Progress on Single-Feed Quality Wideband Linear Wire Array

Abstract

This paper presents the latest developments regarding the single-feed Quality Wideband Linear (QWL) wire array antenna, known for its broadband and high-gain electromagnetic characteristics and robust design. A systematic review of recent advances in relation to the QWL antenna is provided, covering its driven element, director, reflector, low common-mode current interference connector, and array series-feed configuration. For the first time, an analytical expression and a quick design formula for the input impedance of the QWL antenna’s driven element, the linear Wideband High-gain Electromagnetic Structure (WHEMS) antenna, are presented. Theoretical analysis demonstrates the potential for broadband performance using the WHEMS antenna. The rugged design of the QWL array antenna offers engineering advantages such as simple feeding, low wind resistance, a lightweight construction, low cost, and structural robustness. The QWL antenna has already found applications in various industrial sectors, with potential for broader use in the future, contributing to further advancements in antenna technology.

1. Introduction

Linear antennas have played a significant role in the early development of antenna technology, with extensive applications in fields such as maritime and aviation communication. Their simplicity, reliability, and adaptability have ensured their continued use across various domains. From their early applications to modern communication systems, linear antennas have demonstrated their importance and versatility.

Yagi-Uda and log-periodic antennas are among the most widely used wire array antennas. These popular designs, along with the corrugated horn antenna, microstrip antennas, and more recent meta-antennas, were all developed by pioneering antenna engineers. These antennas have become staples in this field due to their unique advantages and versatile applications.

The classic Yagi-Uda antenna, invented a century ago, is a renowned linear wire array widely utilized in various applications, including communications, radar systems, and commercial TV reception [1]. This antenna operates as an end-fire array, comprising a driven element, typically a folded dipole; multiple short dipole parasitic elements, known as directors, arranged linearly in the direction in which the antenna’s radiation pattern beam faces; and a longer dipole reflector. The directors and reflector enhance antenna gain and reduce the back lobe, resulting in a high-gain, low-back-lobe directional beam pattern.

The Yagi-Uda antenna is a single-feed linear array whose parasitic dipole elements exhibit zero voltage at their centers. These elements are typically connected by a metallic rod in direction of the beam, leading to a robust, low-wind-load, and lightweight design. This construction makes the Yagi-Uda antenna cost-effective to manufacture.

The Yagi-Uda antenna first found application in an English-captured flight in Singapore during World War II. It was used for radar and communication systems, showcasing its effectiveness and contributing to its widespread adoption in various military and commercial applications. This early use demonstrated the antenna’s high gain and directional capabilities, which were crucial for effective radar detection and reliable communication.

As a wire array antenna, the primary limitation of the Yagi-Uda antenna is its relatively narrow impedance bandwidth, typically around 10%. With advances in communication and radar technology, there is an increasing demand for antennas with much broader operational bandwidths. To address this limitation, antenna manufacturers often need to produce a series of Yagi-Uda antennas, each designed to cover a specific portion of the wide spectrum of frequencies required for modern applications [2]. In most cases, a single antenna is preferred to meet the bandwidth requirements of the system.

A log-periodic antenna is a type of broadband, directional antenna that can operate over a wide range of frequencies. It consists of multiple elements of varying lengths, arranged in a specific pattern that repeats periodically on a logarithmic scale. This design allows the antenna to maintain consistent performance across its entire frequency range [3,4,5]. Log-periodic antennas are commonly used in applications such as television broadcasting, radio communications, and electromagnetic interference testing, where wideband capability and reliable performance are essential. The typical antenna gains for a log-periodic antenna range from about 6 to 8 dBi.

Numerous studies have been conducted to refine the design of the Yagi-Uda antenna, resulting in optimized and quasi-Yagi-Uda antennas. Researchers have put extensive effort into increasing the operational bandwidth of the Yagi-Uda antenna, which typically operates within a bandwidth of 10%. Research on broadband Yagi-Uda antennas began with the quasi-Yagi antenna in 1999, wherein [6] introduced a wideband planar antenna on a high-dielectric substrate, compatible with microstrip circuits, achieving 3 dBi to 5 dBi gain over a 48% bandwidth. Innovations in feeding techniques, such as microstrip-to-waveguide transitions, have expanded the relative impedance bandwidth of quasi-Yagi antennas to over 90% [6,7,8,9,10,11,12,13,14,15,16,17]. Although these antennas, typically designed for millimeter-wave bands above 3 GHz, have gains 3 dB to 5 dB lower than those of classic Yagi-Uda antennas, their broadband improvements have revitalized the Yagi-Uda design.

