Design of Tight Coupling Antenna to Realize Ultra-Wideband Function

Abstract

In this paper, according to the tightly coupled antenna theory, the dual-polarized tightly coupled antenna of 15–40 GHz is designed. The antenna size is 3.75 mm × 3.75 mm × 2.38 mm, and the spacing between the elements is 3.75 mm. The media used from bottom to top are Rogers5880, PP adhesive, RogersTMM10, and feed line using coaxial line directly, with a coaxial line inner core radius of 0.2 mm. Based on the simulations, we find that the antenna can easily cover ±45° and maintain good active standing wave performance. The antenna can support ±60° maximum Angle scanning and maintain good active standing wave performance.

1. Introduction

With its high data transmission rate, good concealment, and high resolution, UWB signals are widely used in wireless communication, modern radar, and electronic countermeasures; UWB antennas and their arrays gradually show important scientific research value and application prospects [1]. With the development of wireless communication technology, the requirement for information data transmission rate is higher and higher. According to Shannon’s theorem of wireless data transmission, the channel capacity of the communication system is proportional to the system bandwidth, which means that the bandwidth requirement of the communication system will be wider and wider [2]. Wideband reconnaissance can detect enemy radar signals in various frequency bands and then jam and attack targets. The application of wide band signals in radar can realize frequency agility [3]. Moreover, the bandwidth of frequency agility conversion is very wide, making our signal not easily detected by the enemy and improving the radar’s anti-jamming ability [4].

With the increasing demand of data transmission capacity and transmission rate in modern wireless communication systems, communication systems have put forward higher requirements for antenna bandwidth. Some representative wideband antenna array technologies emerge at the historic moment. Among them, wideband array antenna is used more and more widely [5]. Generally, the design idea of wideband antenna array is to start from antenna unit, design antenna unit structure with wideband performance by using wideband technology, and then assemble the wideband antenna unit according to different array forms [6].

As an important part of wireless communication systems, antenna is the premise of realizing effective transmission of wireless communication signals [7]. In various fields of wireless communication systems, it is necessary to carry out the design of various antennas according to the transmission and signal requirements, of which the design of ultra-wideband antenna play a very important role [8].

Lots of work has been done in the design of an antenna to realize ultra-wideband [9,10,11,12,13,14]. In [9], a novel water-drop-shaped monopole antenna based on the model from nature is designed to realize ultra-wideband. In [10], a single-feed ultra-wideband circularly polarized antenna with high front-to-back ratio is presented. In [11], a new wideband multiple-input/multiple-output (MIMO) antenna system is proposed for fifth-generation (5G). In [12], a compact concentric structured monopole patch antenna for super wideband (SWB) application is proposed and investigated. In [13], a novel compact spanner-shaped ultra-wideband (UWB) antenna is designed for multiple input multiple outputs (MIMO) system applications. In [14], the design of a wideband single-layer partially reflective surface (PRS) for a circularly polarized (CP) resonant cavity antenna is provided. All the electrical parameters of an antenna are functions of frequency, where the frequency characteristic of an antenna can be expressed by its characteristic parameter-working frequency band or frequency bandwidth (bandwidth for short).

At present, the research progress of tightly coupled antenna is rapid, and many new tightly coupled forms have been proposed by scholars [15,16,17,18]. However, the tightly coupled antenna is a new antenna technology—the research is not perfect and there are still many problems to be solved. Due to the continuity of the current on the tightly coupled array compared with the traditional array, the edge effect of the finitely large tightly coupled array is obvious, the reflected wave is more intense, and the reflected wave spreads more widely on the array, which leads to the difficulty in matching the tightly coupled array simulation. In addition, bandwidth is one of the most essential characteristic parameters of a wideband antenna. Therefore, the design adopts a frequency-independent or frequency-scalable geometric structure, which can avoid the influence of frequency on electrical performance to achieve ultra-wideband performance.

Through the above analysis of the application of traditional wideband antenna array, we find that there are some limitations in the traditional technology. To solve these problems, this paper focuses on the design of tight coupling antenna. We analyze the basic theory of tight coupling antenna and design a tight coupling antenna to realize the ultra-wideband function. The main contributions are summarized as follows:

• According to the tightly coupled antenna theory, a dual-polarized tightly coupled antenna of 15–40 GHz is designed. The designed antenna can achieve ultra-wideband performance.
• The antenna size is 3.75 mm × 3.75 mm × 2.38 mm, and the spacing between the elements is 3.75 mm. The media used from bottom to top are Rogers5880, PP adhesive, RogersTMM10, and feed line using coaxial line directly, with a coaxial line inner core radius of 0.2 mm.

