Compact Microstrip-Fed Square Loop Antenna for DTV Applications

Abstract

A design method for a compact square loop antenna fed by a microstrip (MS) line for indoor digital television (DTV) applications is proposed. The proposed antenna consists of a square loop, circular sectors, and an MS line. The square loop combined with circular sectors is printed on one side of a substrate, and a 75-Ω MS line is printed on the other side. The circular sectors are used as a wideband balun or transition to connect the MS line and the square loop. A prototype of the proposed square loop antenna operating in the DTV band (470-806 MHz) is designed and fabricated on an FR4 substrate. Experimental results show that the proposed antenna has the desired impedance characteristics in the frequency band of 464-1,220 MHz (89.8%) for a voltage standing wave ratio (VSWR) of <2 covering the DTV band, and a broadside gain of 0.8-3.3 dBi in the DTV band. developed [7]. For the planar tapered-loop antenna, a PCBtype balun was used for transforming the high input impedance of the loop, making antenna fabrication more complicated. Therefore, a loop antenna with a simple integrated balun or transition needs to be developed.In this paper, a design method for a microstrip (MS)-fed compact square loop antenna with circular sectors for indoor DTV applications is presented. The circular sectors are used as a wideband balun or transition to connect a 75-Ω MS line and the square loop [8]. Note that a 75-Ω line is used instead of 50 Ω as the reference impedance for the feed, because a 75-Ω coaxial cable is used for TV broadcast applications. The antenna size is reduced using a square loop instead of a circular one and the slits introduced at the four corners of the loop. A prototype of the proposed antenna was fabricated on an FR4 substrate, and its measured performance was compared with the simulation results. The results in this work were obtained using a CST Microwave Studio and were validated by a measurement of the input voltage standing wave ratio (VSWR), gain, and the radiation patterns tested in an anechoic chamber. II. ANTENNA GEOMETRY AND DESIGNThe geometry of the proposed compact square loop antenna fed by an MS line is presented in Fig. 1. It consists of a square loop with circular sectors and an MS feed line. A square loop combined with circular sectors is printed on one side of a substrate, and an MS line is printed on the other side. The end of the MS line is connected to the upper circular sector by using a via. Since the width of the 75-Ω MS line is very narrow and is similar to the diameter of the via, a circular patch with a diameter much larger than the MS line width is printed on the end of the MS line in order to connect the MS line and the via securely. The two circular sectors are used as a wideband transition to connect the 75-Ω MS line and the square loop.To reduce the antenna size, the square loop is used instead of a circular loop, and four edge slits are appended at the corners where the square loop and the circular sectors come into contact with each other in order to further reduce th...