MICRF001天线设计指南,Every wireless system is composed of the following five

MICRF001天线设计指南Every wireless system is composed of the following five components:? Data encoder? Baseband-to-RF transducer? Antenna system? RF-to-baseband transducer? Data decoderThis is illustrated in the block diagram of Figure 1.The MICRF001 UHF receiver IC, developed by Micrel, providesa low-cost solution for the RF-to-baseband transducerin Figure 1, for applications in the 300MHz to 440MH***requency band. Integrated and discrete solutions also readilyexist for the data encoder/decoder functions and for the baseband-to-RF transducer (commonly called the transmitter).Undeniably, of all the elements in Figure 1, the antennasystem is the most difficult to design and optimize. There areseveral reasons for this. First, many designers lack sufficientworking experience with antennas to gain an intuitive feel,especially in low-power, low-cost applications. Antenna measurementand characterization requires sophisticated andexpensive test equipment, which may not be readily available.Also, antenna analysis often relies on simplifying assumptions,which may not hold in all cases, and often leadsto measurement inconsistency.Reading this application note will not make one an antennaexpert. Antenna design and optimization is too complex anddriven by variables which are often beyond the designer’scontrol. To add insult to injury, the entire problem is furthercomplicated if the antenna is located remotely from thereceiver through a transmission line. In these cases impedancematching networks may need to be designed.Fortunately, the problem of selecting an appropriate antennais not as overwhelming as it seems. Most low power remotecontrolwireless applications are sensitive not only to range,but to cost and packaging constraints as well. And the mostappropriate antennas for these applications are fairly simplestructures. They can be easily characterized and comparedMicrel, Inc. ? 1849 Fortune Drive ? San Jose, CA 95131 ? USA ? tel + 1 (408) 944-0800 ? fax + 1 (408) 944-0970 ? http://www.micrel.comusing some basic terms. The problem is further simplifiedbecause antenna systems in these applications are usuallyconnected directly to the transmitting and receiving units.It has been determined that the best overall antenna for suchapplications is simply a “piece of wire”. Certainly no antennais less expensive, especially when the “wire” is built into theelectronic circuit board. It only remains then to choose theform factor of this “wire.” By this we mean whether the wire isstraight, coil, or a single loop. In many instances even theform factor is dictated by product packaging constraints. Forexample, when the package must be very small and completelyenclosed, a coil or loop will be the preferred choice,assuming the range constraint can also be met.The MICRF001 UHF receiver is designed to be connecteddirectly to the antennas described above and achieve rangeperformance adequate for most applications. Other highperformanceantennas exist, but cost constraints prohibittheir consideration in all but the highest-performance applications.This application note will only discuss relative performancecharacteristics of the three most popular antennas—straight wire (monopole), (helical) coil, and loop—in thecontext of what is generally important to the user (rangeperformance, size, and ease of design). For a more thoroughtreatment of the theory, consult one or more of the referencesin the bibliography.The intent of this application note is to provide the user withsufficient guidance to develop an antenna system for theMICRF001—simply, quickly, and with a reasonable degreeof performance—especially for inexperienced users. If afterapplying the concepts discussed here, rage performance stillis not adequate, further antenna optimization may be attempted;however, one should not expect significant rangeimprovements to come from these further efforts. Antennasystem optimization is closely linked to the “law of diminishingreturns.” This simply means that one can derive most of theoptimum antenna performance with a modest amount ofeffort, and some simple guidelines. Beyond this point, incrementalimprovements become increasingly costly, and yieldonly marginal range benefit.