CHAPTER 1 INTRODUCTION
1.2 Background of This Research and the Related Literature 1
E
ver since the first invention of the first directional antenna, a lot of techniques have been developed to steer the antenna main beam. Gimbaled type antenna system was developed in the beginning to direct the antenna main beam with fixed pattern to a desired direction. Theadvance of the antenna in a radar system was detailedly recorded in [2]. Those mechanical scanning radar systems are inherently characterized by consuming considerable dynamic response time, and may not be acceptable for some systems, which require specifically fast reaction time.
Electronic beam steering has been developed for a long time by using a phased array antenna through shifting the phase in each array element [3-8]. The electronic beam-steering system possesses a fast switching rate; however, the mechanical type has a consistent radiation beam pattern during the beam scanning [9-10]. Electronic beam steering needs, generally, expensive ferrite or solid-state phase shifters or a complex beam-forming network.
Therefore, such a system is not cost-effective and is mainly applied in military or space-borne systems. Nowadays, because of the demands for beam-steering antenna system in wireless communication system and ITS (Intelligent Transportation System) applications, a low-cost and robust beam-steering system attracts considerable attentions.
Various novel beam-steering techniques have been proposed in the past. To mention a few, an actuator perturbing the propagation constant of a microstrip line was used as a phase shifter [11-13]. A moveable grating was used to perform a beam-steering leaky wave antenna [14]. They also used the moveable metal plate to perturb the dielectric image line and then to change the phase constant of the transmission line [15-17]. The grating with PIN diodes or MEMS-based switch was utilized to switch the direction of radiation main beam between two angles [18-20]. A re-configurable photonic band-gap structure was employed to serve as a phase shifter in a phased array system [21]. A slotted-microstrip line perturbed by metal strips was developed to act as a multi-functional device including the phase shifter and feeding network [22]. Some researchers used the photo-induced semiconductor plasma layer as a re-configurable grating antenna to dynamically change the phase constant and then scan the beam pattern [23].
Waveguide-based leaky-wave antennas have been developed for many years. For example, the slits on the metal waveguide sidewall were implemented theoretically and experimentally [24-31]. The slitted leaky waveguide array for mobile reception of DBS (direct broadcast from satellite) were successfully designed and implemented by Ando [32-35]. This paper theoretically and experimentally demonstrates the feasibility of a novel low-cost beam-steering antenna based on a leaky-wave structure. The structure under consideration is a rectangular waveguide cut with a slit in the center of the waveguide narrow wall. The theoretical formulation was well developed by using model transmission line method [28-30].
To add the beam-steering capability to the leaky-wave antenna, we added a dielectric slab longitudinally inside the slitted waveguide. Moving the dielectric slab along the transverse direction, the electric and magnetic fields in the waveguide are perturbed. Consequently, the propagation constant, including the phase and attenuation constants of the leaky mode, varies accordingly. Furthermore, the leaky-wave beam patterns could be changed.
In the theoretical analysis, we model the structure as a slitted waveguide filled with inhomogeneous medium. The transverse resonance equation was employed to calculate the dispersion relation of the waveguide. The finite integration method (FIT), CST Microwave Studio, was also used to compute the radiation pattern. Besides, we fabricated an antenna and measured its far-field radiation pattern under various perturbation conditions (displacement of the dielectric slab away from the waveguide center). Three samples of dielectric slabs were used to examine the beam-steering characteristics. Good agreement between theoretical and experimental results verifies the present beam steering mechanism.
CHAPTER 2. STATEMENT OF PROBLEM AND METHODS OF ANALYSIS
2.1 Basic Theory of the Leaky-Wave Antenna
R
adiation from a continuous uniform slitted waveguide has been known as a traveling wave with a complex propagation constant(
kz =βz− jαz)
. This traveling wave propagates along the waveguide with its phase velocity greater than that of light, and its amplitude being continuously attenuated as it travels. Thus, the waves feature having continuous leakage of energy and is known as leaky waves. In figure1, a leaky wave above a partially open waveguide together with its coordinate system is given here to demonstrate the leaky wave phenomenon [36-37]. Let the wave-number in the x, y, and z directions be , and , respectively. And assuming the system is y-axis independent, that is is zero, such thatkx ky kz
The imaginary part of the above term must vanish, so
z In the region x>0, the leaky structure radiates with wave amplitude variation given as,
(4)
This corresponds to an exponentially increasing of wave amplitude in the x direction.
Though it may seem unphysical, it does represent the field solution in the restricted region of an open waveguide. Again in figure 1, the semi-infinite uniform leaky waveguide is fed by a closed waveguide. The radiation slot of the leaky structure lies in the x=0 plane. It starts radiation at z=0 and the field may extend in the positive z direction. That is, the leaky wave
ever increases its amplitude as it propagates in the x direction. Despite the improper transverse characteristics, leaky wave can validly represent the radiation field within a sector of space, that is, the portion of space lined in figure 1. And they can never reach infinity unless the source is placed infinitely far away, which means the feeding waveguide is infinitely far away (this is the case not physically realizable). To show the confined region where the leaky wave validates the representation of the radiation fields, let θ be the angle of radiation departed from broadside, and φ =π −θ
2 be the angle departed from end-fire of the leaky wave antenna, given as ,
λg
the free space wave-number and wavelength, respectively. Since the amplitude of the leaky wave decays as the wave propagates along the z direction, the amount of radiation per unit length decreases with increasing z. Note that at any point z>0, the field intensity increases along the x direction up to x= z⋅tanφ. In this region, the field amplitude depends on x as given in equation 4. For x> z⋅tanφ the field intensity must decrease drastically in this region. So, confined in the region x< z⋅tanφ, the leaky wave has finite amplitude and is a valid representation of the radiation field.