Design of a Segmented Half-Loop Antenna Payam Nayeri 1,2 , Darko Kajfez 2 , and Atef Z. Elsherbeni 1 1. Electrical Engineering and Computer Science Department Colorado School of Mines Golden, CO 80401, USA 2. Department of Electrical Engineering The University of Mississippi University, MS 38677, USA payam_nayeri@ieee.org, eedarko@olemiss.edu, aelsherb@mines.edu Abstract—A half-loop antenna is considered, placed perpendicularly to an infinite ground plane. By partitioning antenna’s conductor into short segments interconnected with lumped capacitances and/or inductances it is possible to produce an omnidirectional radiation pattern in a vertical plane and at the same time match the input impedance(s). The described segmentation-design procedure is convenient for the printed- circuit antennas and it can also be applied to other types of printed-circuit antennas. I. INTRODUCTION As sketched in Fig. 1, the half-loop antenna (HLA) is a conducting loop located in the x-z plane. If the circumference of the loop is very small in comparison with the operating wavelength, the resulting radiation pattern is omnidirectional in the x-z plane like being produced by an ideal magnetic moment. Such a pattern may be desirable for communication with non-stationary satellites. An example of such application is the geo-positioning system (GPS), where the signal level should not depend on the elevation angle. Fig. 1. Half-loop transmitting antenna above the ground plane. As long as the conductor length of the HLA is shorter than one-quarter wavelength, the input impedance has a negligible real part in comparison with the imaginary part so that an efficient impedance matching is impossible to achieve. At higher frequencies, the imaginary part of the impedance becomes smaller and the real part larger, hence the matching can be achieved. The problem is, at those frequencies the radiation pattern is no longer omnidirectional, and may exhibit nulls. This is mainly caused by the fact that the phase of the current is not uniform. The goal of the present investigation is to modify the HLA in order to achieve an omnidirectional radiation pattern in the x-z plane and also to assure an effective match to the input impedance of the order of 50 Ω. It will be shown that such a goal can be achieved with the so-called segmentation, a procedure that can easily be realized with the printed-circuit antennas. II. SEGMENTATION DESIGN PRINCIPLE It has been shown recently that an open loop antenna of circumference about one wavelength can be designed to produce an omnidirectional pattern with the use of segmentation [1-2]. A HLA with its image against a perfectly conducting ground plane in the x-y plane, radiates in the upper half space an identical pattern as a full loop antenna presented in [1]. Therefore, the same segmentation procedure may be applied to HLA. Figure 2 shows a HLA divided into 7 segments. We consider the short gaps between the segments to represent ports, where we can insert lumped circuit elements, typically capacitances or inductances. For an operation as a transmitting antenna, there are two possible ways of excitation: as a one-port or as a two-port. The one port excitation is shown in Fig. 1, where port 1 is short- circuited to ground and port 8 is excited by a voltage source. We will consider here the two-port excitation, where the ports 1 and 8 are simultaneously excited by two equal and opposite voltage sources. Fig. 2. Half-loop antenna partitioned into segments. The preliminary antenna design is started as follows. An electromagnetic (EM) simulation software is used to generate the impedance matrix of the 8-port shown in Fig. 2. A straightforward circuit analysis can solve for the input impedances at ports 1 and 8, given any combination of loading lumped reactances placed at the gaps. From the known current values at the ports it is possible to make reasonable assumptions about the directivites of the HLA in the z and x directions. An objective function is then defined that minimizes the input reflection coefficients and also equalizes the directivities in the x and z directions. The values of the individual loading reactances are then determined by optimization. The optimization is based on the circuit analysis of a constant antenna impedance matrix. Therefore, the 37 978-1-4799-3540-6/14/$31.00 ©2014 IEEE AP-S 2014