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IoT Crout eSIM Starter-Kit

Crout IoT eSIM – der letzte Austausch der SIM-Karte

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Energy saving with NB-IoT and LTE-M

In this blog, we take a closer look at the energy consumption of NB-IoT and LTE-M. Any energy savings do not come by themselves. The IoT developer has to choose the right AT commands and the right protocol with the right strategy. Sometimes constraints mean that we also have to do without NB-IoT and LTE-M and take a different path. Have fun reading and learning.

Table of contents

  1. NB-IoT / LTEM versus GSM
  2. LWM2M versus MQTT
  3. NeoMesh as a complement to NB-IoT and LTE-M

NB-IoT / LTEM versus GSM

In a nutshell: GSM was developed for voice communication and NB-IoT / LTEM was developed for data transmission.

So what can GSM do?

In 1979, the 900 MHz frequency range was reserved for a mobile radio system for voice communication without a name. This range was later extended to 1800 MHz in Europe and 850 MHz and 1900 MHz in the USA. In 1982, the CEPT (Conférence Européenne des Postes et Télécommunications) set up a working group called the „Groupe Spéciale Mobile“, or GSM for short. The goal was to develop a European mobile radio system. With the later worldwide spread of the technology, the meaning of GSM was changed to „Global System for Mobile Communications“. It was not until 1995 that fax, Circuit Switch Data (CSD), Short Message Service (SMS) and other functions were added in a second phase for data transmission. The packet-switched data service General Packet Radio Service (GPRS) was only added in 2000. GPRS enabled access to the Internet. After that came EGPRS. The original idea of GSM was a digital radio network for voice communication without the known problems from the analogue radio networks. All carriers for transmitting data were added later.

Saving energy with GSM/GPRS

In order for a GSM/GPRS device to communicate, it must be switched on. If an IoT device based on GPRS stays switched on in receive mode, it requires a constant 5 mA current. With this current being continuously drawn, the battery is drained after a few days. The only way to reduce energy consumption is to switch off the device. If the IoT device is switched off, an energy-hungry re-registration to the GSM/GPRS network is necessary. An IoT device based on GPRS only knows the operations: switched on with receiving, switched on with transmitting and switched off.

So what is NB-IoT?

NB-IoT is a radio network within a radio network. It is operated in the same frequency band as LTE. NB-IoT was specified from the start to operate on battery power and cannot transmit voice communication.

The history of NB-IoT from 2014

In 2014, Neul in the UK came up with the idea for a scalable, secure, robust and economical region-wide radio technology for network operators. For this purpose, the radio protocol for the licence-free band called Weightless was changed. The pre-existing protocol was adapted to the licence-required bands of mobile radio. The goal was to provide network services for small devices with low power consumption and connection to the cloud. Compared to GPRS, this new technology had a 20 dB higher link budget and thus better network coverage. The radio technology could operate in any frequency band below 1 GHz. Such a Neul-Weightless network required a frequency spectrum of only 180 kHz as the network within the network. A GSM subcarrier or 200 kHz within the LTE spectrum was enough. Neul offered small, powerful radio modules and the base station.

If you look closely, you can see the NB-IoT specification in the wording. NB-IoT uses only 180 kHz in a free spectrum within the LTE frequency band for 12 channels. The 20 dB greater link budget over GPRS is also found in NB-IoT. Huawei recognised the benefits of Weightless and bought Neul in September 2016. The world’s first NB-IoT chipset was launched by Huawei subsidiary Hisilicon called Boudica 100 in December 2016. In 2022, we have already arrived at the third generation NB-IoT chipset.

Below we explain how NB-IoT saves energy.

Power Save Mode PSM

In PSM mode, the radio module is not completely shut down and the memory with the data for logging into the NB-IoT is not erased. With the memory active, the NB-IoT ddevice requires only 3 uA of continuous current. Completely shut down, it also requires approximately 3 uA.

The NB-IoT module informs the NB-IoT base station that it will be shut down and stores the registration data. The NB-IoT base station acknowledges the request to shut down and also stores the registration data. The NB-IoT then enters PSM mode and is no longer accessible. The NB-IoT device can be woken up by the microcontroller at any time in the case of an event. In contrast to the first registration, the new registration (called reattach) requires significantly less energy than the first registration (attach).

