AN105: 315 MHZ INTERNAL ANTENNA DESIGN
Contents
1. APPLICATION NOTE 105 1 Green Smart Wireless enocealy ABSTRACT This guide is intended to provide a brief overview of the key considerations when selecting an antenna or looking to implement a custom antenna design Contains additional informa tion to STM 300C TCM 3x0C user manuals especially for designing specific 315 MHz internal antennas overview and specific antenna solutions are presented 1 1 References 1 2 3 4 5 6 7 A User Manuals STM 300C http www enocean com en enocean_ modules 315mhz stm 300c TCM 300C http www enocean com en enocean modules 315mhz tcm 300c TCM 310C http www enocean com en enocean modules 315mhz tcm 310c ANOO1 RANGE PLANNING ANO0OO07 FINAL UNIT TEST FOR RADIO PERFORMANCE AN101 Power Supply Layout Layout considerations for Line Power Supplies AN102 Basic Antenna Design Considerations for EnOcean based Products AN103 EXTERNAL PASSIVE ANTENNAS AN507 Radio Approval Of Finished Units Please make sure that you always use the current EnOcean documentation Up to date documentation download http www enocean com 1 2 Revision History NO Major Changes APPLICATION NOTE 105 Green Smart Wireless eEnocean 315 MHZ INTERNAL ANTENNA DESIGN 2 IMPORTANCE OF ANTENNA DESIGN While the antenna is one of the most important aspects of RF design it is also probably the most overlooked part of an end product The range and performance o2. In other implementation cases the modules are not 7 directly matched with this helical antenna Thus a matching filter Co has to be placed between the 50 Q output of radio module and Fig 3 antenna The impedance can be easily matched using two passive components capacitance and inductivity or two inductivities Never the less it is recommended to place three components for matching filter one in series and two parallel connected to GND This M filter structure is shown in Fig 4 315MHZ_HELIX L1 RF_50R GND GND Fi g 4 Matching filter schematics Normally L2 is not required thus it will be positioned only in layout but not in real assembly L2 is needed if special antenna matching is required due to housing properties if the design rules given in following chapter cannot be fulfilled The matching elements for orthogonal design Please use RF_Whip output no additional matching in this case The matching elements for integrated antenna design L1 56 nH L3 15 nH 4 L2 not assembled The given matching is in a first approximation a compromise for best performance matching review based on measurements with network analyzer if available under end product use case conditions is required Two realization examples based on the EnOcean 315 MHz helical antenna are shown below Blue area is one of PCB layers reserved for GND flooded fully Please do not place any traces in GND layer Use four layers design the mi
3. application conditions and match it correspondingly You could even realize a de fined metallic underground in your design to reduce the later application specific en vironment dependency E Antenna size defines working characteristics For antennas not discussed in this applica tion note o AS a general physical rule the antenna s efficiency is directly proportional to its vol ume while the dimension of an antenna including the ground plane is directly re lated to the wavelength The smaller the antenna volume the lower the antenna effi ciency will be EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 10 13 APPLICATION NOTE 105 Green Smart Wireless enocean 315 MHZ INTERNAL ANTENNA DESIGN 6 ADDITIONAL NOTES SEE ALSO 5 E Never forget the ground plane is the other half of your antenna more is better E Do not use metallic housing around antennas Most problems with 315 MHz antennas usually involve its intrinsic size respectively insuffi cient free space around the antenna If the available space is sufficient use a monopole for best efficiency Advantage you don t need any additional FCC device approval because the STM 300C TCM 3x0C modules already have modular approvals If not consider a helical alternative In a small housing a well designed helical can achieve better performance than a poor Shaped standard monopole w
4. the antenna shall be internal and small as possible There is no ideal antenna solution for all use cases and no antenna has at the same time small dimension high gain and large bandwidth Appropiate internal Antenna types Comparison Table Monopole Standard modular approval available low Quite large at 315 MHz not always easy to i cost good efficiency if properly integrated lintegrate no matching needed as PCB version reproducible and stable Good efficiency smaller very good overall Higher costs compared to the monopole compromise for 315 MHz size and Narrow band therefore easily detunable performance specific matching needed FCC device approval required with one exception see 4 2 2 PCB Integrated no additional component Large PCB area needed at 315 MHz Antenna environmental dependent EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 2 13 APPLICATION NOTE 105 Green Smart Wireless eEnocean 315 MHZ INTERNAL ANTENNA DESIGN 3 LOCAL REQUIREMENTS 3 1 USA Generally EnOcean based radio products fall under the category of RF Products for unlicensed operation In the United States this device group is regulated under FCC Federal Communica tions Commission CFR47 Part 15 http edocket access qpo gov cfr 2007 octatr pdf 47cfr15 231 pdf The 15 231 regulates unlicensed transmitters in the 315 MHz band The content of da
