Wednesday, January 5, 2011
【 Weak current College 】 for handheld devices, select the appropriate antenna 】
Antenna is affecting all wireless communication system reliability and performance is critical in a number of factors. Select to completely satisfy the system performance of the antenna is very important.
Antenna is affecting all wireless communication system reliability and performance is critical in a number of factors. Select to completely satisfy the system performance of the antenna is very important. However, today's small handheld devices, or to the antenna design engineer challenges, antenna to make thin, compact structure, performance, but also to meet various technical standards. Time-to-market and cost also manufacturers to consider two important factors. For the system to select the best antenna with two main criteria are the antenna of the electrical and mechanical characteristics. These specifications are subject to equipment design and mechanical structures. Basic electrical characteristics should be considered basic electrical characteristics is the antenna operating frequency, bandwidth, maximum gain, average gain, efficiency, return loss, voltage standing-wave ratio (VSWR), and the polarization direction, direction, side-lobe and the back flap of radiation intensity, phase than before and after, direction map, impedance and rated power. Antenna structure materials and actual radio frequency RF design also determines the antenna end electrical properties. Antenna structure materials should have very low loss and good conductivity. Operating frequency range of working frequency by the type of application. For example, Wi-Fi 802.11b/g, ZigBee and BlueTooth are using the same 2.4GHz ISM band, the band's bandwidth is about 80MHz (2.4 ~ 2.48GHz). Commercial use of L1 GPS system, its bandwidth 1.575GHz band as 2MHz (1575.42MHz ± 1MHz). GSM system using 850/1900MHz band, or 900/1800MHz band, specific uses which band depends on the appropriate area of the operator. 3 g system also uses a different frequency band, was also associated with the region. For example, Europe's WCDMA systems use 2.1GHz band.
Four-or five-band mobile phone antenna is mainly used for global roaming, as well as the realization of different communication system, communication, as shown in Figure 1. They can be in four band GSM system and W-CDMA2100 system transceiver signals to achieve in all mobile frequency bands within the interchange. Other applications also include WiMax, UWB, ISM900, ISM5/5.8GHz, DVB-H, MediaFLO, DECT, RFID, UHF, VHF, AM and FM, etc.
Figure 1 built-in, hybrid Mobile five-band antenna is typically used for global roaming and different communication technology system of communication between multiple antenna systems, such as diversity and multiple antenna systems (MIMO) is used in the need to increase the data rate of application. When using multiple antenna systems, design and characterization of the antenna system is crucial, including antennas and antenna isolation and relevance test, etc. The maximum gain and efficiency gains, the average gain and efficiency determines the antenna bandwidth and performance. These indicators, the higher the antenna bandwidth and performance. Moreover, the antenna in its operating frequency range should have sufficient VSWR. Typically, the return loss 10dB (2.0: 1.0 VSWR) or better is expected. In order to obtain the overall performance of the antenna for better phase diagram, you should consider the efficiency of the largest gain or the gain on average. But don't be stuck into the only consider one of these properties to draw a conclusion. Maximum peak gain is measured antenna directivity of a good target, but if you take it as a regular antenna performance of the main criteria would be misleading. Typically, the more complex equipment, gain more low, so that line was installed in a real device may result in losses. This is due to the high peak gain always means point to an intensity of, and may cause the antenna in a direction to gain more low, this is because in a certain phase orientation diagram direction of zero strength. People should choose a recommended DB value above there is margin for antenna to ensure it meets the actual needs of the system in your environment. Most of the wireless system has 50 Ω impedance, the antenna should, as far as possible, and this value matches, to reduce the system does not match/loss. In order to complete characterization of antenna characteristics and other factors to consider include polarization orientation (vertical polarization, the level of polarization, or circular polarization), as well as phase pattern (zy plane xz plane, and the XY-plane). Most handheld portable devices require a linear polarization antenna, and is used to cover the whole direction 360 ° Omni-directional pattern, but true Omni phase pattern only theoretical. Usually, the influence of mechanical structure of antenna phase pattern shape and direction of the phase diagram generation zero strength and direction. As shown in Figure 2, in many cases, decide the best true antenna performance is a complete 3D antenna radiation efficiency because it shows how many antenna radiation energy is converted into wave transmission went out, and because the antenna impedance mismatches caused by the loss and how much radiation loss. Particularly in small portable or handheld devices, as the actual use of the equipment may be moving in either direction, three-dimensional efficiency is higher than the maximum gain better parameters. In addition, since the maximum gain peak beam can forward the user's body and gain attenuation due to the human body. Mechanical structure layout equipment mechanical structure layout determines the required antenna size and type, and antenna size and type have limitations and decided on the electrical characteristics of the antenna. Therefore, device design of mechanical structure should also be given to the selection of the antenna. Typical characteristics of the mechanical structure is the shape factor (external or internal), structure, dimensions, installation, connection, aesthetic considerations, as well as the mechanical structure of persistence and antenna parts reliability. Equipment and antenna both constitute material selection and design of mechanical structure directly affects manufacturability. Appropriate choice for cost-sensitive, easy-to-manufacture of programmes, particularly for high-volume production. It is important to note that the larger antennas and better antenna performance is not directly related. When you implement the following design rules, compact ceramic antenna confirmed the ratio ofAccumulate greater antenna performance is much better.