Release Assistant Indication RAI

With RAI, the NB-IoT network is informed that no receipt of data is expected after sending. Water meters, bins for recyclables and many other IoT sensors do not require an acknowledgement after sending status. If the status of the meter or the fill level is lost, it doesn’t matter because a new status is transmitted 12 or 24 hours later anyway. Shutting down the receive window saves up to 50 % of the energy of a data packet. If alarm messages with acknowledgements are required in some circumstances, RAI can be switched on and off dynamically.

To use RAI, NB-IoT with UDP or CoAP/UDP must be used. TCP and MQTT are based on TCP and so cannot work without acknowledgement. All other protocols based on TCP are therefore not suitable for energy-saving with RAI.

Extended Discontinuous Reception eDRX

The eDRX operating mode enables continuous receive operation. The procedure is not new and has been used with pagers for decades. A Pocsag pager wakes up briefly every few seconds and checks whether a message is received. If it recognises from the group address that the message is not for it, it does not read in the complete address and immediately aborts the receive operation. With NB-IoT, there is also periodic continuous receiving (paging) with eDRX. In contrast to Pocsag, the reception window for paging is not statically set to seconds but can be dynamic from 20.48 to 10485.76 seconds (~175 minutes). The NB-IoT device informs the base station of the timer for the eDRX, receives an acknowledgement and is then periodically reachable.

Summary of energy saving measures

NB-IoT, like Bluetooth Low Energy (BLE), NeoMesh and Pocsag, requires little energy. The four wireless technologies save energy by switching off in an intelligent way. Switching off and not receiving saves the most energy. NB-IoT, Bluetooth Low Energy, NeoMesh and Pocsag pagers can receive continuously with little energy because the receiving operation is synchronised via precise timers. This is called eDRX in NB-IoT. PSM is just a smart switch off without really switching off. RAI is just a smart send without opening a receive window.

The IoT developer must dynamically switch on or switch over the functions PSM, RAI and eDRX themself to achieve the best balance of minimal energy consumption for a given application’s communication needs. The same applies to searching for the radio network or switching from NB-IoT to LTEM and vice versa. LTEM modules offer the same timers. When to switch and how can be read in my guide to NB-IoT, LTEM and GSM to find the best strategy for your IoT application.

LWM2M versus MQTT

Simply put: MQTT can do practically nothing and LwM2M can do everything.

So what can MQTT do?

Message Queuing Telemetry Transport (MQTT) can, as the name suggests, only transport data with 14 control commands. Unfortunately, MQTT does not transport the data in an energy-optimised way for battery-operated devices. MQTT is a compromise between energy-saving and safety when transporting data. Compared to the relatively energy-hungry HTTP protocol, MQTT requires less energy. HTTP was developed to transfer web pages based on TCP and therefore does not need to save energy. TCP is inherently very hungry because it repeats packets if there is no acknowledgement. Unfortunately, with MQTT, only the complex page description language HTML has been trimmed and the main problem caused by TCP has not been eliminated. MQTT makes it possible to work on the protocol layer with and without acknowledgements. The layer below is TCP and continues to acknowledge. In addition, TCP servers have a time-out that can often not be met via radio protocols. #NBIoT and #LTEM with a latency of up to 20 seconds are difficult to use with high energy consumption. The Power Save Mode (PSM) with up to 310 hours and Extended Discontinuous Reception (eDRX) of up to 40 minutes cannot be operated at all with MQTT. The developers had a good idea in 1999 but unfortunately missed the chance to develop the MQTT protocol on User Datagram Protocol (UDP) instead of TCP.

What can CoAP do?

CoAP was published 10 years later in December 2009 as „CoAP Feature Analysis draft shelby 6lowapp coap 00„. Like MQTT, it can transport data. It was specified as a transmission protocol for use in restricted networks and nodes for M2M applications. These constrained nodes often use 8-bit microcontrollers with little memory. Wireless networks such as 6LoWPAN often have a high packet error rate. The special conditions and packet loss were taken into account in the specification of CoAP by its fathers from the 6LoWPAN world. CoAP used UDP instead of TCP. It has only four control commands (Get, Put, Delete, Post). UDP does not acknowledge in its protocol layer. An acknowledgement is only made one level higher with CoAP. Within CoAP, commands can be sent with or without an acknowledgement. Because the fill level of a tank or a water meter is sent cyclically, an acknowledgement is usually unnecessary. A packet loss is not critical. Since CoAP on UDP or SMS has no time-out, a few minutes delay with SMS or hours and days with NB-IoT PSM and NB-IoT eDRX are no problem. NB-IoT Release Assistance Indication (RAI) immediately switches off the receiver after sending and thus fits perfectly with CoAP without acknowledgement. CoAP is therefore the perfect match NB-IoT and LTE-M. The first version of the specification comprising 14 pages has grown to 110 pages over several adaptations since 2013.

