended power conditions. Set acceptance limits from the system specification.
• Compare measurements with predictions. Measure the multiport response and radiation patterns across the band and scan region, including off-principal-plane directions for planar arrays. Record the configuration so the result can be reproduced.

The practical choice is a pitch that meets the scan requirement with margin, fits the hardware and delivers acceptable matching and patterns in the complete array. Start near λ/2 at the highest frequency, but let the verified array performance determine the final dimension.

References

1. Roger H. Varney, SRI International. Phased Arrays 3: Steering Limits. July 2020. Lecture material hosted by MIT; slides on grating lobes, coupling, cost and two-dimensional steering.
2. NIST. CODATA Value: Speed of light in vacuum. 2022 CODATA values; c is exact. Accessed October 24, 2026.
3. A Study Program on Large Aperture Electronic Scanning Phased Array Antennas for the Shuttle Imaging Microwave System. June 1974. NASA NTRS 19740026528; grating-lobe analysis and waveguide-array design.
4. Evaluation of a Low Sidelobe Phased Array Antenna. November 1969. NASA NTRS 19700002178; 50-element array and spacing selection.
5. Jacob C. Coetzee and Yantao Yu. New Modal Feed Network for a Compact Monopole Array with Isolated Ports. IEEE Transactions on Antennas and Propagation, 56(12), 3872–3875, 2008. DOI: 10.1109/TAP.2008.2007399. Author manuscript hosted by Queensland University of Technology.
6. Low Average Sidelobe Slot Array Antennas for Radiometer Applications. NASA Jet Propulsion Laboratory, NASA Tech Briefs, November 2012, pp. 23–24.
7. South Dakota School of Mines and Technology. Log-Periodic Dipole Array design notes. Undated; Carrel-based design procedure, geometry and recursive element sizing.