Selecting an antenna for an IoT device
Harald Naumann
As the winner of the 5G NTN Antenna Award , I am happy to inform you about my 0 USD antenna concept and more – contact me!
If you want to develop a wireless IoT device, you have to choose a good antenna. Antenna datasheets are full of a lot of technical data, which is often not fully known to the IoT developer. With this edition of IoT M2M Times, we shed some light on the darkness of radiant antennas.
Table of contents:
Typical antenna selection decision process
The following paragraphs are based on experience with wireless IoT projects over the past 20 years. 20 years ago it was called M2M. Today it is called IoT. But the antenna does not care whether it works for an M2M or an IoT device. Not much has changed in the frequency bands in recent years either. There was still no LPWAN with LoRaWAN, Sigfox, Mioty, NB-IoT and LTE-M. The innovative Sub-GHz Mesh-Net NeoMesh from NeoCortec didn't exist yet either. We had GSM 900, GSM 1800, GPS and ISM 868 in our people tracker in 2020. We set up a home zone with 868 MHz to reduce energy consumption. As long as the tracker was in the home zone, the power-hungry GSM and GPS module could be disabled. But there were hardly any suppliers for integrated antennas. So we had to develop the antennas ourselves. Today, there are plenty of suppliers for integrated antennas, but I still develop the antennas myself in some projects because it is cheaper or technically better. Now a little more about the antenna selection decisions made by IoT developers.
Return loss
Most developers look at the return loss of the antenna when making their selection. This is true in the first step. A matching network of many passive components should make one suspicious. Every more component also means more losses in the matching network. The DIY PCB antennas in my antenna study do not need a matching network at all and therefore have no losses in the network and a very high radiated power. The ratio of the injected power to the radiated power is called antenna efficiency. In the simulation, it can be decided whether the fed source has 50 Ohm constant or whether the source matches the output impedance to the input impedance of the antenna. Both are allowed and are not documented in the datasheets of the antennas. The method with the matched impedance gives better values for the same antenna. The method is perfect for determining losses near the antenna, such as the enclosure or battery, more precisely. Study for download: https://www.akoriot.com/white-papers/
Antenna efficiency
The second step should be to look at the efficiency in dB or in per cent in the average value. Here already lies the first pitfall. Some datasheets give the average value of the peak overall frequencies in all directions in 3D. Somewhere on the spatial view compared to the isotropic omnidirectional radiator a peak value is found. All the peak values are then averaged and named the average value. The average value of the peak values is impressively large compared to the other manufacturers, who also name an average value. They call the technically correct average value. The wrong average value thus leads to the wrong choice of antenna. A trained eye of a specialist for antennas recognises such errors immediately. Do not be seduced by a high antenna gain or efficiency. There are known technical limits that no antenna manufacturer can break.
Size of the antenna
Another criterion for selection is often the size of the antenna. The whole unit is planned, the designer of the enclosure does his best, the colour of the enclosure is trendy and the unit is small. Somewhere there is still room for the chip antenna or PCB antenna. If you develop and decide in this way, you have a good chance of not finding an antenna that can do the job. You develop the other way round. You choose the enclosure and antenna and test or simulate them before you develop the whole IoT device. In the simplest case, you 3D print the enclosure put the antenna on the empty PCB and measure the return loss and antenna radiation pattern in 3 axes. Only when this shows function do you continue. Ground plane and efficiency of the antenna If the efficiency in per cent or dB is good, then the second look should always be the size of the ground plane. 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 and 140 mm long. This means that the efficiency in the average value in the table in the data-sheet is correct but may only apply with a very large ground plane. If the IoT developer's device is significantly smaller, it will not achieve the target efficiency. With a smaller ground plane, the frequency bandwidth of the antenna also decreases. This is not noticeable in the tests in our own laboratory. In the certified test laboratory, measurements are taken on the band corners. The small PCB leads to a high return loss and thus to radio waves that run back. These then lead to mixed products at the output amplifier of the radio module. Harmonics are created and RED / FCC is not granted.