What can LwM2M do?

LwM2M, unlike MQTT and CoAP, is much more than just a transmission protocol for data. In order for LwM2M to perfectly serve the new low-energy radio protocols NB-IoT and LTE-M, CoAP/UDP and SMS were chosen in version 1.0. This ensures that NB-IoT can be used perfectly with PSM, eDRX, and RAI. In the newer versions of LwM2M, LoRaWAN, CoAP/TCP and even MQTT have since been added. Devices with MQTT can thus be easily integrated into LwM2M. One of the key features of LwM2M is device management. Since LwM2M was influenced by mobile network operators at the OMA, their experience in managing wireless phones has been taken into account. Registration with the LwM2M server is possible very securely in various ways. The commissioning of a device with bootstrap is also standardised under LwM2M. Necessary FOTA is also managed by the server. The communication of a device is standardised with profiles and resources. Through the mandatory profiles Security, Server, Device and Location, any LwM2M device from any manufacturer can communicate with any LwM2M server on our beautiful blue planet. After successful registration, the LwM2M server interrogates the device and from that point knows whether it is a water meter, presence detector, level meter or tracking device, for example. A device can also support several profiles in parallel.

Summary LwM2M versus MQTT

LwM2M supports many protocols for transmission and was specified in the first version on CoAP/UDP. Contrary to MQTT, it was planned from the beginning for devices with batteries and low energy consumption. LwM2M handles device management, bootstrapping, firmware update over the air and communication via profiles.MQTT does not offer all this. MQTT has never been optimised for the lowest energy consumption and nothing is regulated and standardised except transport. Any company aiming for extensive digitalisation has a good starting point with LwM2M. Perhaps we will soon see the launch of a rocket to Mars, with firmware update via LwM2M. The transit time of about 3 to 22.3 minutes from Mars to Earth is no hurdle for LwM2M. Even signals from Neptune with a 4-hour delay can be solved with this genius protocol.

„Can you hear me, Major Tom?“ 🚀 🌍 😀

PSM, RAI and eDRX is supported by LwM2M. The IoT developer just needs to insert the right AT commands in the LwM2M stack and choose their appropriate strategy.

NeoMesh as a complement to NB-IoT and LTE-M

How do you cleverly complement NB-IoT and LTE-M to:

  • double the 10 years from the battery
  • extend the range to the third floor of the underground garage or the sewer 50 metres below the ground surface

Learn from the experience of Triptec in Germany.

Longer operating time for Triptec’s operating hour meters, data loggers and alarm devices

Triptec HL UG was founded in 2001 and operates thousands of operating hour meters, data loggers and fault detectors with the necessary cloud server. Customers include public utilities, municipalities, industrial companies and energy utilities. The goal of their wireless IoT devices is to monitor and optimise the operation of wastewater plants, district heating power plants and machines as just some of the examples. The operating hour meters are used to determine the time of use of machines and derive any necessary maintenance. The data loggers record measurement data and transmit it periodically to the cloud. The fault indicators monitor measured variables and trigger an alarm in the event of a fault. A combination of alarm and data logger is common. Up to now, Triptec has used GSM/GPRS modules to send messages or receive control commands at the units. GSM/GPRS belongs to the second generation of mobile radio. 3G was shut down in Germany in the summer of 2021. In Switzerland, 2G was taken out of operation in 2020. None of the three German network operators can promise how long GSM will continue to operate in Germany. Triptec’s customers are therefore pushing for future-proof radio technology. At the same time, Triptec wants to reduce the energy consumption of wireless IoT devices during the redesign and reach new market segments.

