Detailed analysis of antenna technology in UWB communications

Analyze UWB antenna performance and design requirements

Introduce several common new ultra-wideband antennas

Requires ultra-wideband antenna bandwidth, high transmission rate, low power, low power consumption, accurate positioning, and strong anti-multipath capability

High requirements for UWB antennas in terms of dispersion, volume and directionality

In recent years, ultra-wideband short-range wireless communication has attracted great attention in the field of global communications technology. It originated in the 1940s. It was initially in the form of pulsed wireless communication, which was mainly used in the military field. It was gradually used in civilian fields since the 1990s. The so-called UWB signal is a signal that requires any relative bandwidth higher than 20% or absolute bandwidth greater than 0.5GHz (FCC definition), whose transmission rate can exceed 100Mbit/s, and meets the requirements of the FCC power spectral density limit, and its working frequency band is defined. At 3.1 to 10.6 GHz. Ultra-wideband baseband narrow pulse signal communication Compared with ordinary carrier communication or spread spectrum communication, the ultra-wideband wireless communication system has high confidentiality, low power consumption, and multi-path resistance in addition to high communication rate and ultra-wide communication bandwidth. Strong fading ability, multiple access and strong penetration capabilities [1,2]. Therefore, UWB technology has broad application prospects in the areas of radar tracking, wireless communication, penetrating obstacle imaging, weapon control systems, ranging, and precise positioning.

Because the radiated signal of UWB wireless access system has two characteristics of ultra-wide band and very low power spectral density, it has very high performance requirements for the receiving system of UWB wireless systems. Its key technologies include pulse waveform design, modulation/demodulation and Multiple access technology, fast capture of narrow pulses, synchronization and detection. In addition, the study of ultra-wideband (UWB) lines is also a very important part. The UWB antenna is still significantly different from the conventional broadband antenna. Conventional broadband antennas are mostly non-frequency-variable antennas and are therefore not suitable for transmitting and receiving UWB signals. For UWB antennas, the fixed phase center and low standing wave voltage ratio are two very important electrical indicators that determine the performance of the UWB antenna.

This article will analyze the design requirements and performance of UWB antennas, and briefly introduce several commonly used UWB antennas and new antennas.

The antenna acts as a transmitter and receiver energy converter and has a series of performance parameters, some of which are more basic.

input resistance

The ratio of the voltage and current at the antenna feed port is referred to as the input impedance of the antenna. defined as,

(1)

Among them, Pin is the input power of the antenna, Iin is the input current, Rin is the input resistance, and Xin is the input impedance.

Within this frequency range, a change of a selected antenna parameter or a set of antenna parameters is acceptable. There are directional bandwidths, gain bandwidths, input impedance bandwidths, etc. Most of the input impedance bandwidth is used. It refers to the bandwidth at which the antenna matches the transmission impedance and the reflected power is less than 10%. For an ultra-wideband antenna, its impedance bandwidth requires 7.5 GHz.

At the same input power, the square of the electric field strength of an antenna at a certain point ( The square of the electric field strength at the same point as the ideal point source antenna without loss of energy ( The ratio of ), known as the gain in the direction of this point, is usually represented by G:

(2)

The directional coefficient of an antenna refers to the ratio of the square of the electric field strength of an antenna generated at a certain point to the square of the electric field intensity of a point source antenna at the same point under the same radiated power, and is usually represented by D:

(3)

The gain of the antenna in the direction of maximum radiation is usually used as the gain of this antenna, and the coefficient of directivity of the antenna in the direction of maximum radiation is used as the directivity coefficient of this antenna. Antenna gain (G) and directionality coefficient (D) are two closely related physical quantities, and their relationship is

G=η.D (4)

Where η is the efficiency of the antenna, which is the ratio of the antenna's radiated power to the input power, ie

(5)

The efficiency of the antenna represents the effectiveness of the antenna in energy conversion.

UWB pulse communication and traditional wireless communication modulation transmission technology is fundamentally different. First of all, its frequency bandwidth is required, and the transmission rate is high; secondly, it requires low power and low power consumption; it requires it to have accurate timing positioning again, and has strong anti-multipath capability. In addition, due to the characteristics of UWB systems, the characteristics of UWB antennas are different from those of general antennas.

(1) According to the requirements of the FCC, the antenna should be able to cover a bandwidth of 3.1 to 10.6 GHz.

(2) It is not an antenna that simply accepts a single-band signal, but a typical multiple narrow-band antenna.

(3) The dispersion of the antenna is very demanding.

(4) It is mainly used in short-distance communication, and has higher requirements for volume and directionality.

In order to realize the broadband of the antenna, there are many mature technical measures. Some of these technologies are suitable for the line antenna, and some of them are suitable for the surface antenna, and some of them are suitable for use: an electromechanical method, a loading method, an impedance matching network, and an integrated method.

At present, when people are researching and designing ultra-wideband, they usually think of ways to broaden their frequency bands to design common broadband antennas, and then analyze their frequency domain and time domain characteristics to verify whether their performance is good, and thus apply to ultra-wideband. system. People adopt these methods based on the traditional theory to design a lot of excellent ultra-wideband antennas.

