Bluetooth Antenna Design National Semiconductor Application Note
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Bluetooth Antenna Design
Bluetooth Antenna Design
National Semiconductor Application Note 1811 Sebastien Mathieu March 2008
This application note intended designers using LMX5251 LMX5252 Bluetooth® radio chips LMX9820A LMX9830 Bluetooth modules. Antenna design various applications described along with theory, matching circuit description, suppliers examples. structure that resonant 2.45 with bandwidth more than efficiency >50% considered Bluetooth antenna. Therefore, countless variety antennas used, they application-specific. Some common types are: Wire Monopole This consists simple wire soldered from which against ground plane. trimmed resonant 2.45 provides good performance high efficiency. disadvantage this antenna that profile because projects above PCB. PIFA Printed Inverted Antenna like monopole printed PCB, ground point feed point along main resonant structure. Helix Similar wire monopole, except that coiled around central core (usually air) making
physical dimensions smaller. provides excellent performance, projects above PCB. Ceramic Surface mount dielectric antennas smallest types antennas available, because they printed high-dk ceramic slab, which makes electric field concentrated allowing antenna made small while keeping high resonant frequency. This application note only describes PIFA ceramic antennas because they most common, low-profile, smallest, inexpensive types available.
Theory
Printed surface-mount antennas have certain common properties. Area around beneath radiating element must kept copper-free. ground plane must placed side radiating element. Bandwidth >100 with VSWR <2.5 efficiency >60%. antenna will detune object placed close near field). This effect pulling frequency, which must retuned 2.45 GHz.
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Bluetooth® registered trademark Bluetooth SIG, Inc. used under license National Semiconductor Corporation.
2008 National Semiconductor Corporation
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oscillating constantly accelerating charge critical producing propagating waves. static non-accelerating charge will result non-propagating electric field. this
only condition radiation. example, consider printed element microstrip, shown Figure
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FIGURE Fringing Field With Full Ground Plane fringing field around microstrip ground plane directly underneath substrate will confined small area. network analyzer connected feed point, would indicate high VSWR narrow bandwidth. This means very little radiation being emitted from microstrip element. increase radiation emission achieve greater bandwidth, ground plane must moved away from microstrip element which makes fringing field cover more distance, shown Figure should noted that ground plane moved far, then fringing field stops altogether, there radiation. Therefore, position size ground plane vital design good radiator.
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FIGURE Fringing Field With Partial Ground Plane antenna could imagined impedance transformer, transforming impedance microstrip line (50) that free space (377), which allows power transferred from guided wave free-space wave. radiation pattern from such antennas which physical size much smaller then wavelength almost symmetrical directions, shown Figure pattern controlled only when similar greater than
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FIGURE Antenna Radiation Pattern
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Input return loss when viewed network analyzer looks like that shown Figure with full band covered with VSWR This gives very good matching into antenna,
however real conditions when antenna detuned handling placement components close VSWR typical.
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FIGURE Return Loss
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Layout
PIFA ANTENNA typical length 2.45-GHz resonant printed antenna depending thickness substrate dielectric constant. Copper clearance required around radiating element which from point along shown Figure position feed used control input impedance into antenna. ground plane
quired side antenna approximately wide. were smaller, will start reduce bandwidth input. Good design practice have three-element matching network going into feed, give some additional tuning ability required. obtain exact dimensions design, input impedance bandwidth would have simulated over frequency band using antenna simulation package. Alternatively antenna manufacturer contacted that capability make such design.
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FIGURE Printed Inverted-F Antenna (PIFA) PIFA placed edge motherboard PCB, shown Figure area around corner kept copperfree, components such shielding that come close PIFA pull frequency. This retuned milling radiating element. LMX5251/ LMX5252 surrounding components need shielding unless they very close radiating element.
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FIGURE PIFA Antenna Placement
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CERAMIC DIELECTRIC ANTENNA ceramic dielectric antenna smaller than PIFA other antenna because active element wound around high-dk ceramic slab, which concentrates elec-
tric field. with PIFA, copper-cleared area ground plane required, shown Figure smaller ground plane used, expense bandwidth efficiency.
