Selecting Antennas for Low-Power Wireless Applications by Audun Andersen Table 1
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Low-Power RF Texas Instruments Incorporated Selecting antennas for low-power wireless applications By Audun Andersen Table 1. Pros and cons for different antenna solutions Field Application Engineer, Low-Power Wireless ANTENNA TYPES PROS CONS Introduction PCB Antenna • Low cost • Difficult to design The antenna is a key component in an RF system and can • Good performance is small and efficient have a major impact on performance. High performance, possible antennas small size, and low cost are common requirements for • Small size is possible • Potentially large size many RF applications. To meet these requirements, it is at high frequencies at low frequencies important to implement a proper antenna and to charac- Chip Antenna • Small size • Medium performance terize its performance. This article describes typical • Medium cost antenna types and covers important parameters to Whip Antenna • Good performance • High cost consider when choosing an antenna. • Difficult to fit in many Antenna types applications Antenna size, cost, and performance are the most impor- Chip antennas tant factors to consider when choosing an antenna. The If the board space for the antenna is limited, a chip antenna three most common antenna types for short-range devices could be a good solution. This antenna type supports a are PCB antennas, chip antennas, and whip antennas. small solution size even for frequencies below 1 GHz. The Their pros and cons are shown in Table 1. trade-off compared to PCB antennas is that this solution PCB antennas will add materials and mounting cost. The typical cost of a Designing a PCB antenna is not straightforward and usually chip antenna is between $0.10 and $1.00. Even if chip- requires a simulation tool to obtain an acceptable solution. antenna manufacturers state that the antenna is matched In addition to deriving an optimum design, configuring to 50 Ω for a certain frequency band, additional matching such a tool to perform accurate simulations can be diffi- components are often required to obtain proper cult and time consuming. performance. Figure 1. Typical antenna solutions (b) Whip antenna (a) PCB antenna (c) Chip antenna 20 High-Performance Analog Products www.ti.com/aaj 2Q 2008 Analog Applications Journal Texas Instruments Incorporated Low-Power RF Figure 2. Radiation pattern Whip antennas If good performance is the most important factor, and size and cost 345° 0° are not critical, an external antenna 15° with a connector could be a good 330° 30° solution. These antennas are often 315° monopoles and have an omni-direc- 45° tional radiation pattern. This means that the antenna has approximately 300° 60° the same performance for all direc- tions in one plane. The whip anten- na should be mounted normally on 285° 75° the ground plane to obtain best performance. For maximum econo- my, a quarter-wavelength piece of 270° 90° wire can provide an effective solu- tion. Antenna parameters 255° 105° Some of the most important things to consider when choosing an 240° 120° antenna are: the radiation pattern, antenna efficiency, and antenna bandwidth. 225° 135° Radiation pattern and gain Gain = 5.6 dBi Figure 2 shows how the radiation Scale = 4 dB/div 210° 150° pattern from a PCB antenna varies Frequency = 2.44 GHz 195° 165° Horizontal polarization 180° in different directions in the plane of the PCB. Several parameters are important to know when interpret- ing such a plot. Some of these parameters are stated in the lower left portion of Figure 2. Figure 3. Spherical coordinate system In addition to the plot information, it is important to relate the radiation pattern to the positioning of the Z antenna. Radiation pattern is typically measured in three ≥θ orthogonal planes, XY, XZ and YZ. It is possible to perform XY = 90° full 3D pattern measurements, but it is usually not done XZ≥ ϕ = 0° because it is time consuming and requires expensive equipment. Another way of defining these three planes is θ YZ≥ ϕ = 90° by using a spherical coordinate system. The planes will then typically be defined by θ = 90°, ϕ = 0° and ϕ = 90°. Figure 3 shows how to relate the spherical notation to the three planes. If no information is given on how to relate the directions on the radiation pattern plot to the position- Y ing of the antenna, 0° is the X direction and angles increase towards Y for the XY plane. For the XZ plane, 0° is in the Z direction and angles increase towards X. For the YZ plane, 0° is in the Z direction and angles increase towards Y. ϕ The gain, or reference level, usually refers to an isotropic radiating antenna, which is an ideal antenna with uniform X radiation in all directions. When an isotropic antenna is used as a reference, the gain is given in dBi or specified as the effective isotropic radiated power (EIRP). The outer smaller circle, the emission level is reduced by 4 dB. circle in Figure 2 corresponds to 5.6 dBi and the 4-dB/div Compared to an isotropic antenna, the PCB antenna will label in the lower left means that for each progressively have a 5.6-dB higher level of radiation in the 0° direction. 