Wideband Printed Dipole Antenna for DTV Signal Reception

Yun-Wen Chi and Kin-Lu Wong

Department of Electrical Engineering National Sun Yat-Sen University, Kaohsiung 804, Taiwan

Abstract- A novel printed dipole antenna capable of generating

a wide operating band for digital television (DTV) signal reception in the 470 ~ 806 MHz band is presented. The antenna is of a rectangular shape and has a size of 20 x 227 mm2, and the antenna comprises two asymmetric radiating portions of arm 1 and arm 2 separated by a step-shaped feed gap with its one open end at the center of antenna’s one long side edge and the other open end at about L/4 away from the center of the opposite long side edge. By the use of the step-shaped feed gap, the proposed antenna can generate two adjacent resonant modes to form a wide operating band of larger than 50% in 2.5:1 VSWR bandwidth, which is much wider than that of the corresponding conventional center-fed dipole antenna with a straight feed gap. The antenna’s first resonant mode is operated as a 0.5-wavelength mode controlled by the antenna length L, while the second resonant mode is a 1.0-wavelength mode controlled by the total effective length of arms 1 and 2, which is about 1.25L in the proposed design. Detailed design considerations and radiation characteristics of the two excited resonant modes for broadband operation of the proposed dipole antenna are discussed in the paper.

I. INTRODUCTION

Digital television (DTV) terrestrial broadcasting has already reached the implementing stage in many countries after more than a decade of intense research and development [1]. The DTV system can offer high-data-rate transmission, provide interactive services, and operate in low power. With these advantageous features, the DTV system becomes very attractive for applications in mobile communication devices such as the laptop computers and mobile phones [2], [3]. Similarly, it is also very attractive for vehicle owners to have their vehicles equipped with a DTV signal reception device [4]. For these perspective applications, it is expected that the requirements of mobile antennas for DTV signal reception will increase.

Here in this paper, we present a novel printed dipole antenna with a step-shaped feed gap which provides a wideband operation. The proposed printed dipole antenna is suitable for DTV signal reception in the 470 ~ 806 MHz band [5]. In the proposed dipole antenna, the step-shaped feed gap is used to replace the simple straight feed gap for the conventional center-fed dipole antenna. By the use of the step-shaped feed gap, an additional resonant mode adjacent to the antenna’s fundamental (0.5-wavelength) resonant mode can be excited. These two resonant modes can then be formed into a wide operating band of larger than 50% (2.5:1 VSWR) centered at about 600 MHz, which is much wider than that of the corresponding conventional center-fed dipole antenna. Over

the obtained wide operating band, the proposed antenna also shows stable radiation characteristics (similar to that of the conventional center-fed dipole antenna) with good radiation efficiency. Also note that the concept of using two adjacent resonant modes excited in the proposed dipole antenna for broadband operation are different from that of using two separate resonant modes excited in the reported dipole antenna with two U-slotted arms or two L-slotted arms for dual-band operation [6], [7]. Another advantage is that the proposed antenna has a planar configuration [8], so it is easy to fabricate with a low cost by printing on a dielectric substrate. Design considerations of the proposed antenna are described, and results of the constructed prototypes are presented and discussed.

II. DESIGN CONSIDERATIONS OF THE PROPOSED ANTENNA

Fig. 1 shows the proposed dipole antenna with a step-shaped feed gap. The antenna is printed on a 0.4 mm thick FR4 substrate and is of a rectangular shape of width 20 mm and length 227 mm (L). The dimensions shown in the figure are the preferred design dimensions. With the selected antenna length L of 227 mm, the antenna can generate a fundamental (0.5-wavelength) resonant mode centered at about 530 MHz, allowing the antenna to cover the lower-edge frequency (470 MHz) of the desired DTV band in the study. However, in order to cover the upper-edge frequency (806 MHz) and the whole DTV band, an additional resonant mode is required. For this purpose, a step-shaped feed gap is used in the proposed design to replace the simple straight feed gap for the conventional center-fed dipole antenna.

Figure 1. Configuration of the proposed broadband printed dipole antenna with

a step-shaped feed gap for DTV signal reception

As shown in the figure, one open end of the step-shaped feed gap of uniform width 1 mm is at the center of one long side edge of the antenna, the same as that of the conventional center-fed dipole antenna. On the other hand, the other open

end of the step-shaped feed gap is arranged to be located away from the center of the opposite long side edge with a distance of t. With the proposed feed-gap arrangement, the antenna is separated into two asymmetric radiating portions of arm 1 (length 113 mm + t or about 0.5L + t) and arm 2 (length 113 mm or about 0.5L). It has been found that, by choosing a proper value of t (about 0.25L in the proposed design), an additional resonant mode can be excited at frequencies close to the antenna’s fundamental resonant mode, thereby leading to a wide operating band achieved for the proposed antenna. This additional resonant mode is identified to be a 1.0-wavelength mode controlled by the total effective length (about 1.25L) of arms 1 and 2.