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FIGURE Ceramic Dielectric Antenna Placement example antenna from Mitsubishi with details ceramic element dimensions shown Figure
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FIGURE Typical Chip Dimensions
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land pattern required mounting shown Figure
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FIGURE Typical Chip Footprint ceramic dielectric antenna behaves similarly PIFA, that detuned, symmetrical radiation pattern, efficiency approximately 70%. EXAMPLES 2.4-GHz PIFA ANTENNAS
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FIGURE LMX5251 PIFA Antenna
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FIGURE LMX5252 PIFA Antenna LMX9820/A ANTENNAS LMX9820 LMX9820A packaged shielded LTCC modules, approximately size. design antenna very similar that LMX5251/LMX5252, shielding makes differences. First, metal shield protects components module from electric field antenna, possible place LMX9820A much closer antenna element. Second, shielding also acts ground plane, less unpopulated ground area required around radiating element. example using ceramic dielectric antenna shown Figure module placed close radiating element. electric field from antenna couples surface shielded enclosure producing propagating radiation. shield well-grounded, there will adverse effects components inside.
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FIGURE LMX9820A Antenna
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LMX9830 Antenna LMX9830 smaller than 9820/A, approximately 9mm, however unshielded within plastic package there some important changes that need taken into account. cannot placed close antenna
tive element, else E-field give rise unwanted coupling effects, also E-field from antenna element will couple though main body module ground plane underneath. ground plane under module therefore important.
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FIGURE LMX9830 Antenna
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Matching
Purchased antennas, such surface-mount ceramic dielectric antennas, will matched input impedance with return loss <-7dB over bandwidth, centered 2.45 GHz. However, this only measured manufacturers test board, free space. Taking this antenna putting application PCB, which ground-plane layout different there detuning components such filters placed nearby, will pull resonant frequency antenna away from 2.45 GHz. antenna therefore needs matching correct frequency. This achieved means three-element network, placed input antenna. Usually capacitor pair inductor, inductor pair capacitor, will give sufficient tuning ability.
There three steps matching: network analyzer must calibrated accurately with electrical delay removed. impedance measurement from 2.300 2.600 return loss Smith-Chart plot. Matching placing capacitors and/or inductors onto impedance changed. NETWORK ANALYZER CALIBRATION network analyzer should calibrated S11, one-port only measurements using open, short, load standard provided. flat line should obtained when standards removed shown Figure
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FIGURE Return Loss, Connection Before soldering semi-rigid cable PCB, connected network analyzer cable, electrical delay adjusted with semi-rigid cable shorted. short cable attachment (less than cm), otherwise electrical delay will long. Electrical delay adjusted until measuring perfect short Smith Chart shown Figure
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FIGURE Smith Chart, Perfect Short MEASUREMENT Attach semi-rigid cable ground point close cable. When measuring input impedance antenna, important have setup wooden non-detuning surface keep your hands away from setup, otherwise measurement will incorrect. example typical return loss measurement shown Figure
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FIGURE Return Loss, Antenna Connected this example, resonant frequency antenna high. desired frequency, return loss only -3.3
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FIGURE Smith Chart, Antenna Connected TUNING IMPEDANCE After taking accurate measurement input impedance, tweaked using matching components three-element network. Marker impedance transformed shown Figure
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FIGURE Impedance Transformation Starting from impedance point that needs matched (point 1.8-nH series inductor move from point shunt inductor will transform impedance point center chart, which normalized
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impedance point. matching network shown Figure
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FIGURE Impedance Matching Network This only theoretical matching circuit. reality, inductors have parasitic resistance capacitance, impedance will transformed cleanly shown Smith Chart. Also, exact values shown above available standard kit. Some trial error required exact match required.
