This chapter is focused on circularly polarized antennas. Key definitions and governing equations of circular polarization are given. Infinitesimal dipole sources are considered to establish circularly polarized radiation. First, radiation patterns of cross dipoles are mathematically reviewed, from which the condition of circularly polarized waves is concluded. Later, the idea is extended to four displaced sequentially rotated dipole antennas, resulting in circularly polarized waves within a wide angular range in space. The extension of the concept to the magnetic source counterparts and Huygens sources is briefly discussed. Other than point sources, also known as one-dimensional current sources, sources of circularly polarized radiation are further investigated for two-dimensional cases, such as microstrip patch antennas, and threedimensional structures, such as volumetric current sources existing in dielectric resonator antennas. For these cases, the creation of circularly polarized radiation using single-feed and dual-feed, perturbed structures and sequentially rotated method is described. As a design example, numerical andmeasurement results of circularly polarized square patch ring antennas are extensively discussed and presented in this chapter. The square-ring microstrip antenna is selected as it closely approximates the sequentially rotated currents, and also it has not been widely studied in the literature.

Circularly Polarized Antennas

소형 안테나 ( Small Antennas )

A novel dual-band printed diversity antenna is proposed and studied. The antenna, which consists of two back-to- back monopoles with symmetric configuration, is printed on a printed circuit board. The effects of some important parameters of the proposed antenna are deeply studied and the design methodology is given. A prototype of the proposed antenna operating at UMTS (1920-2170 MHz) and 2.4-GHz WLAN (2400-2484 MHz) bands is provided to demonstrate the usability of the methodology in dual-band diversity antenna for mobile terminals. In the above two bands, the isolations of the prototype are larger than 13 dB and 16 dB, respectively. The measured radiation patterns of the two monopoles in general cover complementary space regions. The diversity performance is also evaluated by calculating the envelope correlation coefficient, the mean effective gains of the antenna elements and the diversity gain. It is proved that the proposed antenna can provide spatial and pattern diversity to combat multipath fading.

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A Novel Dual-Band Printed Diversity Antenna for Mobile Terminals

The theoretical limit for small antenna performance that was derived decades ago by Wheeler and Chu governs design tradeoffs for size, bandwidth, and efficiency. Theoretical guidelines have also been derived for other details of small antenna design such as permittivity, aspect ratio, and even the nature of the internal structure of the antenna. In this paper, we extract and analyze experimental performance data from a large body of published designs to establish several facts that have not previously been demonstrated: (1) The theoretical performance limit for size, bandwidth, and efficiency are validated by all available experimental evidence. (2) Although derived for electrically small antennas, the same theoretical limit is also generally a good design rule for antennas that are not electrically small. (3) The theoretical predictions for the performance due to design factors such as permittivity, aspect ratio, and the internal structure of the antenna are also supported by the experimental evidence. The designs that have the highest performance are those that involve the lowest permittivity, have an aspect ratio close to unity, and for which the fields fill the minimum size enclosing sphere with the greatest uniformity. This work thus validates the established theoretical design guidelines.

Experimental Validation of Performance Limits and Design Guidelines for Small Antennas

A compact microstrip line-fed ultrawideband (UWB) tapered-shape slot antenna is presented. The proposed antenna comprises a tapered-shape slot and rectangular tuning stub. The antenna is fabricated onto an inexpensive FR4 substrate with an overall dimension of 22 × 24 mm2. The experiment shows that the proposed antenna achieves good impedance matching constant gain, stable radiation patterns over an operating bandwidth of 3-11.2 GHz (115.5%) that covers the entire UWB. The nearly stable radiation pattern with a maximum gain of 5.4 dBi makes the proposed antenna suitable for being used in UWB communication applications.

Compact Tapered-Shape Slot Antenna for UWB Applications

Reduced size microstrip monopole slot antennas with different slot shapes-straight, L and inverted T, and placed on a small ground plane, are investigated. The ground plane size is 50 mm/spl times/80 mm, which is about the size of a typical PC Wireless card. Detailed simulation and experimental investigations are conducted to understand their behavior and optimize for broadband operation. It is shown that, the variation in the slot shape, from straight to L and T shapes, helps in generating additional resonances, which when coupled to the original resonances of the slot, further increases impedance bandwidths. The bent shapes of the L and T slots reduce their height and provide more space on the ground plane for electronics. A mirror image dual L-slot antenna, placed at two adjacent corners of the ground plane, is also investigated and optimized for the polarization diversity. They provide an impedance bandwidth of 87%, with near orthogonal radiation characteristics. The measured impedance bandwidths (S/sub 11/=-10 dB) of up to 60%, 84%, and 80% are achieved for these straight, L and inverted T slots respectively, by suitably selecting their design parameters. The simulation results are in good agreement with the experimental data considering several practical issues.

Bandwidth enhancement and size reduction of microstrip slot antennas

When microstrip antennas are miniaturised, because of high ohmic losses, their gain and efficiency drop significantly. In this study, a new technique is presented to improve these two important parameters of such antennas, where multiple laminated conductors are used to form the conducting patch. The technique essentially reduces the ohmic losses, and consequently improves the gain and efficiency. At first, a planar conductor is studied to show that using multiple laminated conductors, instead of a single conductor of the same thickness, losses can be reduced. The same technique is then used in several miniaturised microstrip antennas, in order to reduce their ohmic losses. It is shown that this process enhances the gain and efficiency of these antennas progressively, by increasing the number of layers in the lamination. Simulation studies on two miniaturised antennas have provided about 1.5 and 2.4 dB improvement in the gain, and from 30 to 40.7 improvement in the efficiency. Experimental investigations are also conducted and confirmed the simulated results. For a microstrip square ring antenna, a measured gain enhancement of 2.16 dB at 2.2 GHz is obtained, by laminating only the ring with four conducting layers.

Gain and efficiency enhancement of compact and miniaturised microstrip antennas using multi-layered laminated conductors

A printed elliptical monopole antenna operating at 1–4 GHz frequency for biomedical imaging applications is presented in this article. To reduce the reflection of the microwave signal from breast, glycerin is used as the coupling medium (er = 9, σ = 0.4 S/m, tan δ = 0.28) whose electrical properties are close to those of breast tissue. Directional patterns and reduced back radiations are achieved using a reflector. The antenna has wide impedance bandwidth from 1 GHz to over 6 GHz (over 140%), suitable for radar-based imaging. The antenna demonstrates S21 > −60 dB from 1 to 4 GHz in the coupling medium. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:808–813, 2014

A planar ultrawideband elliptical monopole antenna with reflector for breast microwave imaging

A unique hybrid perturbation scheme is proposed for circular polarisation of stacked square-ring microstrip antennas, where both positive and negative perturbations are used. It is shown that the method broadens the angular range of circular polarisation. Measured results confirm the simulations.

Saeed I. Latif

Papers

Circularly Polarized Antennas

Bandwidth enhancement and size reduction of microstrip slot antennas

Gain and efficiency enhancement of compact and miniaturised microstrip antennas using multi-layered laminated conductors

A planar ultrawideband elliptical monopole antenna with reflector for breast microwave imaging

Hybrid perturbation scheme for wide angle circular polarisation of stacked square-ring microstrip antennas