Compact low-profile antennas for mobile communications

Abstract

In recent years, countries throughout the world have agreed to allocate the frequency band 1.7 → 2.1 for future personal communications systems (PCS). Handsets are expected to decrease in size and cost as the circuits and antenna components scale down. The size of the handset is primarily limited by the antenna length and the battery. The ability of multiple antennas on the handset to improve data transfer reliability increases the need for compact antennas. Although the PCS system will reduce the antenna length by a factor of 2 from the current 9OOMHz cellular system, the handset will also scale down and the need for small antennas remains. In the past, cellular handsets employed a (shielded) monopole for transmitting and receiving data. Recently, efforts have been focused on several new designs, including the planar inverted F antenna (PIFA) with a resonant wavelength of λ/4. There exist a variety of techniques to reduce the physical length of the antenna. Dielectrics can increase the electrical length of the antenna by [square root]εr, therefore decreasing the physical length by [square root]εr. Increasing the dielectric constant to reduce the antenna's physical length, however, is cost-prohibitive. Another method involves the use of strategically placed inductive and capacitive loads on the antenna structure. An inductive load is difficult to manufacture as the resonance properties are sensitive to the pitch and spacings between the coils. A capacitive load increases the resonant resistance and reactance, decreasing the probability of achieving a 50ω match. In this thesis, we design a new antenna with length λ/8 for PCS applications. The Finite Difference Time Domain method is used in the design process and results from a prototype are used to verify the findings