The state of development of phased antenna arrays (PAAs) technology has undergone considerable advancement over the last four decades along with the development of radar and communication technologies . Modern communication systems all demand faster access, more information, and broadband operation. Satisfying these requirements will demand that the phased antenna array system deliver high data rate in the transmitters .
Radar has been the chief application of PAAs; however, small arrays also find application in communication systems . For example, multiple antennas have been used in terrestrial Line-Of-Sight (LOS) microwave systems to reduce downtime due to fading . Cellular phone service operates with high multipath that produces a Rayleigh probability distribution . The LOS system has slowly varying fades due to changing atmospheric conditions, while cellular phone service has rapid fades as the user moves . However, diversity combining technology improves connectivity in the same manner as the LOS system by providing an alternative path when the signal fades in the first path and reduces signal null depths .
Along with the rapid growth of mobile phone users, their demand for service has nearly saturated existing cellular phone systems and the channel capacity has become a serious problem. By adding PAAs to base stations, they can provide multiple beams to subdivide the cells, thereby allowing improvements without building new sites .
A phased antenna array system typically consists of a large number of radiating elements, which have been arranged in a rectangular or triangular tessellation, with each radiating element including a phase shifter or a time delay device and an antenna element [1-4]. In wave theory, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions [1-4]. The electromagnetic energy received at the in-phase point from two or more closely spaced radiating elements is a maximum. Beams are steered by sequentially shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction as shown schematically in Fig. 1 . The operation bandwidth of the PAA system is limited by the bandwidth of all the radiating elements, especially the phase shifters or delay devices. By using phase shifters, the peak of the main beam is scanned to the desired angle only at the center frequency fo. This results in beam pattern “squint” in which the main beam peak angle is reduced for f > fo and increased for f < fo. Hence, traditional PAAs have typically been built only for relatively narrow band applications . In contrast, true time delay technology is a novel solution for broadband PAAs since the delay time is frequency independent. Representative true time delay methods have been recently developed including digital true time delay technology, dielectric true time delay technology, and variable capacitance nonlinear delay line technology . Among all of these true time delay technologies, RF MEMS varactors employed in the variable capacitance nonlinear delay line technology have a number of advantages, including low loss, low cost, high power handling capability, and capability for analog control, which make this method extremely attractive. Because of these advantages, the focus of this proposed dissertation work is on the RF MEMS varactor based true time delay line technology.
Fig. 1 Schematic representation of a phased antenna array beam steering system 
In addition to their beam steering function from the applications mentioned above, phased antenna arrays can be employed in plasma diagnostic systems for electrical beam shaping application . Beam shaping functions, like focusing/defocusing function are generated by proper arrangement of the delay devices to generate quadratic wave fronts, (concave (focusing) or convex (defocusing) wave fronts), resulting in an electrically controllable lens, as schematically shown in Fig. 2 .
Fig. 2 Schematic representation of a phased antenna array beam shaping system 
A phased antenna array can be used to generate a fixed radiation pattern, or to scan rapidly in azimuth or elevation directions. The phased antenna array system in this work is aimed at the development of the Microwave Imaging Reflectometry (MIR) system  of the plasma diagnostic application, but also finds use in radar applications and space-based communication applications because of their advantages in scanning, reconfigurability, weight, and power.
Fig.3 Phased Antenna Array System
From Fig.3, we can see that we need to design Power Dividers and Antennas as well as the RF MEMS Varactors to form Phased Antenna Array Systems.
A Wilkinson power divider is designed to divide the input signal into two portions. Relatively flat in band response will be the main criterion for the power divider. Figure 45 shows a general schematic of the Wilkinson power divider.
Fig. 4 Schematic of Wilkinson power divider
In the even mode, the excitations on ports 2 and 3 are in phase. Consequently, the voltages across the isolation resistors are zero and they can be ignored. (Fig. 5)
Port 1 Zin, odd ρodd Zin, even ρeven
Fig.5 Wilkinson power divider in even mode
In the odd mode, the excitations on ports 2 and 3 are out of phase. In this case, the resistor values are half of its original value since a virtual ground lies across the axis of symmetry. (Fig.6)
Fig. 6 Wilkinson power divider in odd mode
Based on simulations in ADS with the aid of the spreadsheet on Microwaves101.com , a 6-ring design is necessary to obtain a flat response over the frequency range of 2 – 18 GHz, which has been finished by Jiali Lai in UC Davis Plasma diagnostic group. It is also possible that to make a Wilkinson Power Divider in other designed bands.
Several types of planar antennas are compared in Table 1 
Table 1 comparison of planar antennas
We should choose the end fire type antenna, since we want to fabricate the entire system on a single piece of material for easier handling and lower loss. It would be easier to keep all the components coplanar. When the antenna transmits power out in the z-direction, the whole board can just lay in the z-x or z-y plane, while for a broadside antenna, the whole chip needs to be set in to the x-y plane.
Vivaldi antennas (TSA) proposed by Gibson in 1979, have been widely used in phased and active arrays for radar systems for many years. The Vivaldi is a traveling wave slot antenna having an exponentially tapered profile. They are good candidates for multifunction communication applications because of their stable directional patterns and consistent impedance matching over a very broad operating frequency range without any tuning elements as well as low profile and unobtrusive planar structures. Therefore, a TSA is chosen.
Much of the research has focused on a co-planar antenna structure that exploits a slot line feed. However, Microstrip line is an unbalanced line, while the Vivaldi antenna is fed by a slot line which is balanced. The required balun has to operate over a frequency range of at least two octaves, and up to several octaves . It would be preferable that the balun be frequency independent. A subclass of these antennas, known as Antipodal Tapered Slot Antennas (ATSAs), were first suggested by Gazit in "Improved design of the Vivaldi antenna," IEE Proceedings, vol.135, pt. H, no. 2, Apr 1988 . He realized a tapered transition from microstrip, through parallel strip line, to a symmetric double side slot line and made a double-sided arrangement for the antenna. This arrangement alleviates the difficulties of broadband coupling to slot line.
There are several ways to do the taper, such as linear, exponential, elliptical taper. The schematic of an antipodal elliptically-tapered slot antenna (ATESA) is shown in Fig.7.
Fig.7 Schematic of an antipodal elliptically-tapered slot antenna
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 Hubregt J. Visser, “Array and Phased Array Antenna Basics,” John Wiley & Sons, Ltd., England, 2005.
 Thomas A. Milligan, “Modern Antenna Design,” 2nd edition, John Wiley & Sons, Inc., New Jersey, USA, 2005.
 R. J. Mailloux, “Phased Array Antenna Handbook,” Norwood, MA: Artech House, 1994.
 Chia-Chan Chang, “Microwave and Millimeter Wave Beam Steering/Shaping Phased Antenna Arrays and Related Applications,” Ph.D. dissertation, UC Davis, 2003.
 J. Frank, “Bandwidth Criteria for Phased-Array Antennas,” Phased-Array Antennas, A. Oliner and G. Knittel, eds., Dedham, MA: Artech House, pp. 243-253.
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 Gazit in "Improved design of the Vivaldi antenna," IEE Proceedings, vol.135, pt. H, no. 2, Apr 1988.