Figure 2. Simulated excited surface currents at 530 and 730 MHz on arms 1

and 2 of the proposed antenna

Figure 3. Measured and simulated return loss for the proposed antenna.

Fig. 2 shows the simulated excited surface currents at 530 and 730 MHz (about center frequencies of the excited 0.5- and 1.0-wavelegth modes) on the proposed antenna. The simulated results are obtained using Ansoft simulation software HFSS

(High Frequency Structure Simulator) [9]. First note that, at 530 MHz, the excited surface currents on arms 1 and 2 are in the same direction and very similar to those on the two equal arms of the conventional center-fed dipole antenna excited at the fundamental (0.5-wavelength) mode. On the other hand, for the excited surface currents at 730 MHz, there is a null current on arms 1 and 2 along the step-shaped feed gap, and the surface currents on arms 1 and 2 are in the opposite direction. This behavior is similar to that of the conventional dipole antenna excited at the second resonant mode (1.0-wavelength mode).

Figure 4. Comparison of the measured return loss for the proposed antenna, the corresponding conventional center-fed dipole antenna with a simple feed gap, and the corresponding off-center-fed dipole antenna with a simple feed gap.

Figure 5. Measured return loss as a function of t of the step-shaped feed gap. Other dimensions are the same as given in Fig. 1.

It is also seen that arm 1 has a much longer length than arm 2 and the surface currents on arm 1 are generally stronger than those on arm 2. Thus, in this case, it is expected that the radiation characteristics of the proposed antenna at the second (1.0-wavelength) resonant mode will be dominated mainly by arm 1 and similar to those of the proposed antenna excited at the fundamental (0.5-wavelength) resonant mode. That is, stable radiation characteristics and omnidirectional radiation in the plane orthogonal to the proposed antenna are expected to

be obtained over the wide operating band formed by the two excited resonant modes. More detailed results of the two excited resonant mode as a function of the parameter t will be discussed with the aid of Fig. 5.

There is another major parameter b of the step-shaped feed gap affecting the achievable bandwidth of the proposed antenna. Results have indicated that a small value of b (1 mm in the proposed design) should be selected; that is, the horizontal portion of the step-shaped feed gap is required to be placed close to the long side edge of the proposed antenna. In this case, the width of arm 1 is close to uniform, thus a more uniform excited surface current on arm 1 can be expected. This behavior is helpful in achieving good impedance matching for the proposed antenna. In addition, owing to the more uniformly excited surface currents on arm 1, which has a much longer effective length than arm 2, the radiation characteristics for frequencies over the excited additional resonant mode (1.0-wavelength mode as shown in Fig. 2) are expected to be dominated by arm 1. That is, the radiation characteristics of the additional resonant mode will be close to those of the fundamental (0.5-wavelength) resonant mode of the proposed antenna. This behavior can lead to stable radiation characteristics over the wide operating band achieved for the proposed antenna.

Also note that, to test the proposed antenna in the experiment, a 50 Ω mini coaxial feed line is used, with its central conductor and outer grounding sheath connected to points A and B as shown in the figure. In addition, in order to eliminate the possible unbalanced surface currents flowing back from the dipole antenna to the coaxial feed line to affect the measurement in the experiment, a balun element (not shown in the figure) is also connected in between the feeding points (points A and B) and the coaxial feed line.

III. RESULTS AND DISCUSSION

A preferred prototype with the design dimensions shown in Fig. 1 was first tested. Fig. 3 shows the measured and simulated return loss for the preferred prototype. Good agreement between the measured and simulated results obtained using Ansoft simulation software HFSS is obtained. It is seen that two excited resonant modes are with good impedance matching, and the obtained impedance bandwidth (2.5:1 VSWR) reaches 340 MHz (470 ~ 810 MHz) or about 53% centered at 640 MHz. The obtained wide bandwidth covers the 470 ~ 806 MHz band for DTV signal reception. Also note that for DTV signal reception, the bandwidth definition of 2.5:1 VSWR is good for practical applications. In some cases, the definition of 3:1 VSWR has also been shown to be applicable [4].

