Flexible PCB Antenna with Cable Integration Application Note

Transcription

1 Flexible PCB Antenna with Cable Integratin Applicatin Nte APN /C/JC Page 1 f 10

2 1. BASICS A Flexible PCB antenna with cable is a very flexible, lw prfile, highly reliable, and ecnmical slutin widely used in the wireless industry. Usually cnsisting f plyimide flexible PCB (plyimide), a trace is printed n the substrate t get the desired antenna tplgy. Varius antenna types such as mnples, diples, and printed F antennas can be made. This design applicatin nte is intended t help the antenna integratr understand the relevant parameters affecting the antenna perfrmance. Taglas recmmends that the integratr strictly fllw the guidelines in this applicatin nte. Upn yur device prttype cmpletin, Taglas can prvide further ptimizatin by ffering custm tuning and testing services fr the antenna in yur device. (See Sectin 15 n Page 8 fr further inf n Tuning.) 2. APPLICATIONS A Flexible PCB antenna with cable is suitable fr mbile applicatins where internal antennas are required, but nt much space r vlume is available. It is ideally affixed t the plastic husing f a device directly by duble-sided adhesive. Taglas Flexible PCB antennas cme in single-band r multi-band slutins starting frm 450MHz up t 6 GHz. APN /C/JC Page 2 f 10

3 3. SIZE The larger the antenna surface area (r vlume), in general, the higher the perfrmance in terms f gain and radiatin characteristics. CLR GSM PCS DCS Band VSWR Min. Return Lss db CLR GSM PCS DCS Band VSWR Min. Return Lss db SHAPE AND THICKNESS A Flexible antenna can be made int any 2D shape fr ease f fit int a prduct. Flexible PCB antennas are super lw prfile at 0.5mm. 5. GROUND PLANE EFFECTS The prximity f the antenna t the device grund plane can have a majr impact n perfrmance. Radiatin efficiency is the rati f the ttal pwer radiated by the antenna t the pwer accepted by the antenna. If the antenna is t clse t the grund plane, the radiatin efficiency and radiatin pattern can be diminished because the signal is basically getting trapped by the grund plane. In general, we recmmend at least a 20mm clearance frm the main grund-plane t maintain gd antenna efficiency. APN /C/JC Page 3 f 10

4 6. IMPEDANCE RF circuits in mbile devices shuld be designed fr a 50 Ohm characteristic impedance at the surce (RF mdule), transmissin line (PCB trace r cax cable), and lad (antenna). In practice, the characteristic impedance f the circuit is nt exactly 50 Ohms acrss all bands. This requires the antenna impedance t be changed t match the actual characteristic impedance f the circuit. This can be dne by measuring return lss with a netwrk analyzer r using TRP (Ttal Radiated Pwer) and TIS (Ttal Istrpic Sensitivity) numbers as a guide. See Sectin 15 fr mre infrmatin n tuning. 7. BANDWIDTH Bandwidth is defined as the frequency band belw -10dB return lss. Taglas PCB antennas, in general, are rated at a minimum f -7.5dB average return lss fr the targeted applicatin bands (fr example GSM 850/900/1800/1900). A return lss f belw -10dB is targeted fr the center f the bands and -5dB return lss at band edges where it rlls ff. 8. VSWR VSWR (Vltage Standing Wave Rati) is a functin f the return lss, which describes the pwer reflected frm the antenna. A lwer VSWR means the antenna has a better match t the transmissin line and mre pwer is delivered t the antenna. A VSWR f 1.0 represents an ideal match. In principle, the target is t be belw 1.5. In practice, a VSWR f 3.0 is reasnable fr multi-band antennas in challenging envirnments. APN /C/JC Page 4 f 10

5 9. GAIN The gain f the antenna is clsely linked t the surface area r vlume f the antenna. The larger the surface area r vlume f the antenna, the higher the gain. Care must be taken that clearances f at least 4mm are kept frm ther metal cmpnents in the device r metallised substances which will absrb r reflect the electr-magnetic radiatin, substantially reducing the gain. The larger the clearance, the better the radiatin characteristics f the antenna. We recmmend 20mm r mre fr best gain and radiatin efficiency. 10. EFFICIENCY Antenna efficiency is the rati f the ttal pwer radiated by the antenna t the pwer accepted by the antenna. Efficiency is a gd verall measurement f an mni-directinal antenna fr mbile cmmunicatin systems such as GSM and WLAN. A PCB antenna can be designed t have high efficiency (>30% t 50%+) if the antenna surface area is large enugh. It is mre difficult t achieve high efficiency fr mre than tw bands r in very cmpact frm factr husings. Efficiency f the antenna directly relates t the TRP/TIS results f a device in OTA testing if the radi mdule has 50 Ohm impedance. APN /C/JC Page 5 f 10