Implementation with NeoCortec Sub-GHz-Meshnet in combination with Quectel BG95-M3 (NB-IoT, LTE-M and GPRS) at Triptec

As we have discussed, NB-IoT and LTE-M, unlike GSM, were specified from the start for data communication with low energy consumption. NB-IoT and LTE-M use various timers that enable an IoT device from Triptec to be connected bidirectionally. For example, the devices listen to the cellular channel every minute to 175 minutes to check if a message arrives. The device can go to sleep, receive nothing and still remain logged on. In the event of a fault, the cellular radio module BG95 is woken up. There is no need to register again in the cellular network, thus saving energy. Furthermore, NB-IoT and LTE-M offer up to 20 dB more link budget than the previously used GPRS. 8 dB corresponds approximately to doubling the range or roughly the attenuation in a wall in a building. With 20 dB, the user can penetrate 2 more walls or reach devices in the basement better.

The low-power mobile technology NB-IoT/LTE-M has been complemented with a Sub-GHz mesh net from NeoCortec called NeoMesh. NeoCortec drastically reduces energy consumption per node through extremely precise timing. Instead of transmitting 1 to 3 km to the mobile phone mast, NeoMesh transmits up to approximately 100 metres indoors. Outdoors, 500 metres are possible. A message hops from the radio node to the next radio node in the NeoMesh. Each node in the NeoMesh is also a router. In addition, each message is acknowledged to the next node. In the event of a fault, the message is transmitted again in the next cycle on another of the 15 channels. If a node fails, the module in the NeoMesh automatically searches for a new path. All nodes synchronise every 1 to 30 seconds. With a pulse of 30 seconds, a node can operate for up to 7 years from 2 alkaline AA cells.

Data flow for stand-alone with NeoCortec NC1000 as pressure and temperature sensor for pipes, multi-sensor for various measured variables, a sensor for air quality and gateway with NC1000 and BG95.

An NB-IoT/-LTE device requires approximately 5200 mWs for a message at long range to the base station (154 dB link budget). One message per hour results in approximately 24,000 to 124,800 mWs per day without acknowledgement, depending on the link budget. With acknowledgement, approximately twice the amount of energy is required. NeoMesh requires only 4200 mWs for the 24 messages including acknowledgement. The gateway is also an edge computer. The MCU is woken up every 30 seconds by the NeoCortec module, writes the measured value to the memory and makes a target/actual comparison. If a limit value is exceeded, the BG95 is woken up and the message is transmitted to the server within 2 to 10 seconds. Since the fault message is the exception, the gateway usually sleeps and transmits the data to the cloud once or twice a day if desired.

Comparison of energy consumption of NeoCortec and NB-IoT for 24 messages per day. Source for the measurement series with the LPWAN technologies. Download of the study: https://www.akoriot.com/white-papers/

Wilhelm Oelers, CTO of Triptec says: „We compared the known Mesh-Net protocols and in the end found that most of them do not support Sub-GHz. In addition, with BLE-Mesh, ZigBee, Thread or 6LoWPAN, the routers and also the gateway module are not allowed to sleep. NeoMesh from NeoCortec on Sub-GHz helps us to penetrate the walls well. All participants (gateway, router, node) go to sleep at the same time. This allows 2 AA cells to operate for years. In this way, NeoCortec meets Triptec’s demand for minimum energy consumption with a good range in the licence-free 868/915 MHz band. The NeoMesh can be stretched almost as deep as desired into the ground. In a multi-storey car park, sensors in the third basement and deeper are no problem. A sewer is no longer a hurdle. The combination of NB-IoT/LTE-M with NeoMesh means there are no more radio holes for us.“

Summary of NeoMesh combined with NB-IoT and LTE-M

PSM, RAI and eDRX of NB-IoT and LTE-M can be combined with NeoMesh from NeoCortec to produce a solution that has great reach, interoperability and low power consumption. The IoT developer just needs to combine the right control commands in NeoMesh with the AT commands of NB-IoT and LTE-M.

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An emergency button on NB-IoT / LTE-M / GNSS

For years, I have been receiving requests for small, cellular devices the size of a matchbox or a wristwatch. To minimise the flood of requests from desperate IoT developers, I’m disclosing the secret in this blog: wireless devices are made smaller by doing away with the annoying, expensive antennas! Each chip antenna or Flex PCP antenna drives up costs and takes up space. Small devices such as wristwatches, tracking devices for dogs or even the Apple Air Tag use LDS.

Laser Direct Structuring (LDS) is a process by which an antenna can be applied three-dimensionally to a plastic carrier. This carrier can be a separate plastic part or an existing integral part of the enclosure. The advantages are low tooling costs, quick layout changes and very fine structures. The plastic parts are solderable. LDS is not science fiction. The world’s leading manufacturer for LDS develops and produces its machines 30 minutes away from my office in the neighbouring town of Garbsen.

