44 RF MEMS Circuits -conocimientos.com.ve: the True Time Delay Phased Antenna Array

Beam Steering

The fundamental principles underlying the concept of electronic beam steering are derived from electromagnetic radiation theory employing constructive and destructive interference. These principles can be stated as follows: The electromagnetic energy received at a point in space from two or more closely spaced radiating elements is a maximum when the energy from each radiating element arrives at the point in phase. Controlling the phase through the many segments of the antenna system allows the beam to be rapidly directed in different directions. Figure 1 shows a 4-element linear array of antennas with constant phase difference between neighboring phase shifters, where θ0 is the scan angle, w1 to w4 are amplitude weights, and d is the spacing between adjacent antenna elements.

The angle θ0 of the beam with respect to the antenna axis is determined by the operating wavelength of the microwave signal, the spacing between the antenna elements that is usually half of a wavelength, and the phase shift between the signals in the individual elements. It is given by

(1)

where l is wavelength, d is the inter-element array spacing, nd is the location of the particular radiating element n being investigated, and f is the phase shift located at the nth element. It is seen that changing the frequency results in a change in steering angle q0, if the phase shift f is fixed. Therefore, the beam squinting arises from this distortion.

Instead, the group time delay t only depends on beam position angle and array length, but not frequency, which is shown in Eq. (2).

Beam shaping

Although steering capability is the most common function, a phased antenna array can also provide beam-shaping capability by appropriate arrangement of the feed signals. A radar system with variable beamwidth can produce a wide beam for the acquisition of targets and a narrow beam for subsequent high-precision tracking. Additionally, a broadcast satellite antenna with variable beamwidth can achieve efficient coverage of irregularly shaped geographical service areas based on the environment or traffic conditions, which is an important feature for communication systems.

The major objectives of beam shaping are to minimize pattern ripples, to reduce sidelobe levels, change null positions, or control output power levels, etc. To vary the antenna beamwidth, there are many methods which may be employed. In this research work, phase-only adjustment is used to vary the beamwidth. Adding a quadratic phase error on the array aperture moves the array's phase center and changes its focusing distance (see Fig. 2). However, for a given size of an array aperture, there is a limit to the movement of the phase center. Beyond a certain value of phase taper across the array aperture, beam bifurcation on the plane of the linear array occurs. In addition, the sidelobe level of an array with equal power feeding increases rapidly as the value of phase taper increases. In order to increase the useful range and control the sidelobe level, a Taylor N-bar amplitude taper is introduced across the array aperture in this design.

True Time Delay Technologies

The bandwidth of a phased antenna array is affected by many factors, including change of element input impedance with frequency, change of array spacing in wavelengths that may allow grating lobes, change in element beamwidth, etc. When an array is scanned with fixed values of phase shift, provided by phase shifters, there is also a bandwidth limitation as the position of the main beam will change with frequency, which is called beam squint. In Eq.1, It is obvious that changing the frequency results in a change of the scan angle for fixed element spacing. In contrast, when the array is scanned with true time delay, the beam position is independent of frequency to first order.Three representative true time delay (TTD) technologies are being investigated in our group. They are:

· Piezoelectric-Bender Controlled Delay Line;

· RF-MEMS Extended Tuning Range Varactor Delay Line;

· Liquid Crystal Phase Shifter