In this paper, the latest developments of the single-feed Quality Wideband Linear (QWL) array antenna are presented. The QWL antenna stands out due to its broadband, high-gain electromagnetic characteristics and structurally robust design. This antenna incorporates a wideband driven element, directors, and reflectors, drawing on principles similar to those applied to the Yagi-Uda antenna. A key feature of this design is the WHEMS-driven element, which provides bidirectional radiation and enables a wide operational bandwidth. The directors, arranged on the same plane, and a reflector designed for impedance matching further enhance the antenna’s gain and bandwidth.

What distinguishes this antenna from existing designs is its ability to achieve superior performance while maintaining a compact form factor, simple feeding mechanism, low wind resistance, lightweight construction, cost-effectiveness, and structural durability. These engineering advantages make it ideal for a wide range of applications, including wireless communications, satellite systems, and radar technologies. Additionally, the QWL antenna’s compact and lightweight design holds significant potential for integration into space-constrained systems such as UAVs and portable devices, paving the way for promising future technological advancements.

Section 2 provides a comprehensive overview of the QWL antenna. In Section 3, the impedance characteristics and radiation features of the QWL antenna’s driven element are discussed. Section 4 presents the latest advancements in the research on the QWL antenna’s directors and reflectors. Section 5 details the design approach for the QWL antenna’s low-common-mode-current rugged structure. Section 6 introduces the series-fed array configuration of the QWL antenna. Section 7 outlines the design methodology for the QWL antenna. Finally, Section 8 offers a discussion and conclusions, along with future research directions for the QWL antenna.

The main contributions of this paper are as follows:

(1)

A systematic review of the development history of the QWL antenna is presented for the first time, covering advances in its driven element, director, reflector, low-common-mode-current interference connector, and array series-fed feed design.

(2)

The analytical expression and a quick design formula for the input impedance of the QWL antenna’s driven element, the linear WHEMS antenna, are provided for the first time.

2. QWL Antenna

Despite the many advances, achieving a wide bandwidth while maintaining all the mechanical advantages of the classic Yagi-Uda antenna remains a significant challenge. To the best of our knowledge, this challenge has yet to be fully addressed in current research. The development of a new and high-QWL array has been proposed to substantially increase the bandwidth of a single-feed linear array while maintaining its rugged design. One of the primary factors that limit the bandwidth of a linear array antenna is the driven element. To overcome this limitation, a wideband structure was proposed by Yihong Qi, and more recently, Lidong Chi and his colleagues increased the bandwidth of a bidirectional antenna element to 158% [18,19]. This antenna element has been termed the WHEMS [20,21,22,23,24,25,26,27,28].

3. Driven Element

The WHEMS antenna serves as the driven element for the QWL antenna and is fundamental to the broadband and high-gain performance of the QWL antenna. The following will introduce the input impedance characteristics and radiation properties of the linear WHEMS antenna and, for the first time, provide the analytical expression and a quick design formula for its input impedance.

3.1. Impedance Characteristic

Figure 1 shows the basic forms of loop antennas, slot antennas, and loop-slot antennas, where the blue parts represent metal, and the red arrows represent the lumped ports. The WHEMS antenna has a loop structure and can be classified as a loop, slot, or slot loop antenna, but it has a unique feeding structure and characteristics that differ from those of a typical loop antenna [29]. A slot antenna is typically characterized by a large ground plane [30], while a slot loop antenna is made up of two separate, unconnected pieces of metal [31]. The novel slot antenna has an irregular geometrical design, new materials, and a co-planner waveguide feeding mode [32,33]. Figure 1 illustrates examples of typical loop, slot, and slot loop antennas, while Figure 2 shows the WHEMS antenna [19].

