Selecting an antenna for an IoT device -Part 1
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!
Anyone who wants to develop a wireless IoT device must select an antenna. Antenna datasheets are full of a lot of technical data, which is often not fully known to the developer. With this issue of IoT M2M Times, we shed some light on the darkness of antennas.
Table of contents
Typical antenna selection decision process
The following paragraphs are based on experience with wireless IoT projects over the last 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. We set up a home zone with 868 MHz to reduce energy consumption. 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. The lines will tell more about the antenna selection decisions made by IoT developers today.
Antenna selection based on return loss
Most developers look at the return loss of the antenna when making their selection. This is correct in the first step. In the second step, the view of efficiency in average in dB or in percent. Here already lies the first pitfall. Some datasheets state the average of the peak value over all 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. The average is impressively large compared to the other manufacturers who also name an average. They use a different and technically correct average. The wrong average thus leads to the wrong choice of antenna.
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 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 like this, you have a good chance that no antenna can do the job.
Selection by the efficiency of the antenna
If the efficiency in percent or dB should be considered, then the second view should always serve 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 or 140 mm long. This means that the average efficiency in the table in the data-sheet is correct but may only be valid for a very large ground plane. If the IoT developer's device is much smaller, he will not achieve the avoided 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 edges. 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 occur and RED / FCC is not granted.
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 target is normally an efficiency of 50 % on the band edges. At -3 dB efficiency, 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. This is then a 10 dB loss in the antenna or the matching network. The losses in the enclosure or objects in the vicinity are 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. 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 must be significantly larger. The large battery increases the price and the small unit 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 middle is not specified further. 20 % at the corners or 20 % in the average 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 radiator as a reference. It could also be a d for the dipole radiator. 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 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 telco 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 dark red in the first diagram stands for 3 dBi 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 to 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 will see that dark red is always selected 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 manufacturers knows no bounds.
Selection on the basis of 2D directional diagrams
A popular trick is to change the reference point in the directional diagram for antennas. If you change the reference in the middle, the diagram with clear nulls becomes almost omnidirectional radiation. The diagram on the right shows the same antenna three times. The other diagram shows the evaluation of the query about the antenna here on Linkedin . The majority with 51% have considered the almost circular diagram to be the best antenna and have been misled by it. 26% have guessed correctly. A few of the 26% are antenna experts. They are not fooled. A few of the 26 % guessed it right. 74% got it wrong. That 74% got it wrong is not a big deal. The 74% are experts in other fields.
The honest antennas
The 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 within the 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 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 more honest antenna is not only honest 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 could not even go into series production 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 are not even mentioned. The data always applies only to the manufacturer's reference board. 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 accompanied the radio approval 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 the project and have been requesting all antennas from us since the rescue. In the meantime, some customers only use the proven 0 USD PCB antennas if possible. The antennas are always selected and tested first.
Services around antennas
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. 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 services are welcome at harald.naumann (at) lte-modem.com ??????
Imprint
Product / Growth / Blockchain
2 年Thanks for sharing
Digital transformation through IoT & AI
2 年@s4iot We choose based on CMW500 performance testing report, radiated mode.