Industrial internet increases requirements for antennas
Wireless devices are becoming more common due to industrial internet. This means an increasing number of devices with an antenna. The antennas are usually covered and hidden inside the devices. Industrial internet devices are also heading for a direction where they become smaller, cheaper and less power consuming all the time. Often they are also battery-operated. All this causes headache for a person responsible for wireless (radio) link performance and operation.
The operating environment may pose the product many types of requirements which affect also the antenna design: The product needs to withstand, for instance, dust, moisture, cold, hot and various types of mechanical stress. This is why its electronics may have to be potted; meaning that the electronics is entirely over molded in epoxy. The antenna needs to work even if it’s surrounded by a thick layer of mass.
What should be the goal?
Industrial internet solutions can typically utilize only a limited amount of energy. The operating environment may also contain a lot of undesirable electromagnetic radiation. This type of undesirable radiation may originate from, for instance, other radio systems, low quality power supplies or electric motors. For antenna design, this means that as much of the power supplied to the antenna as possible needs to be radiated out into the desired direction and only containing the wanted frequencies.
The antenna performance is often characterized by three parameters: efficiency, gain and selectivity. When considering these parameters it is worth noting that the antennas are typically reciprocal: they work the same way both in transmit and reception direction.
The efficiency of an antenna tells simply how much of the supplied power the antenna is able to radiate out. If, for instance, the supplied power is one watt and the radiated power is half a watt, the efficiency of the antenna is 50%. Efficiency is often described in decibels; for example an efficiency of 50% equals to -3 dB and an efficiency of 25% equals to -6 dB.
The effect of the antenna efficiency or gain on the radio range is easy to estimate in decibels. A weakening by 6 dB halves the range in free space.
Based on their directivity, the antennas can be divided into two groups: directional antennas and omnidirectional antennas. Industrial internet solutions often require as omnidirectional antennas as possible, as they may consist of many devices, whose mounting locations and directions are not known in advance.
Antenna gain means the radiation capability of the antenna into its main lobe or the best direction. The gain is typically announced in dBi or dBd units. dBi means the antenna gain compared to the ideal, spherically omnidirectional (isotropic) antenna, and dBd respectively compared to the ideal dipole antenna. The radiation pattern of a dipole antenna resembles a doughnut. This kind of doughnut-like radiation pattern is often a desired feature in industrial internet solutions – the radiation is efficient horizontally, but not vertically.
Antenna selectivity can be defined as the ability to provide poor efficiency for undesirable frequencies. This limits the access of disturbing frequencies into the radio receiver.
A descriptive example of the importance of the antenna selectivity is the liberalization of the 800 MHz frequency band for the use of the new 4G network in 2014. The highest television frequencies are found right below 790 MHz and the new 4G network frequencies right above it. Some antenna TV households have had to install extra filters between the antenna and the TV, or to change the whole antenna into a more selective one, in order to get rid of the noise appearing in the TV signal.
Does the antenna gain need to be as high as possible?
Efficiency is easy to understand correctly, but antenna gain can easily be misunderstood. One might automatically think that the higher the gain the better the antenna.
High antenna gain may lead into a situation where the antenna is very directive and works fine into a specific direction, but when slightly turned the connection breaks down. High antenna gain does not guarantee good efficiency either. Hence, it is important to deal with both efficiency and antenna gain together when estimating the quality of an antenna.
High antenna gain may also pose a problem for the authorities. Authority requirements may limit the power to be radiated by the antenna. The gain of the antenna in use may be so high that the transmitting power has to be reduced, not to violate the authority requirements. This causes the transmitting power to decrease also in all other directions.
Typically, a good antenna for industrial internet applications has the gain of approximately 0 dBi and efficiency of at least 50%. Such an antenna is usually an omnidirectional one, which is suitable for mobile or wall-mounted products where it is normally not known where the receiver is located, and the antennas do not need to be mutually directed. An omnidirectional antenna is then the best alternative.
External factors play a role
Several factors are impacting the antenna signal path. The starting point is the transmitter power and the gain of the antenna at the transmitting end. At the receiving end, the gain of the receiving antenna and the sensitivity of the receiver are important.
Distance has a significant impact on signal strength. When the distance doubles, the received power drops to 25%.
The frequency of the signal affects the signal strength as well. Lower frequencies suffer less path loss, but in other hand, physical size of antenna tends to be larger. For instance, if the frequency is lowered from 1800 MHz to 900 MHz, the received signal strength quadruples.
Antenna polarization (vertical, horizontal or circular polarization) has an impact on the signal strength too. If the antennas of both the transmitting and receiving product share the same polarization, there will not be any loss due to polarization. If not, then the polarization loss could be tens of decibels in the worst case.