Table of contents

  • Apple Air Tag with LDS
  • LDS demonstrator from 2012
  • An emergency button on NB-IoT and LTE-M and antenna by LDS

Apple Air Tag with LDS

One of the best-known devices with antennas based on LDS is the Apple AirTag. In this innovative IoT device, the Bluetooth antenna and the U BB antenna were printed on the outer edge of the plastic enclosure. The NFC antenna was also applied to the enclosure using LDS. The inner workings of the device were revealed by Adam Catley via reengineering: https://adamcatley.com/AirTag.html

LDS demonstrator from 2012

To demonstrate the possibilities of LDS, the enclosure in the photo in 2012 was printed with a few tracks and antennas. What impresses the world today in the Apple Tag was possible ten years ago. At that time, it was still possible to have prototypes printed with LDS in Garbsen. The machines have since been sold to a company in the south of Germany. It is still possible to have prototypes printed there using LDS. Original post: http://www.gsm-modem.de/M2M/m2m-componets/do-not-buy-antennas-just-print-it/

An emergency button on NB-IoT and LTE-M and antenna by LDS

The akorIoT Group has a development team with many years of experience in wireless data transmission which today is termed “Wireless IoT”. The integration of antennas into enclosures is another area of expertise for the team; in addition to standard antennas such as solderable chip antennas or flex PCB antennas, customer-specific antennas are often used in their designs.

The akorIoT Group also counts within its software development team experts in MCU firmware, smartphone apps or web-based applications. Product designers are also represented.

They asked designer Thomas Klimek to develop an enclosure concept for an IoT button. The Amazon Dash button and the NB-IoT button from Deutsche Telekom were provided as examples of IoT buttons, these two buttons are very similar in shape. An additional button from Finland called “BTTN” was also provided.

The most challenging example was the button for Gillette for ordering razor blades. No specifications existed. So he was free to imagine the new product with one button, several buttons in a circular, rectangular or triangular housing. Thomas Klimek was at liberty to give free thought to his creative ideas.

Creative ideas of a designer with experience in wireless IoT products

The result after a few days’ work was impressive. In the first iteration, two different IoT devices were created, each with a push-button for operation. One of the two devices is portable and has a belt clip.

In the first design step, hand sketches are made. After review, these are then converted into software 3D models. These models can be further processed and also printed in 3D. Since the designer has experience from other joint wireless projects and builds radio-controlled drones himself, he understands battery and antenna design. Antennas and batteries require installation space and their requirements must be considered during the design phase.

The rounded shapes in the designs lead to a loss of installation space for the battery which is generally square. Other shapes for batteries are possible but lead to high tooling costs. Not all shapes can accept batteries. The two designs create unused installation space in the enclosure. A problem often arises when the design is ignorant of the optimum installation space. For consumer products, on the other hand, look and feel are important. A compromise must be found between design and optimal technical implementation.

Development of the antenna concept for the emergency button

Due to the many round edges, a customer-specific antenna is necessary. Square chip antennas for NB-IoT and LTE-M do not fit into the round shape of the enclosure. The antenna is best integrated into the plastic enclosure as a Flex PCB or by Laser direct structuring (LDS).  For initial tests, the enclosure itself can be coated and the antenna structure worked out. Once the antenna concept has been clarified, the metallization of the inner surface of the enclosure shell is ordered and the desired antenna structure is scratched into the metal surface by hand. Once the structure has been worked out, it can be transferred to the inside of the housing shell. The advantage is clear – there are no tooling costs, and the antenna structure can also be adapted in later stages of product development without major costs.

Size of the device and technical feasibility of the cellular antenna

The antenna system consists of the enclosure, antenna and surface of the circuit boards. What we “see” as an antenna is only half the truth. Dipole antennas have two antenna segments (We know are familiar with this from traditional V-shaped television antennas). Monopoles have the second segment of the antenna located in the ground plane. The size of the ground plane in a monopole antenna therefore cannot be reduced at random to accommodate the cosmetic design. In addition, the distance between the antenna structure and the ground plane cannot be reduced to any arbitrary level. Sometimes a few millimetres distance between the antenna and the ground plane affects whether the antenna will function or not. Nobody can cheat physics. The battery or even a display will also strongly influence the construction of the customer-specific antenna. The first step is to develop such an IoT device on an almost empty PCB with the selected antenna, battery, display and other components in the electrical field of the antenna. Only when the concept has been successfully tested can the development of the complete PCB begin. The almost empty PCB can also contain wireless modules. These are then controlled with USB cables from the PC. If the return loss is good, this does not mean that the antenna really radiates the power. The radiated power must be carefully measured in all three axes.