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PI-NETWORK MATCHING popular type matching network PI-network, consisting shunt components with series component middle. This provides flexibility retuning detuned antenna. Even though only components normally
used matching load source, allows putting shunt component either before after series component. example, consider data point Smith Chart: (10.2 j30.1) shown inFigure
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FIGURE Single Frequency Data Point There methods matching load. first technique move around constant resistance circle from position adding series capacitance then from around constant conductance circle adding shunt capacitance, shown Figure
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FIGURE After Resistance-Conductance Tuning
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second technique move around conductance circle then resistance circle adding shunt series capacitance respectively, shown Figure
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FIGURE After Conductance-Resistance Tuning matching networks methods shown Figure
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FIGURE Possible Matching Networks allow both types matching, PI-pad must used with redundant bridged using zero- link. However, have only matched single-point frequency case real passive device such
antenna, entire Bluetooth band matched closely possible least three frequency points have matched, shown Figure
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FIGURE Broadband Match difficulty with making broadband match frequency point that other will even further out! example, 2.483 brought closer adding shunt capacitor, 2.400 point will move around conductance circle creating even larger mismatch lower frequencies. compromise must found that will suit entire band. This normally involves using simulation software such HP-ADS (advanced design systems). this possible, then manual tweaking needed concentrating center frequency point. First, input impedance detuned antenna measured using network analyzer saved S-parameters block, i.e. frequency points impedance points across Bluetooth band. This then entered into along with network, shown Figure
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FIGURE Network
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make model more realistic, more effective real components with added parasitics rather than just pure
inductance capacitance. models components with parasitics shown Figure
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FIGURE Component Models Data parasitic values obtained from component manufacturer. Starting with best possible values used match 2.445 GHz, model entered into ADS, optimization procedure reduce much possible iterative steps from 2.400 2.483 GHz. simulation finely tweaks PI-pad component values measures S11. lower, then components tweaked again same direction until best optimized solution found. same procedure used larger matching networks even active networks, which yield better results. However larger circuits will have higher insertion loss parasitic resistance present within components. MATCHING NON-50 ACTIVE SOURCE/LOAD IMPEDANCE source load impedances both non-50, then they matched much same impedance shown Figure
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FIGURE Matching Non-50 Impedance this example load source j35. series capacitance shunt inductance required transform load impedance that source. Because these non-50, they anywhere Smith Chart must measured using vector network analyzer (VNA) determine their exact value. Measuring input impedance receiver passive antenna simple. calibrated will display value screen. However, when measuring output impedance power amplifier transmitter, this technique cannot used because power being transmitted will completely disrupt reading. technique conjugate matching using variable load must used. Figure variable load attenuator provide desired load impedance Therefore, influence output power which measured using power meter attached coupled port directional coupler. When perfect conjugate match applied output power measured power meter will maximum, which signifies best power transfer conditions.
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FIGURE Setup Determining Output Impedance When best power transfer achieved, variable load attenuator fixed that their impedance cannot changed. detached from directional coupler attached input directional coupler measure input impedance this point. measured impedance conjugate output impedance impedance measured then output impedance will definition. Definition: term conjugate match means that direction from junction impedance then opposite direction impedance will condition maximum power absorption load, which impedance seen looking toward load point transmission line complex conjugate that seen looking toward source. 4.5.1 LMX5252 Impedance Match case LMX5252 where impedances different slightly options available antenna designer, either assume point simpler design, which case small miss-match will result causing small degradation Tx/Rx power. make matching network described above between non-50 points. first instance where input/output impedance assumed power loss that will results follows; Worse case input impedance VSWR this point Reflection coefficient Return Loss 10LOG(S11) Through transfer coefficient SQRT(1-[S11]^2) 0.98 Power transferred [S21]^2 0.96 Meaning that power received antenna will transferred receiver even with this miss-match. achieve higher power transfer efficiency than this non-50 must used described above.
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Interference Rejection
FILTERING additional function passive components front antenna provide filtering. They used create 83-MHz pass-band window centered 2.44 rejecting unwanted signals outside band that impair received signal quality. Figure shows such
filter would look displayed Network Analyzer. three main features: within pass-band unwanted Insertion Loss (IL) which attenuates transmit receive signals, outside pass-band wanted rejection which attenuates interference, edges pass-band filter roll-off which should steep possible form sharp cut-off between pass-band rejection-band.