Figure 3 shows the basic form and parameters of the WHEMS antenna, where the part in blue is metal. The WHEMS antenna can be regarded as two parallel end short-circuited lossy transmission lines, and the radiation can be equivalent to the loss of the transmission line. The input impedance of the WHEMS antenna is derived below. Assume that the metal arm of the WHEMS antenna here is infinitely thin. The lossless characteristic impedance Z₀ of the linear WHEMS antenna is shown in Equation (1):

$$Z_0=120(\ln {\displaystyle \frac{L\cos \alpha }{d/2}}-1).$$

(1)

Lossy characteristic impedance is given by Equation (2):

$$Z_c=Z_0(1-j{\displaystyle \frac{\sigma }{\beta }}).$$

(2)

Equation (3) is the loss factor:

$$\sigma ={\displaystyle \frac{R}{2Z_0}}={\displaystyle \frac{2R_s}{Z_{0}L(1+\cos \alpha )(1+\frac{\sin \beta L(1+\cos \alpha )}{\beta L(1+\cos \alpha )})}}.$$

(3)

Among these, the loss resistance, that is, the radiation resistance of the antenna, can be calculated using Equation (4):

$$R_s={\displaystyle \frac{1}{2I_0^2}}\sqrt{{\displaystyle \frac{\mu }{\epsilon }}}{{\displaystyle {\int }_0^{2\pi }{\displaystyle {\int }_0^{\pi }\left|H_{\varphi }\right|}}}^2{r}^2\sin \theta d\theta d\varphi .$$

(4)

The $H_{\varphi }$ is given in [19]. Therefore, the input impedance of half a WHEMS antenna is shown in Equation (5):

$$Z_{in}/2=Z_0{\displaystyle \frac{sh2\sigma l+\frac{\sigma }{\beta }\sin 2\beta l}{ch2\sigma l+\cos 2\beta l}}+Z_{0}j{\displaystyle \frac{\sin 2\beta l-\frac{\sigma }{\beta }sh2\sigma l}{ch2\sigma l+\cos 2\beta l}}.$$

(5)

Assuming that α = 60 deg and when f = f₀, βl = 2π, the input impedance of the linear WHEMS antenna can be calculated according to Equations (1)–(5), as shown in Figure 4. The following can be gleaned from the figure:

• The WHEMS antenna theoretically has infinite bandwidth, and the module value of input impedance gradually converges to half of the characteristic impedance of half the WHEMS antenna with the increase in frequency;
• The first matching resonant point of the WHEMS antenna is at the first parallel resonant point, that is, when half the WHEMS antenna is equivalent to a half-wavelength short-circuit transmission line. At this point, half the WHEMS antenna has a perimeter of one wavelength.

Figure 4. (a) Real and imaginary parts of the calculated and simulated input impedances of the WHEMS antenna. (b) Input impedance of wire WHEMS antenna.

Figure 4. (a) Real and imaginary parts of the calculated and simulated input impedances of the WHEMS antenna. (b) Input impedance of wire WHEMS antenna.

3.2. Radiating Characteristics

The WHEMS antenna is a bidirectional radiation antenna that can achieve an effective bidirectional gain of over 2 dBi within a bandwidth exceeding 158%, making it the widest-known bidirectional effective gain bandwidth [19]. The WHEMS antenna element can be represented by three electrical dipoles within a wideband of low frequencies (over twice the frequency bandwidth) [25,26,27,28], as illustrated in Figure 5. These three dipoles are aligned to function as a small in-phase antenna array, which can achieve up to 7 dBi of antenna gain in the broadside direction [25]. This bidirectional feature, as shown in Figure 5c, provides better antenna gain and lower sidelobe potential as a driven element. The equivalent dipole array also allows designers to incorporate reflectors and directors to form a linear array, further improving the antenna gain.

4. Reflectors and Directors

To generate a unidirectional radiation pattern, a flat reflector was used [32]. The distance between the reflector and the driven element can impact antenna gain and match, so a stepped reflector was used to achieve a wider bandwidth [33]. These results have inspired further research into designing wider-bandwidth reflectors. The use of a W-shaped reflector resulted in a bandwidth of over 70%. The W-shaped reflector not only formed a unidirectional beam but also improved the impedance bandwidth while optimizing the distance in the feeding zone. In order to reduce the antenna’s wind load, a wire array arranged in a W shape was used to replace the metallic W plate, and similar results were obtained [27,34].