Antenna gain
The bad news in advance. There is no gain. There are always losses. An antenna without LNA is a passive component and cannot amplify. The antenna only redistributes the energy. An omnidirectional antenna such as a cellular chip antenna can never radiate completely spherically and is compared to an ideal omnidirectional radiator. This is called an isotropic omnidirectional antenna. A perfect antenna such as the one in the DIY antenna study comes very close to the spherical radiator and is better than any chip antenna. Chip antennas are already a compromise between size and efficiency. Download the study: https://www.akoriot.com/white-papers/
A good example of high gain is Yagi antennas. This type of antenna radiates primarily in one direction. The good gain is due to the poor gain in the other direction. In sum, the radiated power is less than the injected power. The gain is the gain at the peak and in an IoT device like a tracker you need an antenna gain that is as uniform as possible in all directions.
Size and price without attention to technical data
Sometimes people just select with size and price. The technical data are not taken into account at all. The datasheet says the efficiency of 10%, the price is OK and so is the size. The target is an efficiency of 50 % on the band corners. At -3 dB efficiency, 3 3 dB of the output power of 23 dBm are lost and 20 dBm are radiated. At 6 dB, the power is reduced to 17 dBm and 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 says 10 % efficiency. That is a 10 dB loss in the antenna or matching network. The losses in the enclosure or objects in the vicinity come back. To compensate for the 10 dB loss, transmit with 10 dB higher power. 3 dB is a doubling of the power. Since the voltage at the radio module remains the same, the current must double. At 10 dB, the current must become more than eight times as large. The battery is discharged more than eight times faster or the battery has to be significantly larger. The large battery increases the price and the small device becomes larger again. Another manufacturer's datasheet states that a frequency band should have at least 20 % efficiency. Whether it is 20 % at the corners of the band or 20 % in the average value is not specified further. 20 % at the corners or 20 % in the average value over the whole band is a big difference. The same manufacturer uses terms that do not exist. On top of that, dB is used for efficiency and not dBi. The i at the end of dB indicates the isotropic antenna as a reference. It could also be a d for the dipole radiator. Let's compare it with temperature. A temperature in degrees says nothing. Only by specifying Kelvin or Celsius does it become a valid value. Sea level is given in height above sea level. The only question is 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 conference call on 2 continents, we need to name the time zone so that we come back together at the end. Is it dBi or dBd? Are they peak values or honest average values in the antenna data sheet?
3D radiation patterns of antennas
More complicated are the 3D radiation patterns. As already mentioned, there are two permitted methods for simulation. In one method, the output impedance is set to 50 Ohms and the antenna is fed over the entire frequency band. Since the antenna reaches its requested impedance of 50 Ohms only at one point in the middle of the band, 25 Ohms and 100 Ohms at the corners of the band are still good and the desired 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 then looks 3 dB better at the band corners. Sometimes the colour scale is changed within a datasheet. The colour dark red stands for 3 dBi in the first diagram and a similar red for 0 dBi. The 3 dB difference is not always easy to read. The scale then changes to orange, yellow, green and blue. The sleight of hand is used in the following 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 can see that dark red is always chosen for the maximum value of the respective band. The 3D diagrams cannot be directly compared via the colours. The crowning glory is colourful 3D diagrams without naming the colour scale in the datasheet. The creativity of the antenna manufacturers has no limits. I cannot show the pictures of the quoted antennas here because I would then need permission from the manufacturers. However, I can go into more detail in a video call or in a webinar.
领英推荐
2D radiation patterns of antennas
A popular trick is to change the reference point in the radiation pattern. If you change the reference in the middle, the diagram with clear nulls becomes almost omnidirectional radiation. The diagram shows the same antenna three times. The other diagram shows the evaluation of the query about the antenna here on LinkedIn. The majority have considered the almost circular diagram to be the best antenna and have been misled by it.
The honest antennas
The honest antennas, of course, also find. 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 within the diagrams. The peak value of 1.28 dBi antenna gain in the three-dimensional diagram is reflected in the table on the first page. If one converts the antenna efficiency in dB into the antenna efficiency in per cent, one finds 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 datasheet 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 which mechanical changes to a PCB antenna, the PCB itself or the enclosure have which influence. 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 that generates a well-documented custom antenna for €600 based on four standard antennas. The generator is fed with the information from the antenna order form. Afterwards, an antenna with the desired parameters of the IoT developer is generated with default settings. This first antenna is usually already very close to the goal. The first design is then changed manually and an antenna is generated again. Partly, the manual changes influence each other. Parameter A influences parameter B and vice versa. This means that the antenna is changed several times with the default settings 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. 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 but also technically better.