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FIGURE Filter Performance 5.1.1 Filter Types simplest type passive filter capacitor inductor series. visualize this works, consider response single capacitor inductor series frequency sweep. Looking capacitor response Figure very high frequency increases decreases. inductor opposite behavior; very this increases frequency increases. frequency which capacitor inductor response changes will dependent value, rate which changes will dependent quality factor. High Q-factor means rapid response change (steep roll-off) given frequency. selecting correct value capacitor inductor, filter formed desired frequency. important note that higher frequency filter, lower will Q-factor hence roll-off. 1-pF capacitor series with 3.3-nH inductor forms filter with center frequency 2.44 GHz. Using high-Q components yields better roll-off out-of-band rejection.
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FIGURE Filter Response
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However, even well designed filter does provide very good roll-off out-of-band rejection. Typically, will provide rejection below above GHz, however interference signals will through closer frequencies. better more expensive solution ceramic chip filter. These purchased from manufacturers such Murata M/A-COM. example Murata filter LFB212G45SG8A166. Table 1lists electrical specifications. TABLE Chip Filter Specifications Specification Nominal Center Frequency (fo) Bandwidth (BW) Insertion Loss Insertion Loss Attenuation (AbsoluteValue) Attenuation (AbsoluteValue) Attenuation (AbsoluteValue) Attenuation (AbsoluteValue) Ripple VSWR Value 2450 max. 25°C max. +85°C min. min. 1710 1910 min. 2110 2170 min. 4800 5000 max. max.
blocking signal stepped intervals from 12.75 GHz. Several thousand test points used, each these points error rate (BER) wanted signal must remain under 0.1%. total exceptions allowed, because very difficult pass test points. Failures insufficient front-end filtering, either direct saturation front low-noise amplifier (LNA) able tolerate more commonly mixing products entering pass-band. Unwanted products caused blocking signal mixing with harmonics other signals present near front end, such clocks local oscillators. eliminating interfering signal using filtering, blocking failures reduced. Good layout techniques also help avoid mixing products. 5.2.1 Blocking Qualification Testing During Bluetooth qualification, Bluetooth Qualification Task Force (BQTF) uses TS8960 test link device under test (DUT) place fixed 2460-MHz receive channel. signal generator combiner used produce interfering signal. whole setup controlled with automated test equipment (ATE), because there several thousand points test. This takes days continuous measurements. Failures counted when exceeds 0.1%, however times certain blocking frequencies goes high that link dropped, link must initialized before testing resume. When this happens, more failing frequencies reported. responsibility manufacturer test these failing frequencies manually determine whether additional filtering required. During link failure re-establishment, system sometimes logs more failures than actually present, manual testing will also confirm whether these failures genuine. RECOMMENDED FRONT-END LAYOUT MATCHING front-end layout shown Figure Figure recommended provide best matching filtering while same time providing flexibility modifying circuit needed meet Bluetooth testing requirements. Figure ceramic filter blocking capacitor with good outof-band rejection, dimensions shown here non-exact. Alternatively layout shown Figure maybe used form simpler cheaper filter allow matching antenna.
Characteristic Impedance (Nom.) Power Capacity Min. Operating Temperature Max. Operating Temperature -40°C +85°C
slightly worse than filter, out-ofband rejection significantly better dB). filter approximately size, rated over full automotive temperature range (-40 +85°C). noteworthy unwanted feature in-band ripple. specification which means that varies within pass-band. ripple will worse input/output impedances presented filter deviate from this will give rise variable sensitivity output power across band. BLOCKING Bluetooth receiver compliance test measures receiver performance under effect strong out-of-band interfering signal. wanted signal 2460 above reference sensitivity, interfering signal applied levels shown Table TABLE Blocking Signal Level Frequency Interfering Signal Frequency MHz2000 2000 MHz2400 2500 MHz3000 Power
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FIGURE Front-end Layout Using Ceramic Filter
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FIGURE Front-end Layout Using
Antenna Vendors
Table lists vendors off-the-shelf antenna products custom designs. TABLE Antenna Vendors Vendor gigaAnt Products Contact Information
Small ceramic chips larger gigaAnt Ideon Science Technology Park S-223 Lund Sweden surface-mount antennas suitable mobile phones, headsets laptops, printers, etc. example, 3030A5839-01 Leftside, Phone:+46 Web: www.gigaant.com E-mail: 3030A5887-01 Rightside. info@gigaant.com
Mitsubishi Materials
Small ceramic chips surface-mount. Mitsubishi Materials Corporation Advanced Products Strategic Suitable mobile applications Company Sales Group, Electronic Components 1-297, Kitabukurocho, Omiya-ku Saitama-city, Saitama, 330-8508 Japan Phone: 5991 Fax: 5562 Web: www.mmc.co.jp E-mail: devsales@mmc.co.jp
Tyco Electronics
Large high-gain printed antennas applications such access points.