Fig. 4 shows a comparison of the measured return loss for the proposed antenna, the corresponding conventional center-fed dipole antenna with a simple feed gap (width 1 mm), and the corresponding off-center-fed dipole antenna with a simple feed gap (width 1 mm). Note that the center-fed and off-center-

fed dipole antennas are with the same length (227 mm) as that of the proposed antenna. It is first seen that, for the center-fed dipole antenna, a single resonant mode (0.5-wavelength mode) is excited at about 580 MHz, and the obtained impedance bandwidth (2.5:1 VSWR) is about 165 MHz only or about 28% centered at 580 MHz, which is much smaller than that of the proposed antenna and cannot cover the whole DTV band. In addition, for the center-fed dipole antenna, results show that the 1.0-wavlength mode cannot be excited. On the other hand, for the off-center-fed dipole antenna, two resonant modes (0.5- and 1.0-wavelength modes) at about 600 and 1200 MHz are excited; however, poor impedance matching for frequencies over the 0.5-wavelength mode is observed. By using the proposed step-shaped feed gap, which can ensure good excitation of the 0.5-wavelength mode and, moreover, the excited 1.0-wavelength mode can be shifted to lower frequencies close to those of the 0.5-wavelength mode to form a wide operating band for the proposed antenna. In this case, the 1.0-wavelength mode is mainly controlled by the total effective length (about 1.25L) of arms 1 and 2 in the proposed antenna.

Figure 6. Measured return loss as a function of b of the step-shaped feed gap.

Other dimensions are the same as given in Fig. 1.

Effects of the parameter t of the step-shaped feed gap on the proposed antenna were also studied. Fig. 5 presents the measured return loss for t varied from 0 to 58 mm. From the results, it is seen that the antenna’s fundamental resonant mode is in general slightly affected; on the other hand, the antenna’s second resonant mode is strongly affected by the length t. With an increase in t, the second resonant mode is quickly shifted to lower frequencies close to those of the fundamental resonant mode. This result is expected, since the length of arm 1 (113 mm + t) increases as t increases, and so is the total effective length (226 mm + t) of arms 1 and 2. Results also indicate that, with t chosen to be 58 mm (about 0.25L), the antenna’s two excited resonant modes can be formed into a wide operating band, with good impedance matching achieved for frequencies over the whole DTV band of 470 ~ 806 MHz.

Effects of the parameter b were also analyzed. It is observed that the width b has strong effects on the impedance matching

of the proposed antenna, especially on the impedance matching for frequencies over the second resonant mode. Results show that a smaller width b (that is, a more uniform width for arm 1) will lead to enhanced impedance matching for the second resonant mode. For this reason, the parameter b is selected to be 1 mm in the proposed antenna.

Figure 7. Simulated antenna gain and radiation efficiency over the operating

band of the proposed antenna

Radiation characteristics of the proposed antenna such as radiation patterns, antenna gain and efficiency were also studied using Ansoft simulation software HFSS are presented, which are expected to provide reliable results for the proposed antenna. Fig. 7 presents the simulated antenna gain and radiation efficiency over the operating band of the proposed antenna. Good antenna gain and radiation efficiency are obtained. The antenna gain varies from about 1.5 to 2.8 dBi, and the radiation efficiency is all larger than 60% for frequencies over the band. Other results of radiation characteristics like radiation patterns will be discussed in detail in the presentation.

IV. CONCLUSION

A novel printed dipole antenna with a step-shaped feed gap for DTV signal reception in the 470 ~ 806 MHz band has been proposed, fabricated and tested. The proposed antenna has a simple and planar configuration, making it easy to fabricate with a low cost. The wide operating band of the proposed antenna is formed by two excited resonant modes: one 0.5-wavelength mode controlled by the antenna length L and one 1.0-wavelength mode controlled by the total effective length (about 1.25L) of the antenna’s two radiating arms separated by the proposed step-shaped feed gap. A parametric study of the step-shaped feed gap on the impedance matching over the obtained wide operating band has also been conducted, and preferred design dimensions of the step-shaped feed gap has been determined. Moreover, over the obtained wide operating band, stable radiation characteristics have been obtained, and similar radiation patterns as those of the conventional half-wavelength center-fed dipole antenna have been observed. With the wide operating band and good radiation characteristics achieved, the proposed antenna is very

promising for DTV signal reception in the 470 ~ 806 MHz band.

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[9] http://www.ansoft.com/products/hf/hfss/, Ansoft Corporation HFSS.