A Modified Planar Monopole Antenna for UWB Applications

Yashar Zehforoosh Department of electrical engineering

Islamic Azad University, Urmia Branch Urmia, Iran

y.zehforoosh@iaurmia.ac.ir

Ramazanali Sadegzadeh Department of electrical engineering

Khajenasir Tosi University Tehran, Iran

sadeghzadeh@kntu.ac.ir

Navid Mirmotahhary

Department of electrical engineering Islamic Azad university, science &research branch

Tehran, Iran n.mirmotahhary@srbiau.ac.ir

Abstract— In this paper, we present a novel design of printed monopole antenna for UWB applications. The radiating element of the proposed antenna is composed of an octagonal patch fed by a 50 ohm microstrip. It has a very compact size of 20mm×12mm, which can be integrated easily with other RF front-end circuits. The antenna parameters and performances have been investigated through a large amount of EM simulations. It has been demonstrated that the proposed antenna provides an ultra wide bandwidth from 2.95 GHz to 13.35 GHz, completely covering the range set by the Federal Communication Commission (FCC) for UWB operations (3.1GHz to 10.6GHz). It also enjoys advantages such as low profile, low cost, high gain and satisfactory radiation characteristics.

Keywords—planar antenna,monopole,UWB

I. INTRODUCTION As a candidate for the so called “short fat pipe” Ultra wideband (UWB) radio has recently been researched for the possibilities of providing a short range (10m or less) high bandwidth (> 1 Gbps) communications link. The average emission limits as defined by the FCC for the regulation of indoor UWB systems [1] shows that UWB is essentially now a low power technology. This has presented many opportunities and challenges for antenna designers. Several kinds of antennas, such as bow-tie, conical, TEM horn, vivaldi, IRA, porcupine, spiral, fractal and log-periodic antennas have been introduced and they have good impedance stability over a very large frequency band [2-8]. In short-range UWB communications, the trade off between bandwidth, size, radiation efficiency, and low cost should be optimized to obtain an acceptable design. Some trade offs such as size-bandwidth and size-gain are more challenging [9]. Patch antennas are extensively used in

wireless communications because of their light weight, low cost, and ease of fabrication. These features are desirable for both indoor and outdoor handheld UWB antenna applications. Ideally, UWB antennas should be non-dispersive or dispersive in a controlled fashion that is amenable to compensation. Planar dipoles perform well in commercial applications [10]. Among the UWB antenna designs in the recent literatures, planar monopole antennas have become one of the considerable candidates for UWB applications owing to the features of nearly omni-directional radiation pattern. Several designs of planar monopole UWB antennas have been proposed [11-15]. However, some of these monopole UWB antennas only cover partial parts of the UWB band and some designs are difficult to integrate with other RF fronts because of their large size. In this paper, we present a novel planar microstrip-fed modified rectangular monopole antenna that exhibits broadband performance. The design of the proposed structure is based on the simple rectangular monopole antenna, but has a significantly large operational bandwidth. The antenna’s ground-plane is also truncated symmetrically. This antenna designed operates across 2.95 to 13.35 GHz with VSWR < 2. Unlike other antennas reported in the literature to date, the proposed antenna displays a good omni-directional radiation pattern even at higher frequencies. The monopole antenna is analyzed using Ansoft’s High Frequency Simulator (HFSS™) [16]. Simulated results are presented to validate the usefulness of the proposed antenna structure for UWB applications.

II. ANTENNA DESIGN Fig. 1 shows the configuration of the proposed ultra wideband monopole antenna which consists of rectangular structure which three sides of the rectangular patch filled with same size stubs and positioned above a triangular. The ground-plane is truncated, as shown in

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Fig. 1 and envelops the feedline to the radiating triangular patch including two stairs at its both sides. The proposed antenna is constructed from FR4 substrate with thickness of 1.6 mm and relative dielectric constant of 4.4. The proposed antenna is investigated by changing one parameter at a time, while fixing the others. Optimum physical parameters of the antenna presented in table I. To fully understand the behavior of the antenna’s structure and to determine the optimum parameters the antenna was analyzed using Ansoft’s high-frequency structure simulator (HFSS™). The proposed shape of the truncated groundplane acts as an effective impedance matching network to realize an antenna with a very wide impedance bandwidth. This is because the truncation creates capacitive loading that neutralizes the inductive nature of the patch to produce nearly pure resistive impedance present at the antenna’s input [12].

Figure1. Configuration of proposed monopole antenna

Table I. Physical parameters of the proposed antenna

III. RESULTS Fig. 2 shows a comparison of the return-loss characteristics of the proposed antenna with simple rectangular monopole antenna. As shown the simple rectangular monopole antenna has a relatively narrow bandwidth. As observed in Fig. 2 the successive modifications improve the bandwidth of the rectangular monopole antenna. With the inclusion of stubs on the radiation patch and modifying the groundplane, as in Fig. 1, has a significant effect on increasing its bandwidth. Figure 3 shows the antenna gain from 3 to 10 GHz for the proposed antenna. The maximum gain variation is less than 2 dB with the peak antenna gain of about 4.3 dB.

Figure2. S11 plot of a rectangular monopole and proposed monopole Antenna

Figure3. Gain plot of the proposed monopole antenna

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Figure4. Current distribution of the proposed monopole antenna at (a) 4,

(b) 6, (c) 8 and (10) GHz.

The spectrum of the antenna in Fig. 2 shows several resonances. These resonances correspond to the different modes of field distribution. Fig. 4 shows the simulated current distribution at frequency of 4 GHz, 6GHz, 8 GHz, and 10 GHz. Fig. 4(c) and (d) show a strong current distribution located around the radiating patch on the stubs, as well as the current flow directed towards the side stubs. This indicates that the modifications in the radiating patch excite resonance modes at a higher frequency which effectively extend the antenna’s bandwidth. The proposed antenna’s far-field radiation patterns are presented in the two principle planes, y-z plane for the E plane and x-z plane for the H-plane. Figs. 5 show the radiation patterns plots at several different frequencies are stable. It is noticed that the H-plane pattern is omnidirectional at lower frequency and is near omni-directional at higher frequencies.

Figure5. Measured radiation patterns of the proposed antenna in x-z and

yz plane at: (a) 4 GHz, (b) 7 GHz, and (c) 11 GHz.

IV. CONCLUSION In this paper a compact planar monopole antenna is proposed that exhibits ultra wide bandwidth performance and easily satisfies the requirements for UWB applications. The results show that the impedance bandwidth of the proposed antenna is significantly improved with the inclusion of square stubs on the radiating patch, a triangular shape piece on the connection section of feedline and radiating patch, and using a truncated ground-plane. The proposed antenna exhibits an impedance bandwidth of 127.6% over a very wide frequency range from 2.95 to 13.35 GHz with return-loss better than -10 dB also results show good radiation patterns within the UWB frequency range.

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