21 Analog Applications Journal 2Q 2008 www.ti.com/aaj High-Performance Analog Products Low-Power RF Texas Instruments Incorporated As shown by Equation 1, antenna gain, G, is defined as Bandwidth and impedance matching the ratio of maximum-to-average radiation intensity multi- Two common methods to determine antenna bandwidth plied by the efficiency of the antenna. are: 1) measuring the radiated power while stepping a P P U carrier across the frequency band of interest, and 2) mea- GeD=×=×=×rad D rad max , (1) P P U suring the reflection at the antenna feed point with a net- in in avg work analyzer. Figure 4 shows the first method which is where Umax is the maximum radiation intensity, Uavg is the measurement of radiated power from a 2.4-GHz antenna average intensity, and the ratio of these two values is that has approximately 2-dB variation in output power known as directivity, D. Ohmic losses in the antenna ele- across the 2.4-GHz frequency band and has maximum ment and reflections at the antenna feed point determine radiation near the center of this band. This measurement the efficiency, e, which is simply the radiated power, Prad, was done by stepping a continuous-wave signal from 2.3 divided by the input power, Pin. High gain does not auto- GHz to 2.8 GHz. Such measurements should be performed matically mean that the antenna has good performance. in an anechoic chamber to obtain a correct absolute level. Typically, mobile systems require an omnidirectional radia- However, this measurement can be very useful even if an tion pattern so the performance will be about the same for anechoic chamber is not available. any antenna orientation. For an application where both Measurement in an ordinary lab environment can give a the receiver and the transmitter have fixed positions, relative result that shows if the antenna has optimum per- higher performance can be achieved when the antennas formance in the middle of the desired frequency band. are positioned to direct their high-gain lobes toward each The performance characteristics of the receiving antenna other. being used to conduct the measurement will affect results. To accurately measure an antenna radiation pattern, it Therefore, it is important that this antenna has approxi- is important to measure only the direct wave from the mately the same performance across the measured fre- device under test and avoid reflecting waves that could quency band. This precaution will help ensure that the affect the result. To minimize picking up reflected energy, observed relative change in performance across the measurements are often performed in an anechoic cham- measured frequency band is valid. ber or at an antenna range. Another requirement is that The second method to characterize antenna bandwidth the measured signal must be a plane wave in the far field is to measure the reflected power at the antenna feed of the antenna. The far field distance, Rf, is determined by the wavelength, λ, and the largest antenna dimension, DIM, as shown by Equation 2. Since the size of anechoic chambers is limited, it is common to test large, low- Figure 4. Bandwidth of a 2.4-GHz antenna frequency antennas in outdoor ranges. 2DIM× 2 10 R = (2) f λ –0.45 dBm 5 at 2.442 GHz Polarization 0 Polarization describes the direction of the electric field. All electromagnetic waves propagating in free space have –5 electric and magnetic fields perpendicular to the direction –10 of propagation. When considering polarization, the electric- 2.4-GHz field vector is usually described and the magnetic field is –15 Band ignored because it is perpendicular and proportional to –20 the electric field. To obtain optimum performance, the receiving and transmitting antenna should have the same Output Power (dBm) –25 polarization. In practice, most antennas in short-range applications will produce a field with polarization in more –30 than one direction. Reflections change the polarization of –35 en electromagnetic wave. Since indoor equipment experi- –40 ences a lot of reflections, polarization is not as critical as it 2.2 2.34 2.48 2.62 2.76 2.9 is with equipment operating outside with line-of-sight Frequency (GHz) limitations. 22 High-Performance Analog Products www.ti.com/aaj 2Q 2008 Analog Applications Journal Texas Instruments Incorporated Low-Power RF point. Disconnecting the antenna and connecting a net- Antenna reference designs work analyzer with a coax cable to the antenna allows Texas Instruments (TI) offers a wide range of RF products such a measurement.