GNSS and BLE antenna

Integrating GNSS and BLE antennas into an IoT button is straightforward. Both can be designed as ceramic chip antennas. They can also be printed by LDS into the housing or designed as a Flex-PCB. The GNSS antenna can be integrated into the cellular antenna. If the wireless module transmits with 23 dBm, GNSS receiving is not possible. A switch between GNSS and cellular operation can be used.

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A guide to selecting an antenna for an IoT device

Anyone who wants to develop a wireless IoT device must choose an antenna. Antenna datasheets are full of a lot of technical data, produced at the discretion of the manufacturer rather than to an established set of standards. This can make interpretation and comparison a challenge for even the more experienced engineer. in this blog post, we shed some light into the murky world of antenna specifications.

3D gain image

Table of contents

  • Typical antenna selection decision process
  • Antenna selection based on return loss
  • Selection on basis of the antenna size
  • Selection by the efficiency of the antenna
  • Selection by size and price without considering the technical data
  • Selection based on the 3D radiation diagrams of antennas
  • Selection on the basis of 2D directional diagrams
  • The honest antennas
  • Summary of antenna selection
  • Services around antennas

Typical antenna selection decision process

The following paragraphs are based on our 20 years plus of experience with wireless projects. 20 years ago it was called M2M. Today it is called IoT. But the antenna does not know or care whether it is installed in an M2M or an IoT device. There has not been a lot of change to the licensed and unlicensed frequency bands in recent years either. Back in the day there was still no LPWAN with LoRaWAN, Sigfox, Mioty, NB-IoT or LTE-M. The innovative Sub-GHz Mesh-Net NeoMesh from NeoCortec didn’t yet exist either. We simply had GSM 900, GSM 1800, GPS and ISM 868 bands for our design for a people tracker. We set up a home zone at 868 MHz to reduce energy consumption, but with few suppliers for integrated antennas we had to develop our own. Today, there are plenty of suppliers of integrated antennas, but we still choose to develop the antennas ourselves for some projects either because it is cheaper or technically better.

Antenna selection based on return loss

Most developers look at the return loss of the antenna when making their selection. This is a correct first step. It’s good idea to check the number of components in any matching circuit. If there are a large number of passive components you should be immediately suspicious, because more components mean more losses in the matching circuit affecting the radiated power. DIY PCB antennas such as those designed by the Crout team do not need a matching circuit at all and therefore have a very high radiated power. In addition, when considering the return loss, you should check whether the measurements are made with a source that has a constant 50 Ohm impedance, or one that has been matched to the actual impedance of the antenna which may be different.

Selection by the efficiency of the antenna

For the second step, we need to look at the efficiency in average dB or in percent. Here lies another pitfall. Some supplier datasheets state the average of the peak value over all frequencies in all directions in 3D. Somewhere on the spatial view compared to the idealised isotropic omnidirectional radiator peak values are found. All the peak values are then averaged and named the average. The average is thus impressively large compared to the other manufacturers who also name an average. These latter use a different and technically correct average. Picking the highest number without understanding how it was arrived at can result in the wrong antenna choice.

If the efficiency targets in percent or dB are met, then the next point to consider should always be the size of the ground plane used to arrive at these numbers. For the same efficiency at the same frequency, LoRaWAN, Sigfox, MIOTY, NB-IoT or LTEM chip antennas require a ground plane that is 100 mm long or 140 mm long. This means that the average efficiency in the table in the data-sheet is correct and attractive for a design, but may only be valid for a very large ground plane. If the IoT developer’s device is much smaller than the dimensions used for the efficiency measurement given in the datasheet, then they will not achieve the advertised efficiency. With a smaller ground plane, the frequency bandwidth of the antenna also decreases. This is not easy to measure in the developer’s laboratory. In the certified test laboratory, measurements are taken on the band edges. The small PCB leads to a high return loss and thus to reflected radio waves. These then lead to mixed products at the output amplifier of the radio module. Harmonics occur and RED / FCC certification is failed and the unit cannot be sold and a redesign is required that can be expensive in both development cost and lost time to market.