Tyco Antenna Products/Rangestar Metro Park Rochester, 14623
Small ceramic chips surface-mount. Phone: (585) 272-3103 Fax: (585) 272-3110 Web: Suitable mobile applications www.rangestar.com
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Vendor Centurion
Products surface-mount antennas various applications.
Contact Information Centurion Wireless Technologies 82846 Lincoln, 68501 Phone: (402) 467-4491 Fax: (402) 467-4528 Web: www.centurion.com E-mail: sales@centurion.com
Murata
surface-mount antennas example, series ANCM12G45SAA072 ries ANCW12G45SAA110TT1.
Murata International Sales Dept. 3-29-12 Shibuya, Shibuya-ku Tokyo 150-0002 Japan Phone: 5469 6123 Fax: 5469 6155 Web: www.murata.com E-mail: intl@murata.co.jp
Comparison Summary
Table compares features different antenna types. TABLE Antenna Comparison Antenna Type Stub helix monopole Surface-mount ceramic chip Performance Profile Cost Physical Size antenna approximately long, projects. Does need ground plane function. Element approximately long, needs ground area clearance around active region. Element approximately long, needs ground area clearance around active region.
Good bandwidth High: Projects from side High efficiency, does require matching network. Reasonable performance Low: machine g/4. Small bandwidth mounted during assembly, more than thick reduced efficiency. become detuned during handling Medium
Printed inverted-F Reasonable performance Lowest: Printed other printed g/4. Small bandwidth types reduced efficiency. become detuned during handling
Points Consideration
Many types antennas available. Antenna type chosen application. Larger antennas generally have better performance than smaller ones. Ground plane always required with printed ceramic antennas. Cannot metal objects such crystals close antenna without causing detuning. case phone other device will also detune antenna, some tuning adjustment ability needed.
Matching elements have parasitic values that affect their quality; this possible simulate using simple simulation software. Some trail error therefore required when performing match sophisticated simulation tool must used. LMX9820A shielding will ground plane antenna placed correctly. LMX5251/LMX5252 radio chip must shielded from strong electric field only placed close radiating element. Shielding will improve performance always required.
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Examples Antennas Used With LMX5251/LMX5252
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FIGURE Ceramic Chip Antenna Industrial Remote Control with External
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FIGURE Printed Antenna-Monopole Yagi-Array Off-Board Navigation Using
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FIGURE Ceramic Chip Antenna Intelligent Remote Access Lock
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FIGURE Ceramic Chip Antenna Distance Meter
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FIGURE External Antenna-Helix Monopole Automotive Integrated Hands-Free
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FIGURE Printed PIFA Antenna Automotive Hands-Free
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FIGURE Surface-Mount Chip Antenna (Phycomp AN-2700 Murata ANCM12G) Automotive Hands-Free
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FIGURE Surface-Mount Chip Antenna (Mitsubishi Materials 1403) Electronic Whiteboard
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10.0 Popular Antenna Types
Ceramic chip antennas (Mitsubishi, gigaAnt) most popular types being used Bluetooth products. These cost about cents/unit.
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FIGURE GigaAnt Rufa Antenna
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FIGURE Mitsubishi AHD1403 Surface-Mount Antenna second most popular type PIFA. These have lowest cost because they consist trace, larger more design-intensive.
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Bluetooth Antenna Design
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