The whole paper is organized as follows: in Section 2, the principle of tight coupling is introduced. The design method of the tight coupling antenna is given in Section 3. The test and simulations are provided in Section 4, and concluded in Section 5.

2. Principle of Tight Coupling

Wheeler uses the current slice method to analyze the impedance variation of the infinite theoretical array with the Angle of phase sweep and puts forward the ideal element model. The array consists of an infinite current sheet equivalent to an unlimited number of infinitely close units. Wheeler deduced that the radiation impedance for an E-plane electric dipole is proportional to. For H plane, the radiation impedance of an electric dipole is balanced to. Using Wheeler’s theory of the current sheet model, the bandwidth of short dipole arrays with infinite period interconnection can be deduced to approximate the theoretical value.

Generally, the entire array’s feed port impedance (or system impedance) is between 50 and 200 ohms. The designed array should be as close to the port impedance as possible in the whole frequency band. Fifty years after Wheeler proposed the current sheet model, Munk introduced a new concept that revolutionized phased array design. Essentially, Munk’s equivalent circuit model illustrates the working principle of a tightly coupled phased array. Moreover, the comparable circuit model can effectively deal with the antenna matching problem due to the existence of the ground floor.

In Munk’s analysis, the equivalent circuit model can be established under the following conditions: the imaginary array is an infinite array; the electrical size of the element is small; the array has no gate lobe; and only a few principal radiation planes (E plane, H plane) are scanned. This simplification does not prevent the use of the equivalent circuit to guide the design of the entire phased array because, on this basis, the method of periodic moments (PMM) can be used to solve the results when the array scans in other directions. An equivalent circuit is a powerful tool for analyzing the theory of tightly coupled phased arrays and realizing ultra-bandwidth performance. After the Munk equivalent model is established, multiple impedance combinations can be introduced to analyze the input impedance of the short dipole array placed on the ground floor. The total impedance value will be restricted to a small range. If the port impedance is precisely designed within or near the range of this region, a wideband antenna array can be created.

3. Design of Tightly Coupled Antenna

According to the tightly coupled antenna theory, the dual-polarized tightly coupled antenna of 15–40 GHz is designed, as shown in Figure 1. The antenna size is 3.75 mm × 3.75 mm × 2.38 mm; the spacing between the elements is 3.75 mm. The media used from bottom to top are Rogers5880, PP adhesive, RogersTMM10, and feed line using coaxial line directly, with a coaxial line inner core radius of 0.2 mm.

Its electrical performance is shown in Figure 2. It can be seen from the simulation results in Figure 2a,b that the active standing wave ratio of the dual-polarized tightly coupled antenna in the whole frequency band is less than 3 when the scanning Angle is 0~45°. The dynamic standing wave will deteriorate to a certain extent with the increase of the scanning Angle in the H-plane high-frequency band. According to the simulation results Figure 2c,d, the antenna beam coverage is more significant than ±45° at 16 GHz and 33 GHz. Figure 2e shows the variation of the gain in the whole frequency band, and that the simulation analysis can meet the system’s needs.

A double-polarized tightly coupled antenna requires G3PO connector and G3po-Sma2.62 conversion wire, which is expensive and still immature. Given such engineering problems, a single-polarized, tightly coupled magnetic dipole antenna is used instead. The antenna model is shown in Figure 3.

Its electrical performance is shown in Figure 4. From the simulation results in Figure 4a,b, it can be seen that the active standing wave ratio of the tight-coupled antenna in the whole frequency band is less than 3 when the scanning Angle is 0~45°. When the scanning Angle increases to 60°, the active standing wave of surface E and surface H deteriorates to a certain extent, especially in the high-frequency band. As can be seen from the simulation results of the directional pattern in Figure 4c–f, the antenna conforms to the typical directional pattern of the edge-emission antenna, and the beam coverage is more significant than ±40°.

