Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Low-profile ultrawideband (UWB) phased-array antennas are in great demand for a wide range of commercial and military applications, including in communications, reconnaissance, electronic warfare (EW), and radar systems. Traditional antenna elements for UWB phased-array antennas employ tapered slot, Vivaldi, and -rabbit-ear-shaped structures. Although these elements can achieve wide bandwidths, they are not planar for conformal installations and are also too thick for some low-frequency applications.1-3 Additionally, due to the mutual coupling that occurs between antenna elements in an array layout, the broadband performance can be significantly degraded.

Phased-array antennas based on tightly coupled arrays (TCAs) offer solutions for these diverse applications. A TCA with resistive frequency-selective surface (RFSS) can achieve a wide impedance bandwidth by inserting a square-split-ring RFSS between the array and the ground plane. This suppresses the ground-plane reflections and avoids any short-circuited frequency bands. Simulations have shown that a wide impedance bandwidth of 12.8:1 (with VSWR of less than 2.0:1) can be achieved using a TCA with RFSS. Without the RFSS, the array exhibits different characteristics across two bands: The scan impedance bandwidth is 8.9:1 (1.8 through 16 GHz) within a ±30° scan angle, and the antenna array’s gain is reduced by about 0.5 to 2.8 dB due to the loss of RFSS.

A great deal of interest now surrounds TCAs based on the current sheet concept for wideband applications. Instead of trying to suppress mutual coupling, this approach exploits mutual coupling to counteract ground-plane inductance and maintain a stable, mostly real input impedance over a wide bandwidth. Consequently, a TCA can provide a low profile and UWB operation even if placed over a ground plane.4-7 For example, a wideband phased-array antenna with tight coupled octagonal ring elements was presented by Chen,8 and a 4.4:1 bandwidth with VSWR of less than 2.0:1 was realized. Munk9 used tightly coupled dipole elements to develop a phased-array structure with wide bandwidth extended to 9:1 with VSWR of less than 3.0:1 by loading multilayer substrates.

Nevertheless, the TCA approach suffers from ground-plane shorting which can severely limit the bandwidth when placed one-half wavelength (λ/2) from the ground plane. To overcome ground-plane shorting, a novel UWB TCA was designed by inserting a square-split-ring RFSS between the array and the ground plane. To better understand this design approach, the effects of sheet resistivity and mutual coupling on impedance bandwidth were analyzed, and the scan performance will be presented.

For this study, a TCA was designed based on the current sheet concept.10 This design can be described by a tightly coupled dipole array, using the structure and equivalent-circuit model shown in Fig. 1(a). The array is arranged with period, d, and is located a distance, h, from the ground plane. Unlike traditional array elements, placed apart from each other to minimize mutual coupling, the TCA elements are placed within close proximity of each other, with a small gap to increase mutual coupling.

UWB Arrays Employ TCAs, Fig. 1

If the end-coupled capacitance is represented by C and the self-inductance of the dipole is denoted by L, the ground plane is represented by a short-circuited transmission line. The impedance looks toward the ground plane equal to ZGP, or

ZGP = jη0tan(2πh/λ)

and the input impedance of the antenna element, Zin, is

Zin = jωL + 1/(jωC) + η0 || ZGP

where η0 = 120πΩ is the characteristic impedance of free space. Due to the strong coupled capacitance can counteract ground-plane inductance at the low frequency, the phase of input impedance has a nearly flat response around 0 deg. over a wide bandwidth. Eventually, TCA can realize wideband operation and low profile. As can be seen from Fig. 1(b), a TCA can realize a 4.5:1 (2.1 to 9.5 GHz) bandwidth without substrate loading.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.