In addition to reflectors, directors can also be applied to the WHEMS structure to achieve high directional antenna gain [35]. Three simple wire directors can be used to enhance array antenna gain. The positions of the directors are set to match the equivalent source of the driven WHEMS element. The directors exhibit capacitive behavior at lower frequencies and maintain this capacitive behavior across the entire operating frequency range, while the reflectors display inductive characteristics throughout the frequency range of interest. The arrayed directors and reflector improved the aperture efficiency of the QWL antenna. The combination of a wideband driven element, W-shaped reflectors, and directors resulted in a wideband high-gain linear array, as shown in Figure 6, where the arrows in the Figure 6d point to the y-axis corresponding to the values referenced by each curve [36].

5. Rugged Design

Across the wide bandwidth, all rods of the directors and reflectors serve as resonating elements with center voltages equal to zero. The simulated current distribution of QWL antenna is shown in Figure 7, where red indicates the areas of maximum current and blue denotes the areas of minimum current. Additionally, the voltage at the center of the two shorting arms in the driven element is also equal to zero across the operating band, as demonstrated in Figure 7. Since the voltages at the center points of the reflectors, directors, and shorting edges of the driven elements are all zero, a metallic structure can be used to connect all of these zero-voltage points, resulting in a rugged design. The fully designed QWL array is depicted in Figure 8 [19,27].

The QWL antenna is able to more effectively utilize its radiation structure to transmit or intercept electromagnetic waves. When enclosed in a minimal surrounding sphere, the maximum size of the QWL antenna is only 0.74 of that of the Yagi-Uda antenna, as shown in Figure 9. The antennae were compared at a low frequency range, because that is the frequency which determines antenna size.

6. Series-Fed Array Arrangement

To further enhance the gain, the QWL antenna can also be configured as a series-fed array. As shown in Figure 10, [37] proposed a series-fed QWL antenna array based on the WHEMS antenna. This series-fed QWL antenna array includes a reflector, driven elements and directors. The antenna uses a series-fed arrangement in the driven element section, connecting the feed points of two WHEMS antennas in series. The series-feeding method proposed in Figure 10 cancels out the phase difference of π/2 introduced by the feed, achieving equal amplitude and in-phase currents in the two array elements, thereby realizing high gain.

To achieve high gain in the series-fed QWL antenna array, it is also necessary to address the array gain loss caused by mutual coupling between adjacent elements. The red dashed line in Figure 11a represents the capacitive coupling between the two WHEMS elements due to the voltage distribution. The inverse current distribution on the adjacent edges generates differential currents, likening this structure to two transmission lines carrying opposing currents. Therefore, the adjacent side currents of the WHEMS array will cancel each other, resulting in a gain decrease of 2–3 dB compared with the ideal case calculated via array theory. The sides of the two WHEMS antennas that are close to each other are bent in Figure 11b, increasing the isolation between the two WHEMS antennas and further improving the overall gain of the QWL antenna array. Ultimately, this single-feed-point, two-element series-fed antenna can achieve a directional gain of over 14 dBi.

A photo of the series-fed QWL antenna is shown in Figure 12, and its radiation patterns are shown in Figure 13. According to the comparison between the proposed antenna and other related studies in

Table 1. Comparison of the series-fed QWL antenna array with other antennas.