5. the specified modular approval one please consider that for best performance trans mitter settings have to be optimized for each specific device antenna during approval procedure The efficiency of the receiver s antenna is critical to maximize range perform ance Unlike the transmitter antenna where legal operation or energetic considerations may mandate a reduction of transmitted power the receiver s antenna efficiency should be optimized as much as is practical possible Background for transmitters e g sensors with poor antenna efficiency a higher output transmission power could enhance the link budget as well for a very efficient transceiver antenna like an external antenna needed for devices unfavorably mounted behind metal Shielding a lower output transmission power would be mandatory to comply with the ap proval while the receiver side still fully profits from the increased antenna efficiency E If needed also consider using of external 50 Q antennas if the use case or location re quires allows TCM 300C provides 50 Q output and TCM 320C soldering pads for an alter native SMA connector Please note that the original whip antenna and a SMD capacitor must be removed in this case for more details consult user manual indications and EnO cean additional appnotes 2 and 6 EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 11 13 APPLICATION NOTE
6. to 0 25 mW 6 dBm It is less than devices are allowed to transmit in North America The transmission power is radiated power over antenna The other limitations of Japanese regulations mostly not impacted by antenna design are Spurious emissions are limited to 36 dBm below 1 GHz and to 30 dBm above 1 GHz The 99 occupied bandwidth is limited to 1 MHz but additionally upper edge shall not exceed 315 25 MHz This limits occupied bandwidth to maximum of 500 kHz if transmit ter middle frequency is exactly 315 MHz transmission time control e Duration of transmission and suspension for radio equipment which transmission is not periodic duration of transmission 5 sec or less However 90 sec or less for radio equipment which transmission is manually performed e Periodic transmission duration of transmission at a time 1 sec or less duration of transmission suspension 30 times or more of transmission duration and 10 sec or more However in the case this radio equipment is used only to ensure a Safety driv ing of automobiles or other vehicles Additionally the radio equipment shall be inside a case in this case an antenna terminal is not installed and cannot be opened easily Thus modular approval is not possible 3 4 Other Countries For usage in not mentioned countries please consult local regulation authorities APPLICATION NOTE 105 Green Smart Wireless enocean 315 MHZ INTERNAL ANTENNA DESIGN 4 RECOM
7. 105 Green Smart Wireless enoceal 7 PERFORMANCE VERIFICATION The antenna for transmitter and receiver can be measured using the same measurement pro cedures due to reciprocity principle Generally the transmission performance is easier to be measured 7 1 Transmitter radiation pattern measurements The transmitter radiation pattern measurements should be done with complete device The best approach to measure device transmission pattern is to do it in reflection loss environ ment which could be anechoic chamber or 3 10 m free space open air measurement place The setup must be calibrated to perform reliable measurements For calibration the refer ence transmitter antennas are used The DUT must be therefore programmed to transmit Carrier Wave CW signal please con sult AN507 7 The radiation pattern measurements are done by rotating DUT 360 in several orientations captured data is used to plot the charts The transmitter power should be close below allowed FCC limits especially in the direction away mounting side The lowest acceptable EnOcean transmitter power should be no more than 10 dB below the maximal approved limits In ideal case not only completely assembled device should be measured The measurements of devices in real application environments deliver knowledge of transmitter reliability in real world As the walls windows or ceilings could be very difficul ty fit in a low reflection location it is recomme