6 11. POLARIZATION Plarizatin describes the rientatin f the wave scillatin. All Taglas cellular and bradband antennas are linearly plarized t mst efficiently match with the signals bradcast frm the linearly plarized antennas munted n cellular base-statins and ht-spts. Whether it is hrizntally r vertically plarized just depends n hw it is munted frm yur frame f reference. Standing directly in frnt f the antenna, the linear plarizatin is hrizntal if the antenna is placed in a hrizntal psitin and vertical if the antennas are placed in a vertical psitin. In practice, the radiatin emitted and received by internal antennas will be crss-plarized due t reflectins frm the envirnment and scattering in the atmsphere t sme degree. APN /C/JC Page 6 f 10

7 12. ADVANTAGES f Flexible antennas with cable Adheres directly t inner shell f plastic husing f device N space needed n main-bard f device Extremely lw prfile Design Flexibility size, shape, cable, cnnectr all fully custmizable with minimum tling cst Quick turn-arund time n new designs 2 weeks t prttypes Multi-band frequencies easily accmmdated Ecnmical pricing 13. MOUNTING Taglas flexible PCB antennas use 3M 467 duble-sided adhesive fr the mst reliable munting slutin. Ideally, the Flexible PCB antenna is munted clse t the uter husing f the device t allw it t radiate utwards and receive signals withut bstructin frm internal cmpnents in the device. The antenna can be slid int a slt, screwed dwn, r affixed with dublesided adhesive. The cable exit pint can be mved t different lcatins n the antenna, if needed, please Cntact us. Generally, the rientatin f the antenna is nt critical, as lng as clearance frm metal cmpnents is maintained. APN /C/JC Page 7 f 10

8 14. ENVIRONMENTAL CONSIDERATIONS Clse prximity t cmpnents r husing affects the electrical perfrmance f all antennas. When placed within the enclsure, in mst cases, there shuld be a clearance f 20mm in all directins frm ther cmpnents, metal, and grund plane fr maximum efficiency. A reductin in antenna efficiency and a shift in tuned frequency will be bserved if these clearances are nt maintained. Prximity effects will als have an adverse effect n the radiatin pattern f the antenna. Device husings shuld never be metal r have metal materials when using internal antennas. Mst Taglas flexible antennas are designed t be placed n 2-3mm thick ABS. Slight shifts in tuned frequency may be bserved with ther husing materials r ther thicknesses. Sme husing clrants, such as carbn black, have RF absrptive prperties and can severely degrade antenna perfrmance. Husing materials that absrb misture, such as nyln, are als nt recmmended. 15. TUNING The frequencies f Flexible PCB antennas with cable are easily shifted when clse t ther cmpnents r if the cable is bent mre than 30 degrees. This phenmenn is called detuning. Taglas ffers Flexible PCB antenna samples t custmers n the understanding that they be tested utside the device as a benchmark perfrmance. The actual prductin antenna will be custmized fr each custmer. This is why Taglas always ffers a tuning service t its custmers at the prttype integratin phase. We tune the antenna t its surrunding envirnment within the custmer's device at ur lab. Samples are sent back APN /C/JC Page 8 f 10

9 t the custmer within 2 weeks f receiving the device alng with a test reprt shwing antenna Return Lss/VSWR, Gain, and Radiatin Characteristics in the custmer device. Please Cntact us fr service pricing. Further services, such as ptimizing the RF perfrmance f the whle device t enable the prduct t pass netwrk apprvals, regulatry cmpliance, r specific perfrmance targets, are ffered as well. 16. ISOLATION Islatin is a measure f cupling between tw different antennas. Fr example, in a CDMA diversity antenna plan, the target is t get -10dB islatin between the main and the auxiliary antenna. Greater islatin can be achieved by using different plarizatins n the tw antennas. Fr example, the main antenna has hrizntal linear plarizatin and the auxiliary antenna has vertical linear plarizatin. In practice, this is difficult fr Omni-directinal internal cellular antennas as there is crss-plarizatin f the waves ccurring. The typical slutin is t keep the distance between antennas as large as pssible. The antenna cables shuld nt crss ver r cme clse t the ther s antenna. Islatin testing is perfrmed by sending a signal in ne antenna and measuring the pwer f the signal at the ther antenna. There shuld be at least a 10dB difference between the transmit signal and the receive signal. The easiest methd is t keep increasing the distance between the tw antennas until the target islatin is achieved. APN /C/JC Page 9 f 10

10 17. CABLE & CONNECTOR A 1.13mm diameter micr cax cable is preferred in mst cellular antenna prjects as the mst ecnmical slutin. Cable lss is nt a big factr if cable length is kept belw 150mm. The cable needs a gd grund. Try t rute the cable as clse t the device grund as pssible. The cable shuld nt be lped because it will cause frequency shifts and create magnetic fields which will interact with the main antenna magnetic field. The cable shuld be kept away frm emitting cmpnents such as LCD driver chips r CPUs. It is preferred t use cnnectrs n the cables fr higher cnnectin reliability, as ppsed t slder. The mst ecnmical cnnectr slutin is the IPEX line f cnnectrs which is cmpatible with Hirse U.FL and W.FL industry standards. Taglas ffers any cable and cnnectr slutin fr the custmer. Taglas als ffers the n-bard mating cnnectr and cable jumpers. APN /C/JC Page 10 f 10