Ref.	Bandwidth (%)	Efficiency (%)	Size (${\mathit{\lambda }}_{\mathit{c}}^{\mathbf{3}}$)	Process	Wind Load	Gain (dBi)
[38]	32.7	90–92	0.88 × 0.88 × 0.29	PCB + Metal	high	8.0–9.0
[39]	35.3	NG	0.71 × 0.62 × 0.29	PCB	low	6.5
[40]	26.7	NG	0.73 × 0.44 × 0.10	PCB + Metal	high	5.1–8.0
[41]	55.3	NG	0.93 × 0.93 × 0.25	PCB	high	8.2–9.1
[42]	31.6	87–92	1.05 × 0.66 × 0.22	Metal + Plastic	high	9.8
[43]	5.4	NG	1.83 × 1.83 × 0.54	All-metal	low	15.1
[44]	14.4	NG	2.48 × 0.67 × 0.23	All-metal	low	12.1
[45]	25.1	NG	1.63 × 0.69 × 0.23	All-metal	low	10.1
This antenna	19.9	93–100	1.37 × 1.22 × 0.56	All-metal	low	14.2

, the advantages of this QWL antenna array include its high gain and low wind load. Additionally, it does not include a dielectric plate power divider, resulting in low loss and all-weather environmental adaptability.

7. Design Steps of QWL Antenna

The design of the QWL antenna can be initially guided by the following steps:

(a)

Determine the operating frequency range of the antenna.

(b)

Set ${\displaystyle \frac{Z_0}{2}}=50$ Ω in Equation (1) to define the fundamental parameters of the antenna.

(c)

Design the three-dipole array director according to the design principles discussed in Section 4. Following Yagi antenna design guidelines, ensure that the length of each metal rod in the directors does not exceed half of the wavelength at the lowest operating frequency.

(d)

Design the W-shaped reflector according to the design principles discussed in Section 4.

(e)

Design the voltage balun using the method described in [27].

(f)

If implementing a series-fed array antenna, design the series feeding according to the method outlined in Section 6.

(g)

Design the low-common-mode-current connector based on the method provided in Section 5.

(h)

Finally, optimize the antenna’s design based on the specific application requirements.

8. Discussion and Future Directions

8.1. Conclusions and Discussion

This paper provides a comprehensive review of the definition and development of QWL antennas, presenting for the first time an analytical expression for the input impedance of the WHEMS antenna, which acts as the driven element of the QWL antenna. Theoretical analysis shows that the WHEMS antenna possesses infinite impedance bandwidth. Furthermore, the WHEMS antenna can be effectively modeled as a three-dipole array, capable of delivering high side-lobe gain with a single element. Key advancements in the three-dipole array director, W-shaped reflector, and low-common-mode-current connectors are also discussed, highlighting their contributions to the performance improvements in QWL antennas.

The QWL antenna stands out for its wide bandwidth, high gain, compact structure, and robust mechanical design, characterized by low wind resistance and lightweight construction. To achieve even higher gain, this paper introduces the implementation of a series-fed array for WHEMS antennas and decoupling methods to enhance the gain of the QWL series-fed array.

Already adopted in the industry, the QWL antenna has demonstrated its versatility across a range of applications. Its single-feed linear array design has significantly impacted fields such as long-distance communication, radar systems, and wireless networks. The simple design and low cost of the QWL antenna also make it accessible to a wide audience, including amateur operators and hobbyists. The innovations and concepts discussed in this paper are expected to inspire further research and broaden the scope of applications for linear antennas in the future.

8.2. Future Directions

The QWL antenna offers several advantages, including wideband performance, high gain, a single feed point, low cost, low wind resistance, light weight, and a compact structure. As shown in Figure 14, advancements have been made in various aspects of the QWL antenna, including the driven element, feed, director, reflector, and connector. The WHEMS antenna, which serves as the driven element of the QWL antenna, can achieve good impedance matching and over 2 dB of broadside gain across a continuous bandwidth exceeding 150%. The proposed W-shape reflector for the WHEMS antenna, when used as a driven element, can theoretically provide over 150% of 3 dB gain bandwidth. The low-common-mode-current connector enables a more compact structure and further reduces wind resistance for the QWL antenna. Progress in the series-fed array configuration allows the antenna to achieve over 13 dBi of gain with a single feed point. The three-dipole array director effectively directs the electric field on the aperture of the WHEMS antenna, making it a highly efficient director; however, its operational bandwidth is limited, and it is not a high-efficiency broadband director.

Therefore, the future directions for the QWL antenna include the following:

(a)

Designing a broadband high-efficiency director.

(b)

Expanding the bandwidth of the QWL antenna by incorporating a broadband high-efficiency director.