Figure 2 decided a true antenna performance is to examine its overall 3-d radiation efficiency figure select built-in antenna most handheld devices need a built-in antenna, instead of an external antenna, this purely aesthetic considerations. In fact, depending on application and device features different may require more than one antenna. Built-in antenna typical structure is printed metal, PCB, plastic substrate of flexible printed circuit board, bend antenna, LTCC, ceramic antennas, antenna and a built-in four-armed plate antenna. Equipment size and mechanical structures, and electrical requirements, also determines the use of antenna structures and technologies. Most of the built-in antenna installation via SMD technology, SMT technology of Setup, spring pins, pressure spring connection, micro-U.FL/I-Pex connector or direct soldering. When using long cables, signal transmission loss in the cables are very important factors, its value may reach several decibels. If you want to use with multiple band of small handheld address these most difficult design problems, will narrow the antenna and required technical type of selection. These challenges were isolation, the smallest gain and efficiency, arm/body effects and special absorption rate. When the small handheld's multiple antenna location is very near, the isolation is becoming a very important issue. Work in the same or a similar band antenna mutual coupling occurs, performance will be low. Select antenna required minimum gain and efficiency into small devices design challenge, this is because the antenna and other parts are in close proximity, such as LCD, metal shielding shells, batteries and other electronic parts, etc. In the lower frequency range of applications, such as 850MHz and 900MHz band, the challenges also exist, this is due to the operating frequency, the lower the longer wavelength. Human impact on the handheld device will cause problems, when people take the handset closer to the body tissues, arm/body impact will create frequency detuning. Frequency of resonance will deviate from the working frequency, which is due to impedance mismatch and antenna performance decrease and make a great signal attenuation. In addition, close to the handheld device of human tissue will absorb the antenna radiation energy, block signal transmission to the open space. Similarly, because the frequency adjustable demand, SAR indicator should be taken into account. Because of the many special ceramic antenna has a high isolation characteristics and near-field radiation distribution, SAR and the influence of body/arm can be reduced to a minimum, so that even in a small handheld device can get very good isolation. Select ceramic antennas in choosing ceramic antennas, you also need to pay attention to some problems. Ceramic antenna are many different types, such as LTCC antenna, ceramic Monopole antenna and so on, in comparison to certain antenna manufacturer's unique ceramic antennas, people will find the antenna performance and the effectiveness of the application are differentiated. Design and performance indicators of consistency is important because this benefit and antenna company of cooperation, the company has many of the intellectual property rights, strong research ability and engineering capability. If you select the appropriate and in accordance with the antenna design guide, even with the correct design of miniature ceramic antennas, or you can reach 70% to 80% efficiency, 1 ~ maximum gain 2dBi, as well as better than the average gain 3dBi. All antennas must be full screen tests to ensure its quality and reliability. These tests include mechanical structure and reliability testing, such as mechanical and thermal impact tests, vibration, and extreme temperature testing and assembled in front of the mechanical connection design test. Electrical characteristics should also be tested under certain conditions, such as plastic shell package or phone cases packages after testing, close to the human head/arm model tests, etc., just like when people actually use.