8. MENDED ANTENNA TYPES FOR 315 MHZ SHORT RANGE DEVICES 4 1 Monopole The most simple effective and low cost antenna is called wave monopole and is the common antenna on mobile devices e g earlier generations of cell phones Typical anten na material is a simple wire A monopole antenna provides good overall performance and Stability has an isotropic pattern and a wide bandwidth Antenna length is proportional to wavelength the lower the operational frequency the longer the antenna At 315 MHza 1 4 wave monopole is 23 8 cm long The monopole can be shortened and or bent and or traced above the ground plane tilted whip With such techniques it will be more compact but less efficient less power will be radiated it means working range loss When the antenna cannot be straight and perpendi cular to the ground plane the distance from antenna to conducting structures is crucial The antenna wire shall be at least 2 5 cm more is better away from conducting structures and the end of the antenna not soldered away from GND The wire can be made solid or flexi ble recommended with insulation EnOcean modules with FCC modular approval 40 8 tinned typically use a 150 mm shortened monopole 7 wire antenna as a compromise for easier inte gration This antenna has in best case a 5 dBi gain This is however compensated in transmit ter operation by appropriate generated Tx pow er settings of EnOcean module Please consult with EnOcean App
9. PE a my gt d Halfloop over PCB EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 6 13 APPLICATION NOTE 105 Green Smart Wireless enocean 315 MHZ INTERNAL ANTENNA DESIGN 4 2 Normal Mode Helical Antenna A helical element is a wire coil usually wound from copper or brass Compared to monopole which is essentially a two dimensional structure a helical antenna is a 3 dimensional structure but is simply a shorter monopole Helical antennas with physical dimensions considerably smaller than the wave length work as normal mode helical Their radiation pattern is similar to monopole This provides an optimum condition for compact devices A small helical signifi cantly reduces the needed physical size of the antenna and that is important at longer wave however this reduction is not without a price Due to the helical specific higher quality factor Q the bandwidth B is significantly narrower Q F B while the gain is as a matter of prin ciple lower vs a full size elongated wave monopole 4 2 1 EnOcean Helical ANT 300C Art S3035 D300 The dimensions for helical antenna Fig 3 are 7 7 x 8 15 x 31 15 mm WxHxL The Helical antenna impedance depends on posi tioning of antenna within device In implementation case of an tenna orthogonally to TCM300C 310C antenna is matched on Gir RF_Whip pin
10. ddle layers can be used for signal traces See next page EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 7 13 APPLICATION NOTE 105 Green Smart Wireless enocean 315 MHZ INTERNAL ANTENNA DESIGN ITI Free zone no components o Fig 5 Orthogonal design Fig 6 Integrated design Practically a well matched helical antenna will perform better than a bent monopole realizing best overall compromise at 315 MHz Regarding layout recommendations please consult 4 For good performance the distance to mounting plane should be held especially over metallic background Fig 7 Antenna positioning within housing Important note beside their limited bandwidth all electrically small antennas like e g heli cal chip or PCB suffer from low radiation resistance associated with a high reactance Using correspondingly tools and experience a matching network for a specific design can always be found to cancel the reactance and increase the input resistance to a convenient value such as 50 Q The antenna is then tuned at the matched frequency and specific environment How ever a matching network also has disadvantages First adding reactance to the whole an tenna system increases the Q thus lowering further the bandwidth and making it easier to be detuned in field Second loss in the matching network will also lower the efficienc
11. ed ground plane area dimension is directly proportional to the wave length this means lower frequencies need correspondingly larger ground planes E Antenna requires space around it for optimal performance Keep Antenna at least 25 mm from mounting plane When the Antenna is close to device mounting surface perform ance degrades rapidly E Antenna position For best performance place the antenna at the edge of device Away from conducting materials metal and all the electronics on the board o Please follow advice given in the antenna reference designs Fig 2 5 7 E Antenna shape o Helical shape cannot be changed o Monopole wire antenna shape can be changed The antenna bending point should be at least 25 mm from each other please fol low given advice For antenna standing over PCB keep distance between electronics at least 15 mm recommended bigger E Antenna Matching o Take special care of the RF specific layout tracking as short as possible and as good an approximation to a 50 Q strip line as you can achieve 2 7 mm thick tracking on 1 6 mm FR4 board is a good start point for microstrip 1 9mm on 1 mm FR4 board 1 1 mm thick tracking 0 2 mm gaps on 1 6 mm FR4 board for grounded coplanar waveguide 0 9 mm thick 0 2 mm gaps on 1 mm FR4 board o Mostly conductive or dielectric materials in the reactive near field of the antenna are however unavoidable In these cases measure the antenna impedance under real