Environment and obstacles affect significantly the signal transmission. Multipath propagation caused by reflections may cause the received signal to vanish almost completely. Additionally weather conditions like moisture may cause the signal to decrease.
There are also plenty of other signals in the air that can interfere with the reception of the desired signal. All the disturbances mentioned above can significantly reduce the range of the radio transmission. It is not rare to see the radio circuit range dropping to less than 10% of the theoretical range.
Finally, it is good to remember that, for example, the circuit board containing the antenna may form an essential part of the antenna performance. An antenna radiator designed on a circuit board size of a margarine package works probably better than in a circuit board size of a matchbox.
Antenna design along from the beginning
An antenna may be either external or internal to the product case. In industrial internet applications it usually resides inside the case. Sometimes the product is to be installed in such a challenging place that it requires an external antenna connected with a coaxial cable.
Upon selecting the antenna one should know exactly for which environment it is meant, and for which market it has been designed.
At the beginning of the project, sufficient room for the antenna should be reserved. That is why it is good that the antenna designer is involved in the project from the very beginning.
An important part of antenna design is the use of electromagnetic simulation tools. Simulation can effectively be applied to fine-tune the antenna dimensions for optimized performance. Simulation is particularly useful when prototyping with real hardware is time for some reason time consuming. Read more about simulation here: Less prototyping rounds and EMC challenges with simulation.
It is good to know the installation environment of the device at an early stage. It is essential to know in which position the antenna will reside inside the product, so that no extra losses will emerge due to polarization or radiation pattern.
Products possessing various radios and antennas cause also problems for antenna design. Unfavorably located antennas may cause a weak isolation between the radios, which causes the radios to interfere with each other and may even completely prevent each other from communicating.
The circuit board may also contain other types of electronics which cause noise to be connected to the radio through the antenna thus interfering or even preventing the reception.
If the antenna is inside a potting or has been over molded, the antenna design needs to consider the qualities of the material used. The material has an impact on the losses and resonance of the antenna, i.e. the frequency it operates on. The material has two parameters that need to be known: relative permittivity and dissipation factor. Relative permittivity describes how much the propagation of radio waves is slowed down by the material, therefore affecting the resonance frequency of the antenna. Dissipation factor describes how much radio waves dampen when going through the material.
An important part of the antenna design is the equalization of the characteristic impedance of the transmission line and the antenna. If the characteristic impedances of the antenna and the transmission line are different, the power cannot be exploited to the fullest. Part of the power is reflected back. For example, a TV cable with a characteristic impedance of 75 ohm should not be extended with a 50 ohm coaxial cable.
Measuring ensures functionality
Antenna measurement is an extremely important thing to do. Really seldom a product can be put on the market completely without any measuring.
The antenna measurement of a ready-made product should always be conducted in a way that includes all the mechanics of the product, and that takes into account also the potential impact of the installation environment.
The measurement must be able to verify that the product and the antenna are working according to the specifications.
Two different measurement methods are typically used for verifying the antenna design.
The first method includes measuring how much power the antenna can receive without reflecting it back to the measurement device. At that moment it is not yet known how much of this power is radiated to the sky, because power can also be absorbed by the lossy materials surrounding the antenna.
The second method is to use e.g. Satimo Stargate 64 measurement system within a specific measurement chamber. The system can measure the power being emitted by the antenna. This method allows for finding out not only the efficiency, gain and selectivity of the antenna, but also the three-dimensional radiation pattern of the antenna.
If there are several radios or antennas in the product, a separate measurement chamber for measuring the Interoperability (HW IOP) efficiency needs to be used. HW IOP measurement inspects the impacts of the interferences caused by the electronics of the device itself on the sensitivity of the radio receiver.
Important things to remember
1. Introducing an antenna designer in a product development project as early as possible pays off. His expertise should be utilized even in selecting a commercial antenna. Also the selected radio technology may significantly affect the challenges in antenna design phase.
2. Always design the antenna according to the product and its operating environment. Even a ready antenna component can easily get mistuned, unless the environment has been constructed strictly according to the manufacturer’s guides. It is often good to reserve some place for a couple of tuning components in between the radio circuit and the antenna.
3. Reserve space for a large enough antenna and a favorable installation location. In general, the bigger the antenna and the fewer the obstacles around it, the better the end-result.
4. In antenna design, large metal planes can often be utilized, for example the ground plane of the circuit board.
5. Before starting the actual product development project, sufficient radio signal range can be assured in a variety of ways: with architectural design, testing with evaluation kits, building antenna prototypes, making isolation measurements in demanding environments (e.g. measuring penetrations in buildings, wall structures and installation cabinets), and HW IOP measurements.
Etteplan, Embedded Systems and IoT