Summary of antenna decision making
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 mistake 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 could not go into mass production at all because the antenna concept chosen was completely wrong. 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 data for comparison is not mentioned at all. The data always only apply to the manufacturer's reference PCB. In your own IoT device, the supposedly worse 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. My business partners and I offer this as a service. We have reference customers who select the antenna together with us and develop the customised enclosure so that the antenna works perfectly. The antennas were measured and approved in the enclosure and empty PCB. In only 15 months of development, the IoT device with LTE, WiFi and LoRa received radio approval without any problems. We assisted with the radio approval on behalf of the customer. 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 the project and have been requesting all antennas from us since the rescue. In the meantime, only the tried and tested 0 USD PCB antennas are used by some customers, if possible. The antennas are always selected and tested first.
Our services:
On request, we offer personal training on the selection of antennas and show you the known errors in the datasheets. There we can show and explain the wrong tables and graphs. As already mentioned, the public online in a LinkedIn article is unfortunately not possible because the graphics are the intellectual property of the respective manufacturers and they certainly do not give permission to discuss the errors. Enquiries about antenna-related services are welcome at harald.naumann (at) lte-modem.com ??????
About Harald Naumann
I trained as an industrial electronics technician for over 30 years. The focus was on analogue and digital electronics. From op-amps to digital gates to microcontrollers, everything was there. After that, I worked in the German Armed Forces repairing radio equipment and in the evenings I qualified as a professional radio and television technician. With this knowledge, I then worked in the special testing department of a radio equipment manufacturer and completed an evening course to become a state-certified electrical engineering technician. Antennas did not come back in-depth in my training. As the head of development for locating devices, I had my first painful experience with antennas. Only the third developer was able to solve the task of antennas in the locating device with GSM, ISM 868 and GPS in 1999. Since 2000 I collect antenna layouts like other stamps. Add to that reading a lot of PDFs and books plus listening to experienced antenna developers. My IoT M2M Cookbook from 2014 consists of approximately one-third of the book on antenna design. I invested the income in a Vector Network Analyser and a measurement system to measure antenna gain in 3 axes. From 2015 until today I have built up a network of antenna designers. My antenna team can simulate, design and measure antennas in 3 axes. This includes PCB antennas integrated into the main PCB, Flex PCB antennas, stamped metal antennas, spiral antennas, LDS antennas and much more. We can solve almost tasks for modern wireless IoT antennas. We use LDS for chip antennas. We print metal on plastic carriers and pack the chip antenna on the roll. But we also print it directly on the inside of the plastic enclosure. I merge everything into one company for components and services. The first truly global one-stop shopping for wireless IoT developers. Every IoT developer can then get everything from a single source. But you can just order a consulting, order a matching network for 400 Euros or download the free study "Low cost do it yourself antennas for the wireless IoT" from me here: https://www.akoriot.com/white-papers/
No one has to, everyone may. I will present my antenna study at Embedded World in Nuremberg (21 - 23 June 2022). Before and after that, I am available for discussions on the development of wireless IoT devices. Please send appointment requests to harald.naumann (at) lte-modem.com ??????
If you don't want to wait, you can meet me at the Cebit in Hanover in May. And if you're in an even bigger hurry, you can get a video call. See you soon in Nuremberg, Hanover or at the monitor in the video call. And pre-registrations for the daytime webinar on antenna design are also welcome.
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Enquiries are welcome to harald.naumann (at) lte-modem.com ??????.
Imprint
Driving IoT ecosystem evolution at Cre8 IOT
2 年Thank you for your detailed information, it will help alot of startups
Head of Field Management Practice - Divisione Transportation & Logistics di Almaviva SpA | Head of Field & Smart monitoring at Tecnau Transport Division gruppo Almaviva SpA
2 年Very interesting topic