12. f a wireless device are critically dependent upon its antenna Successful antenna design is based on understanding that the entire device is part of the antenna The primary antenna requirement must be effi ciency as the antenna acts as transducer between the radio and the environment Unfortu nately this aspect is often neglected until the end of the design and expected that an antenna fits into whatever space is left no matter how unfavorable to performance that location may be As result such designs will have to accept degraded performance or go through multiple and costly redesigns Often such degradation is not immediately evident in a new installation but it could appear any time in the form of mysterious behavior even through minor envi ronmental changes because of insufficient link budget Various conductive or dielectric materi als in the reactive near field of the antenna PCB housing wall underground of device people and use case specifics are unavoidable producing a downward drift of the initial resonance frequency and impedance Therefore optimal antenna matching must be made under typical application conditions e g as use case PCB with housing placed on concrete respectively dry wall in order to reach the best real compromise The link budget is limited by allowed trans mission power e g by FCC or MIC regulations receiver sensitivity environment noise and interferences On the other side mostly it is required that
13. han 787 kHz 0 25 of operation frequency Bandwidth is measured at points 20 dB down from the modulated carrier All EnOcean modules are within bandwidth limitations regardless to settings 3 2 Canada For usage in Canada the 315 MHz EnOcean based radio modules must fulfill the require ments described in RSS 210 http www ic gc ca eic site smt gst nsf vwapj rss210 issue 7 pdf FILE rss210 issue7 pdf The maximum allowed transmission power is same as for FCC regulations 95 6 dBuV m Spurious emission limitation is the same as well But there are some minor differences in regulations e g the 99 of modulated signal bandwidth shall not exceed 0 25 of opera tion frequency 787 kHz All EnOcean modules are within these bandwidth limitations re gardless to settings dBuV m is field strength dimension dBm power dimension For A wave antenna approximate conversion is 95 6 dBuV m 1 78 dBm dBm 10 log Watts 30 Watts V mxmeters an aBuVv m 120 20 30x10 4B1 10 Conversion in far field V m 10 EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 3 13 APPLICATION NOTE 105 Green Smart Wireless enocealy 3 3 Japan In the Japan 315 MHz band is governed by ARIB Association of Radio Industries and Busi ness STD T93 http www arib or jp english html overview st ej html The Transmission power is limited
14. hile maintaining a very compact size The helical is therefore very popular since well designed it can provide excellent overall performance at small size E When using pre manufactured antennas like e g helical or chip keep in mind that their specified performance depends on the attached reference ground plane The manufac turer s specifications are only achieved if the ground plane including free space around has the same size and shape as the manufacturer s evaluation board In all other cases you have to measure the impedance of the pre manufactured antenna under application conditions and to match it to the needed characteristic impedance E Receiver performance can be seriously degraded by interferences caused by digital circuits Switch mode power supplies PCs or electronic ballasts These interferences are as matter of principle stronger at lower frequencies Always keep antennae away from such paths e g try to keep digital traces short and run them over a ground plane to help confine the electromagnetic field generated by the pulses For more details please consult 4 E Remark you can never design a too good antenna for a receiver only but maybe for a transmitter transceiver Therefore please make sure that the final device e g transmit ter still fulfils the region specific approval with respect to the radiated power E A radio link always has 2 antennas transmitter and receiver If you use any other antenna than
15. ication test Disclaimer The information provided in this document describes typical features of the EnOcean radio system and should not be misunderstood as specified operating characteristics No liability is assumed for errors and or omissions We reserve the right to make changes without prior notice For the latest documentation visit the EnOcean website at www enocean com
