Antenna Fundamentals and Integration Advanced Antenna Topics Informational

How do I design a wideband antenna element for a phased array covering more than 3:1 bandwidth?

Q: Why is a wideband element harder to design in an array than in isolation?

In isolation, the element has a fixed impedance that depends only on its own geometry. In an array, mutual coupling between elements modifies each element's impedance (the 'scan impedance' or 'active impedance' depends on the array lattice, element spacing, and the scan angle). At different scan angles, the mutual coupling changes, causing the impedance to vary. The element must be designed so that its scan impedance remains within acceptable VSWR limits across both the frequency band AND the scan range, which is a much more demanding requirement.

Q: What ground plane spacing is optimal for wideband arrays?

The ground plane (reflector) is placed approximately lambda/4 below the elements at mid-band frequency. At the lowest frequency, this spacing is approximately lambda/8 (ground plane too close, causing high input reactance). At the highest frequency, it is approximately lambda/2 (causing a null in the broadside pattern). A resistive frequency-selective surface (FSS) or magnetic absorber behind the elements can broaden the usable bandwidth by damping the ground plane reflections.

Q: Can I scan a wideband phased array?

Yes, wideband phased arrays support beam scanning to typically +/- 45 to +/- 60 degrees from broadside. The scan range is limited by: grating lobes (at high frequencies, if the element spacing exceeds lambda/2, grating lobes appear at large scan angles), scan blindness (at specific scan angles and frequencies, a surface wave or Floquet mode resonance causes the scan impedance to become very large, creating a blind spot), and the element pattern rolloff (individual elements have a cos-theta pattern that reduces gain at large scan angles).

Designing a wideband antenna element for a phased array covering more than 3:1 bandwidth requires using antenna topologies that maintain a stable impedance and radiation pattern over a very wide frequency range while being compatible with the array lattice spacing constraints (the element spacing must be less than approximately 0.5 lambda at the highest frequency to avoid grating lobes, which means the elements are very closely spaced relative to the wavelength at the lowest frequency). Successful wideband phased array element types include: Vivaldi (tapered slot) antennas (exponentially tapered slot in a metalized substrate; bandwidth typically 3:1 to 10:1; dual-polarized versions use egg-crate construction; excellent for 0.5-18 GHz arrays), connected dipole arrays (dipole elements with inter-element capacitive or direct connections that create a continuous current sheet at low frequencies; bandwidth 4:1 to 6:1; based on the Wheeler current sheet concept), tightly coupled dipole arrays (TCDA, also called current sheet antennas; very thin dipoles with strong mutual coupling and a wideband balun feed; bandwidth of 3:1 to 5:1 in a low-profile structure), and bunny-ear or flared notch elements (similar to Vivaldi but with a different flare profile; bandwidth 3:1 to 6:1). The key challenge is the array environment: the antenna element's impedance changes dramatically from isolated to in-array (scan impedance varies with scan angle and frequency), and the element must be designed for the array environment, not in isolation.

Category: Antenna Fundamentals and Integration

Updated: April 2026

Product Tie-In: Antennas, Arrays, Feeds

Wideband Phased Array Element Design

Ultra-wideband (UWB) phased arrays are required for electronic warfare (high probability of intercept receivers covering 0.5-18 GHz), multi-function radar (radar, communications, and EW on a single aperture), and next-generation communication systems (cognitive radio, software-defined arrays).

Design Approaches

Vivaldi antenna: A tapered slot etched in a metallic sheet or PCB. The exponential taper provides a smooth impedance transition from the narrow slot (transmission line) to the wide opening (radiator). Dual-polarized arrays use orthogonal Vivaldi elements in an egg-crate arrangement. Bandwidth: 3:1 to 10:1 with VSWR < 2:1
Tightly coupled dipole array (TCDA): Thin printed dipoles on a PCB with strong capacitive coupling between adjacent elements. The coupling creates a continuous current sheet at low frequencies (Wheeler limit), providing stable impedance from DC to the first grating lobe frequency. A wideband balun feed is critical. Profile height: approximately lambda/4 at mid-band over a ground plane
Connected arrays: Elements are physically connected to their neighbors, forming a continuously connected sheet. Provides the widest bandwidth (theoretically infinite for a resistively loaded sheet) but is more complex to feed and fabricate

Wideband Array Element Parameters

Array element spacing: d < lambda_min/2 (highest frequency constraint)
At lowest frequency: d ~ lambda_max/6 to lambda_max/10 (very small electrically)
For 2-18 GHz array: d < 8.3 mm (lambda/2 at 18 GHz)
Scan impedance: Z_scan(theta) varies with scan angle theta
VSWR target: < 2.5:1 over full bandwidth and scan range

Common Questions

Frequently Asked Questions

Why is a wideband element harder to design in an array than in isolation?

In isolation, the element has a fixed impedance that depends only on its own geometry. In an array, mutual coupling between elements modifies each element's impedance (the 'scan impedance' or 'active impedance' depends on the array lattice, element spacing, and the scan angle). At different scan angles, the mutual coupling changes, causing the impedance to vary. The element must be designed so that its scan impedance remains within acceptable VSWR limits across both the frequency band AND the scan range, which is a much more demanding requirement.

What ground plane spacing is optimal for wideband arrays?

The ground plane (reflector) is placed approximately lambda/4 below the elements at mid-band frequency. At the lowest frequency, this spacing is approximately lambda/8 (ground plane too close, causing high input reactance). At the highest frequency, it is approximately lambda/2 (causing a null in the broadside pattern). A resistive frequency-selective surface (FSS) or magnetic absorber behind the elements can broaden the usable bandwidth by damping the ground plane reflections.

Can I scan a wideband phased array?

Yes, wideband phased arrays support beam scanning to typically +/- 45 to +/- 60 degrees from broadside. The scan range is limited by: grating lobes (at high frequencies, if the element spacing exceeds lambda/2, grating lobes appear at large scan angles), scan blindness (at specific scan angles and frequencies, a surface wave or Floquet mode resonance causes the scan impedance to become very large, creating a blind spot), and the element pattern rolloff (individual elements have a cos-theta pattern that reduces gain at large scan angles).

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