The physical appearance of the Ka array antenna is shown in Figure 5, which is composed of a copper plate, multi-layer PCB, and SMP connector. The visual observation method is adopted to observe that the appearance of the Ka array antenna has no burr or damage, the copper plate has no mechanical impurities, the copper layer of the multi-layer PCB has no oxidation phenomenon, the line width, line thickness, and line distance meet the simulation requirements, and there is no heating, short circuit or open circuit. The product’s appearance has no deformation, no noticeable repair marks, or other defects. The appearance color is uniform, flat, and smooth, and the appearance is qualified.

4. Test and Simulations

4.1. Standing Wave Test

We can use the following procedure to proceed with the test:

(1)

Connect the device under test and the test instrument according to Figure 6;

(2)

Start the test instrument and preheat it for 30 min;

(3)

Set the center frequency of the vector network analyzer to 33 GHz and the frequency range to 26.5–40 GHz, and test 401 frequency points. The level is set to 20 dBm, port 1 is connected to the receiving array element, port 2 is connected to other array elements in turn, and the unconnected array element is connected to the load;

(4)

The amplitude and phase of S11 and S21 parameters are stored, and the active S parameter matrix is synthesized to obtain the standing wave of the antenna unit to be measured.

Figure 6 shows the active standing wave test results of the antenna. It can be seen from the effects that the active standing wave of the antenna is less than 3 in the frequency band of 27.5~40 GHz, which can meet the system’s needs.

Figure 6. Ka array antenna standing wave test results.

Figure 6. Ka array antenna standing wave test results.

4.2. Pattern Test

We can use the following procedure to proceed with the test:

(1)

Connect broadband horn antenna and standard horn antenna AVTAH320;

(2)

Start the test instrument and preheat it for 30 min;

(3)

Set the center frequency of the vector network analyzer to 33 GHz, frequency range to 26.5–40 GHz, level to −5 dBm, port 1 to connect the broadband horn antenna as transmitting antenna, port 2 to join the standard horn antenna AVTAH320 as receiving antenna, adjust the turntable and set the azimuth axis (theta) to −5°~+5°. Test and save S21 amplitude and phase values (horizontal/vertical polarization direction);

(4)

Take off the standard horn antenna and replace it with a Ka patch array antenna. The azimuth axis (theta) is selected from −100° to +100°, and each element’s amplitude and phase values of S21 (horizontal/vertical polarization direction) are tested and saved;

(5)

The gain values of each array of Ka patch array antenna are calculated as shown in (1), where $G_{AUT}$ represents the gain of each array of Ka patch array antenna, $G_{SGH}$ represents the gain of standard horn antenna, $P_{AUT}$ and $P_{SGH}$, respectively, represent the received power levels of each array of Ka patch array antenna and standard antenna.

$$G_{AUT}\left(dB\right)=G_{SGH}\left(dB\right)+P_{AUT}\left(dB\right)-P_{SGH}\left(dB\right),$$

(1)

The array orientation diagram of 0° and ±60° was synthesized by phase assignment. Figure 7 shows the Ka-band antenna array test scene. Figure 8a–h shows the antenna scanning direction results of 26.5 GHz, 28 GHz, 30 GHz, and 35 GHz frequency points under 2 transmitting and receiving states. It can be seen from Figure 8a,b that the antenna can perform a maximum 60° scan at 26.5 GHz. It can be seen from Figure 8c–h that the scanning gate lobe of the antenna will be improved to a certain extent when the antenna performs large Angle scanning in the frequency band above 28 GHz. The scanning gain of the whole band is rolled down to about 4.5 dBi at most, which satisfies the phased-array antenna beam scanning variation law.

5. Conclusions

According to the tightly coupled antenna theory, the dual-polarized tightly coupled antenna of 15–40 GHz is designed. The results show that the proposed antenna has good performance. The antenna can support ±60° maximum Angle scanning and maintain good active standing wave performance. The physical appearance of the Ka array antenna is also given.

Compared with the traditional phased array antenna, the compact coupled antenna designed in this paper will benefit the miniaturization of the array. The compact coupled antenna designed in this paper can be applied to the phased array to obtain the advantages of ultra-wideband, easy miniaturization, easy integration, etc. The tightly coupled antenna designed in this paper can realize ultra-wideband operation, but due to the limitation of time and hardware conditions, it has not been integrated and verified in the actual system. In addition, there is a need to further reduce the processing cost of antenna design.