RF Range. Application Note AN014. of TR-7xDx transceivers IQRF Tech s.r.o. Appl_Note_AN014_ Page 1

Transcription

1 RF Range of TR-7xDx transceivers Application Note AN IQRF Tech s.r.o. Appl_Note_AN014_ Page 1

2 RF range of IQRF transceivers Typical RF range accessible between a couple of IQRF transceivers with small antennas (e.g. the PCB ones optionally built in the transceivers themselves) in common applications without special optimizing is tens of meters in buildings and hundreds of meters in free space. Special arrangements enable ranges in kilometers. RF range can be extended by two main approaches: There are several possibilities how to prolong the physical range accessible between a couple of IQRF transceivers. Much higher effective range can be reached in Mesh network. In this case, a packet intended for a transceiver out of direct range can be delivered via other transceiver(s) in range. Moreover, this hopping with redundant paths has a lot of additional advantages, such as reliability, robustness and immunity against dynamical changes in range (e.g. by moving obstacles, persons or the devices themselves). Intelligent routing and other soft IQ methods are more efficient than simple increasing of output power. Some obstacles can not be penetrated at all, high output power generates high power consumption and undesirable noise, etc. IQRF operating system and additional DPA communication layer provide an easy and efficient way to powerful Mesh implementation (IQMESH), with up to 240 hops per packet. This Application note describes basic methods how to increase physical range between a couple of IQRF transceivers. It is intended as a simple guide relating to practical possibilities applicable to IQRF but not as an exact paper describing RF design and RF signal propagation in general. For more information about such details, refer e.g. to RF propagation set of articles. General design rules Useful information regarding RF design are published by various third-parties. The most recommended is the RF Design guide by Circuit Design. Calculation You can calculate RF signal propagation in your specific environment using the Calculation tools by Circuit Design. Testing The range really accessible with your hardware at given place should definitely be tested. To avoid possible issues in user software, the IQRF basic example E09-LINK is recommended for the first checks as well as for all tests described below. Comparative test If the range reached with your hardware does not meet your expectations, perform the following test to compare the results achieved at your hardware with the range achieved with TR-72DA transceivers plugged in standard DK-EVAL-04A development kits. Compare the ranges of your equipment with the reference system consisting of a pair of TR-72DA transceivers plugged in kits DK-EVAL-04A. If the range of your system is significanlty shorter, the cause is probably in your hardware. Then, to determine the impact of individual sides, you should perform similar test with your device on one side and DK-EVAL-04A kit on the other one, and then the same test with DK-EVAL-04A kit on the opposite side. Tip: But the achievable range can be even about two times longer. DK-EVAL-04A is optimized for space but not for the range. The best reference hardware for range check is with range extenders. See chapter Counterpoise IQRF Tech s.r.o. Appl_Note_AN014_ Page 2

3 Noise test It is possible to create a simple noise detector from any TR transceiver. Upload the application below into a TR (typically TR-72DA) and plug it into the DK-EVAL-04A kit. #define RX_FILTER 0 void APPLICATION() { while(1) { if (checkrf(rx_filter)) pulseledr(); } } Set desired RF band and RF channel in TR configuration. Place the detector to the location where the noise should be checked, e.g. close to an electronic equipment or a power source. The noise is indicated by red LED flashing. Change parameter RX_FILTER to evaluate the noise level. You should also consider that the noise may be generated by the user wireless equipment itself (if the HW is not designed properly). To exclude this case, repeat the above mentioned noise test applied to your equipment. If red LED flashes in your HW but not in DK-EVAL-04A, then the noise is probably generated by your HW IQRF Tech s.r.o. Appl_Note_AN014_ Page 3

