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Printed slot antennas are currently under consideration, especially for use in ultra-wideband (UWB) applications, due to their attractive merits, such as lightweight, ease of fabrication and wide frequency bandwidth.¹ In addition, these antennas are completely planar and are easily integrated with active devices or monolithic microwave integrated circuits (MMIC).² On the other hand, these antennas require a large area for the slot and a much larger area for the conductor plane around the slot.³ Moreover, it can be observed that most broadband designs of a slot antenna are possible, but not with a small size. Therefore, reducing the antenna size, decreasing the cost of manufacturing and also increasing the bandwidth are important goals.⁴ Recently, there has also been growing research activity on many other microstrip-line-fed printed slot antennas, especially printed wide-slot antennas^5,6 because of their favorable impedance characteristics. In these designs, the characteristics of printed wide-slot antennas fed by a microstrip line with various tuning stubs have also been studied. A novel printed wide-slot antenna fed by a microstrip line with a fork-like tuning stub has been presented,⁶ which is good for bandwidth enhancement but not enough for the UWB applications.

In another design of a wide-slot antenna with a fork-like stub and a square-ring slot,⁷ a bandwidth of 114 percent, from 3 to 11 GHz, for a VSWR less than 2.2, has been achieved with dimensions of 100 x 120 mm. A good design of a wide-slot antenna with an octagonal-shaped slot has recently been reported,⁸ which has the compact dimensions of 29 x 30 mm and a bandwidth from 2.9 to 12 GHz, for a return loss lower than 10 dB. Two small notches in the bottom edge of the slot are used, but only for fine tuning the impedance matching characteristics and not for enhancing the total bandwidth.

In this article, a novel design of a printed symmetric wide-slot antenna with small size and good characteristics is proposed and investigated in detail. By using the etched modified octagonal wide slot in the ground plane, three notches in the bottom edge of the slot below the microstrip-fed line and a fork-like tuning stub, an impedance bandwidth from 2.5 to 12.8 GHz with good matching and relatively constant gain can be achieved. In this present deign, small notches are used in order to generate the additional resonant frequencies and also tune the matching simultaneously. Numerical and experimental results for the frequency characteristics, surface currents, radiation patterns, and gain of this proposed slot antenna are also presented and discussed.