Selection on basis of the antenna size

Another criterion for selection is often the size of the antenna. The whole unit is planned, the designer of the enclosure does their best, the colour of the enclosure is trendy and the unit is small. Some arbitrary amount of room is left for the chip or PCB antenna. If you elect to develop in this manner, you have to face the fact that there’s a good chance that no antenna you can find can will do the job and you will have to start again.

Selection by size and price without considering the technical data

Sometimes people just select with size and price. The technical data are not taken into account at all. The datasheet tells an efficiency of 10%, the price is OK and so is the size. The generally accepted target is an efficiency of 50 % at the band edges. With 23 dBm output power, -3 dB efficiency means that 3 dB of the output power is lost and 20 dBm are radiated. At 6 dB, the power is reduced to 17 dBm but the efficiency is then 25%. At 9 dB loss, the efficiency is then 12.5 %. One manufacturer of an NB-IoT / LTEM Eval kit actually states 10% efficiency. This is then a 10 dB loss in the antenna or the matching network. There will be additional losses in the enclosure or objects in the vicinity that must be added to this. To compensate for the 10 dB loss, you have to transmit with 10 dB more power. 3 dB is a doubling of the power. Since the voltage at the radio module remains the same, the current must double. To achieve 10 dB, the current must become more than eight times as large. The battery is there discharged more than eight times faster, or the battery must be significantly larger to provide the same in service lifetime. The larger battery increases the price and the small unit becomes larger again to accommodate it.

Another manufacturer’s datasheet states that a frequency band should have at least 20 % efficiency. However it does not specify whether it is 20 % at the corners of the band or 20 % in the middle. 20 % at the corners or 20 % in the average over the whole band is a big difference. The same manufacturer uses terms are either inadvertently or intentionally ambiguous: dB is used for efficiency and not dBi. The „i“ at the end of dB indicates an isotropic radiator as a reference. It could also be stated in dBd using a d for the dipole radiator. A temperature range that is given in degrees with no reference to measurement scale tells us nothing. Only by specifying Kelvin or Celsius does it become a valid value. The question is only which normal zero is assumed. If we make an appointment at 8 o’clock it is important to name AM and PM. If we arrange a telephone call on 2 continents, then we need to name the time zone so that we end up together.

Selection based on the 3D radiation diagrams of antennas

More complicated are the 3D directional diagrams. There are two allowed methods for simulation. One method is to set the output impedance to 50 Ohm and feed the antenna across the entire frequency band. Since the antenna reaches its requested impedance of 50 Ohm only at one point in the middle of the band, 25 Ohm and 100 Ohm at the band edges are still good and the requested 50 % efficiency is achieved.

In the other method, the output impedance of the generator is matched to the input impedance. If the antenna only shows 25 Ohm, then the impedance of the generator is adjusted to 25 Ohm. The simulation looks 3 dB better at the band edges as a result.

Very often the colour scale is changed within a datasheet. The colour range is set to match the measured values for a particular set of parameters, so for example dark red in the first diagram stands for 3 dBi and a similar red for 0 dBi in the next with a declining scale of colours in both from red to blue. The 3 dB difference is not always easy to read, particularly if the numbers on the scale are small. The sleight of hand is continued in further diagrams. The same antenna shows -6 dBi in another frequency band. In this band, -6 dB is then set as red. The 6 dB worse frequency range looks the same as the good range at first glance. If you look closely, you will see that dark red is always selected for the maximum value of the respective band. The 3D diagrams therefore cannot be directly compared via the colours you must pay careful attention to the scales on each.

The crowning glory is colourful 3D diagrams without naming the colour scale in the datasheet. They look impressive but are completely useless. The creativity of the manufacturers knows no bounds.

Selection on the basis of 2D directional diagrams

Another popular trick is to change the reference point in the directional diagram for antennas. If you change the reference in the middle carefully, you can turn an antenna with clear nulls into one that shows almost omnidirectional radiation. The diagram above illustrates this and shows the same antenna three times with different middle references. The LinkedIn poll on the right tracks the readers‘ ability to spot this. The majority of 51% have considered the almost circular diagram to be the best antenna and have been misled by it. 26% have guessed correctly. If 26 % guessed it right, then 74% got it wrong, hopefully those getting it right are the designers working with antennas and not the larger group.