4 Hardware Besides of basic RF parameters (such as output power, input sensitivity and bit rate) of the transceiver itself, RF range strongly depends on several design and application aspects. Construction The construction of the device is important in every design, but especially when a TR with built-in PCB meander line antenna (e.g. TR-76DA or TR-72DA) is used. Conductive parts Everyone including beginners knows RF shielding and the fact that the antenna must not be housed in a metallic case. However, there are even other important impacts. All conductive objects (ground planes on mother PCB and bulk objects such as metallic components and batteries) in the nearest surroundings close to the antenna have significant influence to RF range due to reflections and interferences. This influence applies for RF output power as well as for RF input sensitivity. It can be either (slightly) positive or (even significantly) negative, depending on particular arrangement and dimensions. But omnidirectional characteristic of PCB or other whip antenna (see Antennas) is deformed depending on the direction of reflections. Due to very complex character of these effects, it is not recommended to utilize such reflections to increase the range. Thus, general recommendation to keep all conductive parts as far as possible from the antenna should be observed. To achieve correct directional radiation, use (a) directional antenna(s), if applicable. See Antennas. The impact of a metallic object is higher if it is located in direction with higher radiation. See Radiation patterns. E.g., a parallel conductive plane impacts the range more than a perpendicular one. Examples The following examples illustrate the change of relative ERP (Effective Radiation Power) vs. antenna spacing to a conductive plane close to the antenna, measured at the location of the receiving TR. The violet lines represent the comparative levels corresponding to ERP in free space (100 %, see the datasheet of given TR transceiver, chapter Electrical specifications, RF range). To eliminate the dependency on RF band, the X axis units are expressed in multiples of RF wavelength [λ]. The following examples only illustrate the trends to be taken into account in RF design relating to conductive objects. Even a small change in arrangement may cause completely different results. X antenna spacing to the edge of a conductive plane perpendicular to TR antenna, in λ units Diagram 1: Perpendicular arrangement. The edge of transmitting TR-72DA antenna 1 cm above the plane 120 mm x 120 mm, receiving TR placed in direction of the blue arrow IQRF Tech s.r.o. Appl_Note_AN014_ Page 4

5 X antenna spacing to a conductive plane parallel to TR antenna, in λ units Diagram 2: Parallel arrangement. Transmitting TR-72DA antenna in parallel with the plane 120 mm x 120 mm, receiving TR placed in direction of the blue arrow. Non-conductive parts Even non-conductive parts close to the built-in meander antenna can impact the range due to losses of RF power as well as RF sensitivity. Even a mainboard PCB under the antenna can significantly degrade the range. This impact depends on parameters of given PCB and is significant up to approx. 5 mm around, inversely related to the distance. To avoid RF attenuation due to the mainboard PCB, it is recommended to mill a mechanical hole in mainboard PCB under the built-in antenna, overlapping the antenna outline for at least 5 mm (if it is possible with respect to general PCB design rules). Examples of correct and incorrect layout of main board PCB under TR-76DA: Correct Correct Incorrect 2018 IQRF Tech s.r.o. Appl_Note_AN014_ Page 5

6 Counterpoise By contrast, utilizing an artificial ground is the right way to increase RF range. A conductive object connected to the ground placed in proper location towards the antenna creates a counterpoise effect improving RF output power as well as RF input sensitivity. This can significantly prolong the range with no side effects and almost no expenses except possible enlarging dimensions. RF signal stays omnidirectional. The counterpoise effect can be tested using the TR-7xDA transceivers plugged in DK-EVAL-04A development kits, either directly or via the RNG-EXT-01 range extenders. More than two times longer range is achieved with the extenders. The RNG-EXT-01 is optimized for use with DK-EVAL-04A only. To utilize the counterpoise at final user-specific product, the distance between the antenna and artificial ground must always be tuned for the particular hardware. Tip: One of the objects best suitable as a counterpoise is a battery. Tip: If a long range is required, it will pay to try to find the best antenna position. Even simple experiments without special RF knowledge and equipment can bring much better range. To achieve optimal results, the best way is to use a third party company specialized in RF for hints or to design the RF part of your product. (One of such companies is also the IQRF inventor / manufacturer.) Diagram 3: Relative RF range vs. level in the setrfpower(level) function, both TR-72DA plugged into DK-EVAL-04A kits with or without range extenders RNG-EXT-01. Refer to IQRF OS Reference guide, function setrfpower() IQRF Tech s.r.o. Appl_Note_AN014_ Page 6

7 Antennas AN014 Crucial effect on RF range has the antenna type (style, dimensions, directionality, ) at the transmitter as well as the receiver. Mutual orientation of antennas should respect polarization and radiation patterns. All antennas mentioned below have linear polarization. Directional antennas provide ranges up to about 10 km. PCB antennas built in TR transceivers Radiation patterns RF output power specified in TR datasheet relates to isotropic radiation (equal in all three dimensions). However, the PCB antenna built in TR-7xDA transceiver is designed as a shortened ¼ wave whip which is not isotropic but omnidirectional in two dimensions. It radiates equal power in all azimuthal directions perpendicular to one axis of the antenna. Radiation patterns create a torus. Signal strength in direction of the axis of the torus is attenuated and the energy saved in this way is focused to the other two dimensions. That is why the RF power radiated in prefered directions is higher than one specified in TR datasheet. Diagram 4: TR-72DA RF output power [in dbm] vs. antenna orientation (radiation patterns) IQRF Tech s.r.o. Appl_Note_AN014_ Page 7