At present, ultra-wideband antennas mainly include TEM horns, dipoles, spiral antennas, double-cone antennas, Vivaldi antennas, etc. They have different performances and different principles. Here is a brief introduction to some of them.

The dipole antenna is the most basic antenna, essentially a narrow-band antenna, but it can significantly widen its operating frequency band by loading and other technical measures. The dipole load can be either impedance loaded or reactance loaded, and can be continuously loaded or discretely loaded in order to change the impedance per unit length of the antenna along the antenna arm according to a specified rule. In 1965, Wu and King proposed a continuous resistive loading of a dipole antenna, and thus obtained an outward traveling wave on the antenna within a very wide frequency band, which is called a Wu-King dipole antenna. The general dipole antenna is only a receiving antenna.

The basic structure of a TEM horn antenna consists of two triangular metal plates with a corner between them. TEM horn antennas are widely used in UWB applications. It has good impedance characteristics and waveform fidelity. By increasing the length of the horn and increasing the opening angle of the horn, the wave impedance of the horn will be closer to the free-space wave impedance, the reflection at the mouth will be reduced, and the distortion of the pulse waveform will also be reduced. The gain of the TEM horn antenna ranges from 5 to 15 dB. This range is used in directional base station antenna applications [3].

In 1943, Schelkunoff proposed the biconical antenna shown in Figure 1 [4]. Bi-conical antennas and other deformed antennas, including disk cones, are widely used in the ultra-wideband field.

The diameter of the vibrator of the double-tapered antenna and its corresponding distance between the two arms are kept constant so that the characteristic impedance along each point along the line remains unchanged. When the antenna is infinitely long, the input impedance is equal to the characteristic impedance of the vibrator, and the antenna is then Electrical characteristics are independent of frequency. The impedance of the double cone antenna is only related to the size of the cone angle. When the cone angle is close to 90°, the input resistance of the antenna is approximately 50Ω, and the antenna can obtain a wide impedance bandwidth.

The Vivaldi antenna consists of a narrow slot line transition to a wider slot line, which was proposed by Gibson in 1979 [5]. Its slot line changes exponentially, gradually increasing the width of the slot line on the dielectric plate to form an electromagnetic wave that the bell mouth radiates outward or receives inward. At different frequencies, different parts of it transmit or receive electromagnetic waves, and the electrical length of each radiating part with respect to the wavelength of the corresponding different frequency signal is constant. The same beam width in the design band. In addition, it also has good time domain characteristics, and the time domain received waveform has non-dispersive characteristics, so it is a very promising ultra-wideband antenna.

With the civilization of UWB systems, more and more UWB devices are used for short-range wireless communication. Such devices are usually small in size and require a simple antenna structure, small size, and low cost. In recent years, there have been many studies on antennas used in short-range wireless communications, and many new types of UWB antennas have emerged.

The microstrip antenna has the advantages of small size, easy processing, and easy integration of active devices, but it has one of the most obvious disadvantages is its narrow-band characteristics. In order to meet the requirements of ultra-wideband antennas, special microstrip antennas have been developed.

The UWB antenna is implemented using a microstrip antenna. The basic method is to use a printed monopole antenna or a modified printed dipole antenna.

The printed monopole antenna has a variety of structures, such as triangles, circles, ellipses, etc., as well as some special shapes for fractal results. The impedance and bandwidth of the distorted dipole antenna have a large relationship with the width of the vibrator. Using a printed dipole antenna, the vibrator of the antenna can be made flat, and the connection between the antenna and the feeder can be controlled at the same time. Match to achieve the purpose of broadening the frequency band. Using this method, both the directional pattern and the reflection coefficient are satisfactory. Literature [6] proposed a rectangular monopole microstrip antenna.

A slot antenna is a basic form of antenna that is grooved in metal and uses a coaxial line. Microstrip lines, waveguides, etc. are excited to generate radiation. The slot antenna replaces the vibrator antenna in the microwave to solve the shortcoming of the vibrator is too small, difficult to make and feed.

The UWB bandwidth is achieved through a slot antenna. Its basic principle is to use ultra-wideband feeds to achieve ultra-wideband performance using special wide slots such as ellipses, circles, and rectangles. The bandwidth of the slot antenna has a large relationship with the size of the slot. When using a wide rectangular gap, the aspect ratio of the gap has a lot of influence on the bandwidth of the antenna, and the major and minor axes of the elliptical gap have a greater influence on the bandwidth.

Through the research on UWB antennas, we can know that there are still some difficulties. For example, how to avoid ringing in ultra-short pulse transmission. In addition, how the UWB antenna maintains the constant gain of the entire bandwidth, improves the antenna efficiency, increases the antenna gain, and broadens the working frequency band of the antenna are also challenging tasks.

Although the development of ultra-wideband antennas currently faces many challenges, we have reason to believe that there will be greater breakthroughs in the coming years. The development of super-short-band antennas in civilian ultra-wideband and mobile communication devices will also develop in the direction of miniaturization, high efficiency, stable gain, broadband, and ultra-short fast impulse response.