Honest antennas

Honest antennas, of course, can also be found. One supplier of honest antennas, for example, is akorIoT www.akoriot.com. Why are the antennas from this company honest antennas? Well, the datasheets for these antennas were created by Harald Naumann. All parameters in the datasheet are briefly explained on the last page of the data sheet. Furthermore, a scale is chosen in all two-dimensional and three-dimensional diagrams which enables the IoT developer to read the values in the diagrams and compare them between diagrams. The peak value of 1.28 dBi antenna gain in the three-dimensional diagram is shown in the table on the first page. If you convert the antenna effectiveness in dB into the antenna efficiency in percent, you will find that the two curves are congruent. How to convert one curve into the other is described in the datasheet. The datasheet of the antennas is not only a data sheet but also a small document with help and references.

Even more honest is the study called „Do it yourself PCB antennas for IoT devices“ by Harald Naumann. This 80-page DIN-A4 study explains in detail how mechanical changes to a PCB antenna, the PCB itself or the enclosure influence performance. After reading the free study, every IoT developer has the freedom to copy and use well-documented antennas.

The result of the study, funded by NeoCortec in Denmark, is a semi-automatic antenna generator software model that generates a well-documented custom antenna for €600 based on four standard antennas. The generator is fed with the information from the customer’s antenna order form. Afterwards, an antenna with the desired parameters of the IoT developer is generated by the software with default settings. This first antenna is usually already very close to the goal of the designer. This first-pass design is then changed manually and an antenna is generated again by the software model. Because these manual changes influence each other (Parameter A influences parameter B and vice versa and so on) the antenna is changed several times until the optimal result of antenna performance is achieved. Such an antenna then has 50 Ohm at the feed point in the middle of the band and no longer needs a matching network. Eliminating the matching networks avoids the problem that the more components there are in a network, the higher the losses become. Furthermore, the tolerances of the components add up. If these tolerances run against each other, higher losses occur in the matching network than in an ideal network without tolerances. Therefore, antennas without a matching network or with only one component in the matching network are clearly superior to chip antennas with 3-7 components in the matching network. The honest antenna is not only honest through its truthful data sheet but also technically better.

Summary of antenna selection

The examples for selection are not complete. However, the examples show that there are many ways to select the wrong antenna. Choosing the wrong antenna is an expensive mistake. Sometimes the error can be corrected by replacing the antenna. In the case of a chip antenna, this means redesigning the PCB. Sometimes the error can be fixed by a custom antenna. In some cases, the device cannot go into mass production because the antenna concept chosen was completely wrong and no working design in possible.

The developer of the wireless IoT device should be aware that there is no standard for antenna datasheets. It is therefore not possible to compare the data without experience. In part, the comparison is impossible because the ways the data you are using for comparison is derived are not even mentioned. The data always applies only to the manufacturer’s reference board. In your own IoT device, the supposedly worse performing antenna may end up being the better one. To find out, only a test setup with measurement of the return loss and, most importantly, the radiated power can help.

It is cheaper to get consulting from an external expert when choosing antennas. Crout offers this as a service. We have reference customers who selected the antenna together with us and developed the customised enclosure so that the antenna works perfectly. The antennas were measured and approved in the enclosure and on an empty PCB. In only 15 months of development, the IoT device with LTE, WiFi and LoRa of our customers received radio approval without any problems. We attended the radio approval test on behalf of the customer. In order to save costs and surprises, we make control measurements at the critical points before the radio approval.

We have reference customers who are very grateful for the rescue of their projects and have been requesting all custom antennas from us since. Other customers take advantage of our proven 0 USD PCB antennas wherever they can.

Services around antennas

  • Consulting on antennas and analysis of the data sheets of all manufacturers
  • Test set-ups and measurements with the selected antennas in the enclosure with an empty PCB
  • Customised antennas in different manufacturing processes
  • Antenna matching of antennas from all manufacturers
  • Webinars, seminars and workshops on wireless IoT
  • Support for IoT developers in the development of PCBs
  • Pre-tests for RED / FCC and execution of measurements in the test laboratory
  • Hardware-related consulting for the development of firmware („mini“ BIOS for IoT devices)

On request, we offer personal training on the selection of antennas and show in more detail known errors in the datasheets such as those we have discussed here. We have to do this face to face because the graphics are the intellectual property of the respective manufacturers and they certainly do not give their permission to discuss the errors.