8 Arrangement Examples of correct and incorrect arrangement of TR-72DA pairs: Correct Correct Incorrect Incorrect Incorrect Wrong radiation angle Mismatched polarization Wrong radiation angle Mismatched polarization Tip: In a hand-held remote controller, locate your PCB antenna in transverse but not in longitudinal position. External shortened ¼ wave whip antennas Similar characteristics and radiation patterns as for the PCB meander line antennas built in TR-7xDA transceivers are also valid for external IQRF (shortened) ¼ wave whip antennas. Directional antennas AN-11 AN-06 AN-07 AN-D01 Directional antennas in low power packet-oriented RF communication are still a bit undervalued. There are a lot of not space-constrained applications where directionality is not a problem, and, in many of them it can be even an advantage. It may be useless (and sometimes undesirable) to spread RF signal to areas where no TRs are placed. Moreover, radiation of some directional antennas in not preferred direction is not negligible. Planar antennas like AN-09 have wider transmitting angles compared to Yagi ones. In addition to preferred direction with the range in kilometers, there is also a wide angle where transmitting signal has higher strength in comparison with small omnidirectional antennas described above. See Diagram 5. Thus, these antennas are suitable as a convenient trade-off between omnidirectionality and very long range. Of course, they need not be used at every wireless end device (individual sensors etc.) in the project, but only at the devices where there is a reason for it, e.g. at network Coordinator, gateways or dedicated routers. A reasonable number of such routers placed in appropriate locations can significantly improve RF network as a whole. The advantages can be great: Larger area covered with less transceivers Less network traffic, higher throughput, less routing zones, faster Discovery etc. See IQRF User s guide, chapters Routing and Discovery. At the receiver, a noise incoming from other directions is automatically attenuated. Less noise transmitted to unwanted area AN-09 Planar AN-08 Yagi Radiation patterns of AN IQRF Tech s.r.o. Appl_Note_AN014_ Page 8

9 Diagram 5 shows radiation patterns of planar directional antenna AN-09 in comparison with small IQRF omnidirectional antennas. E.g., there are only two < 40 º areas where RF range is lower for AN-09 than for the antenna built in TR-7xDA. Units: dbm Diagram 5: Radiation patterns of antenna AN-09, AN-011 and antenna built in TR-7xDA (either with or without range extender RNG-EXT-01 at DK-EVAL-04A). Shaded areas illustrate the only spaces where the range of AN-09 is shorter than at TR-7xDA IQRF Tech s.r.o. Appl_Note_AN014_ Page 9

10 First Fresnel zone Environment Obstacles may attenuate or destroy RF signal, firstly due to absorptions and reflections. Their influence may be important even if they are out of the signal path line-of-sight. To achieve the longest RF range, the 1st Fresnel zone should be free. It is an ellipsoidal space between and around two transceivers defined according to Diagram 6. Diagram 6: 1st Fresnel zone TR Transceiver D Distance between TRs P Any point between TRs D1 Distance between P and one TR D2 Distance between P and the other TR R Radius of the 1st Fresnel zone corresponding to point P λ RF wavelength. Approx. values: 0.35 m for 868 MHz 0.32 m for 916 MHz 0.69 m for 433 MHz The boundary of the 1st Fresnel zone corresponding to any point P is defined by radius R = λ (D1 D2)/D. E.g., for 868 MHz band, D = 500 m and point P in the middle of both TRs, the approximate resulting radius is R = 43 m. It is almost (or absolutely, in indoor applications) impossible to achieve the 1st Fresnel zone completely free in practice. The only cases approaching this ideal are TRs placed on two points (such as masts or skyscrapers) much taller then all the terrain between and around. In other cases, the trade-off should be found as best as possible. Reference measurement All reference measurements to evaluate RF range specified in TR datasheets are accomplished not in free 1st Fresnel zone but just on the free plain according to Diagram 7. Both transceivers are 1.6 m above the ground with cut grass surface. Diagram 7: Arrangement for reference measurement of RF range (top view) 2018 IQRF Tech s.r.o. Appl_Note_AN014_ Page 10

11 Ground influence AN014 One of the important phenomena impacting RF range is ground influence. The antennas should be placed as high as possible above the ground surface. The dependency is important especially up to 1.8 m. The ranges specified in IQRF TR datasheets assume the height 1.6 m for both TRs. Diagram 8: TR-72DA relative RF range vs. antenna height above the ground, 868 MHz and 916 MHz band. Absorption In addition to essential attenuation in free space, significant losses appear when penetrating possible obstacles. Estimated attenuation for various materials [in dbm] can be found at various sources, e.g. at The Basics of Signal Attenuation white paper. Useful hints may also be included in simulation software web tools. Some materials have unexpectedly high losses. For example, a multilayer window glass is comparable to a wall. Another hardly passable object is a common mirror. Reflection Reflection (and other similar phenomena such as refraction and diffraction) at possible obstacles and terrain protrusions may also strongly influence the range. The straight and reflected signal components are vectorially added. Thus, the result of such interference can be either negative or positive, depending on the signal phase at given location. E.g., the range could be prolonged over a large water surface or a snowy terrain IQRF Tech s.r.o. Appl_Note_AN014_ Page 11