16. lication engineers bared twisted and tinned On receiver side antenna losses cannot be compensated unfavorable antenna layout leads to losses of working range Special care of an tenna layout shall be taken when designing a transceiver device The 150 mm monopole antenna is matched internal in TCM 3x0C series modules Thus no additional matching is required If another length whip antenna is used please consider adding matching filter This is not covered he reby Also note that in this case a new FCC modular approval will be required Cu wire LiY according to VDE 0812 or 0245 CI 5 or Cl 6 Insulation PVC mixture TI2 according to VDE 0207 4 orange EnOcean specification of used and recom mended 315 MHz wire antenna Fig 1 L 150 mm 315 MHz AWG24 wire composed of 7 strands AWG32 Material is PVC according to DIN VDE 0207 Self extinguishing and flame retardant according to EN 50265 2 1 IEC 60332 1 Some adequate more practical configurations are shown in the Fig 2 antenna efficiency decreasing from a better to d poorer depending on the available place The space from antenna to mounting plane as this can be conductive is important Fig 1 EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 5 13 APPLICATION NOTE 105 Green Smart Wireless 315 MHZ INTERNAL ANTENNA DESIGN a Straight monopole a teal a
17. nded to test transmitter in typical application environment 7 2 Pragmatic test Please consult AN507 7 APPLICATION NOTE 105 Green Smart Wireless enoceal 8 REGULATION COMPLIANCE ENOCEAN SPECIFIC RADIO SETTINGS Usually for certification testing procedure several device samples types have to be prepared E Devices programmed to transmit carrier signal for field strength and spurious emissions measurements If manufactured device does not have battery holder it should be equipped with as small as possible temporary battery for operation needs Please keep the battery as far away as possible from antenna as it is not part of device and the influence to radiation pattern should be minimal Also keep the connection wires as short as possible See below two realisation examples E Best you have three identical devices with 3 different radio settings from most powerful to the less powerful and starts with the most powerful device In case that this exceeds the compliance limits take the next one The goal is to keep the specific final device as close as possible under the approval requirements for transmitted power AN507 describes how the special SW containing these individual radio settings can be compiled E Device programmed to transmit EnOcean sub telegrams Refer to Table 1 AN507 7 E Device as for field pictures documentation Fig 9 DUT equipped with small battery Fig 10 DUT prepared for certif
18. ta sent defines which limits are applied to transmitters for this frequency Two categories of inten tional operation are defined a Restricted to the transmission of control signals data can be transmitted with control Signal 15 231 a Periodic transmissions are not allowed only with exception for se curity and safety applications for polling or supervision transmissions within limit of two seconds per hour The field strength of fundamental wave is limited to average 6042 uV m 75 6 dBuV m measured at 3 meters distance Spurious emissions limited to average 604 uV m 55 6 dBuV m 15 31 0 The limits at 315 MHz are 95 6 dBuV m for funda mental wave and 75 6 dBuV m for unintended spurious emissions when EnOcean pro tocol is transmitted b Any other type of operation for example periodic data transmission without control action 15 231 e Additionally the devices are limited to maximum single transmis sion length of one second and the silence period should be at least 30 times the dura tion of the transmission but never less than 10 seconds The field strength of fundamental wave is limited to average 2417 uV m 67 6 dBuV m measured at 3 meters distance Spurious emissions limited to 241uV m 47 6 dBuvV m The peak limits are 87 6 dBuV m for fundamental wave and 67 6 dBuV m for unintended emissions Additionally 15 231 c defines the bandwidth limitations For 315 MHz transmissions the bandwidth shall not be wider t
19. y EnOcean www enocean com Subject to modifications Christian Bach Darius Draksas Oliver Sczesny Dec 2011 Page 8 13 APPLICATION NOTE 105 Green Smart Wireless enoceal 4 2 2 Helical Antenna Module already FCC approved for STM TCM 300C As exception the STM TCM 300C modules can also be used with the antenna module ANT_315_EO_PRO Fig 8 manufacturer Vicos GmbH A 5020 Salzburg without to need a new modular approval therefore This is already done by Vicos Embedded Helical needs add ground plane Design flexibility Drop in solution Already matched for STM TCM 300C modular approved Details sales vicos at Fig 8 APPLICATION NOTE 105 Green Smart Wireless eEnocean 315 MHZ INTERNAL ANTENNA DESIGN 5 ASPECTS FOR OPTIMAL ANTENNA DESIGN E Antenna requires proper grounding plane All antennas are resonant RLC an electrical circuit consisting of a resistor an inductor and a capacitor networks and like any elec tronic component have at least two connection points IMPORTANT Monopole Helical Chip or PCB antennas are in opposite with dipole all single ended and therefore ground de pendent Therefore they must have a near and efficient ground plane to work against Since this plane is the other half of the antenna its size and proximity are essential for its efficiency A compromised RF ground plane affects antenna stability performance and resonance frequency Please note the requir
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