12 Application software RF range can significantly be influenced also in application SW. RF parameters (not only the basic ones such as output power and input sensitivity) are under SW control. Additionally, more or less advanced SW options and methods like LBT (Listen Before Talk) or channel switching are available to achieve the best possible wireless communication. However, they are beyond the purpose of this Application note. RF output power RF output power is selectable from 8 levels by IQRF OS function setrfpower(level). Diagram 9: Effective radiation power vs. level in the setrfpower(level) function. This is an illustrative example only. For exact information refer to the diagram in datasheet of given TR transceiver. RF input sensitivity RSSI indication IQRF TR is able to measure the actual strength of the signal incoming to the input of the receiver (total amount of power at given carrier frequency, including possible interferences and noise which can increase or decrease the value depending on the phases). Resulting level is reported as proprietary parameter RSSI (Received Signal Strength Indication) value. The higher RSSI number, the stronger signal. Input filtration RSSI detection To increase the reliability and improve other features such as power consumption and throughput, RSSI can be used for filtering in order to eliminate incoming RF signals weaker than a specified limit. (There is no sense to attempt to receive a weak signal with negligible chance to get the packet properly.) This filtration can be implemented in application SW by OS function checkrf(level) (and applies automatically in LP and XLP power saving modes using the level selected by the user). Of course, the stronger filtration is applied, the higher immunity against interferences is achieved, but the lower effective RF range is reached. The results can differ for some TR types depending on RX mode (STD, LP or XLP). Diagram 10: Relative RF range vs. level in the checkrf(level) function. This is an illustrative example only. For exact information refer to the diagram in datasheet of given TR transceiver IQRF Tech s.r.o. Appl_Note_AN014_ Page 12

13 Preamble detection Another useful method how to recognize a proper IQRF signal is preamble format detection. It is available at IQRF OS v4.00 or higher. See IQRF OS Reference guide, function checkrf(), preamble quality check. The great advantage is that effective RF range is not shortened in this case. Adjacent channels IQRF transceivers operating at different channels do not interfere each other. However, there is an exception if a receiving TR is very close to another TR transmitting at a different channel. Channel spacing should be as high as possible is such cases. The results can be a bit worse in LP or XLP RX modes. Diagram 11: Packet Error Rate (PER) detected at a TR receiving in STD mode at channel 52 vs. distance X of an alien TR transmitting at channel 53 or 54. This example only illustrate the trends to be taken into account in RF design relating to close sources of interferences. Even a small change in arrangement may cause completely different results. RF range shortening Some applications may require shorter range than possible. To keep the best signal-to-noise ratio in such case, the shortening should mostly be accomplished by input filtration rather then by reducing output power. Near field effect IQRF transceivers are not primarily intended for operation at very close distance (the decimeter order of magnitude) each other. In such cases, the following recommendations should be observed: Reduce RF output power to level 1 (setrfpower(1)) to avoid saturation of RF input of the receiver, and use RF RX filter level 15 (checkrf(15)). Avoid close TRs transmitting at very adjacent RF channels. Channel spacing should be as high as possible IQRF Tech s.r.o. Appl_Note_AN014_ Page 13

14 Document history Document information Chapter RF range of IQRF transceivers revised. Chapters Noise test, Non-conductive parts and Reference measurement added. Chapter First Fresnel zone extended First release IQRF Tech s.r.o. Appl_Note_AN014_ Page 14

15 Corporate office Sales and service IQRF Tech s.r.o., Prumyslova 1275, Jicin, Czech Republic, EU Tel: , Fax: , (commercial matters): Technology and development (technical matters): Partners and distribution Quality management ISO 9001 : 2009 certified Complies with directives 2011/65/EU (RoHS) and 2012/19/EU (WEEE). Trademarks The IQRF name and logo are registered trademarks of IQRF Tech s.r.o. PIC, SPI, Microchip and all other trademarks mentioned herein are a property of their respective owners. Legal All information contained in this publication is intended through suggestion only and may be superseded by updates without prior notice. No representation or warranty is given and no liability is assumed by IQRF Tech s.r.o. with respect to the accuracy or use of such information. Without written permission, it is not allowed to copy or reproduce this information, even partially. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. The IQRF products utilize several patents (CZ, EU, US). On-line support support@iqrf.org 2018 IQRF Tech s.r.o. Appl_Note_AN014_ Page 15