This article highlights some of the factors that affect the behavior of antennas and their RF signals and which should be considered in the layout of the PCB.
Placing an antenna in a design always needs care, but when there are two or more antennas in the design, it is even more important to understand how the antennas radiate, how they will perform together, and how their relative positions will impact the signal.
Generally, SMD antennas are designed to co-exist in proximity to other components, provided some basic rules are followed.
The advice is to keep antennas away from other noisy components and to allow a ground plane below the antenna, to allow it to radiate effectively. It is important to keep the space below the antenna clear throughout the PCB stack-up.
To design a device with several different antennas inside, you will want to know how multiple antennas will co-exist and perform in the one single system on the PCB. The requirement for various radio systems to operate with antennas in close proximity to one another is called “in-device coexistence.”
In some cases, two antennas are used to work together on the same frequency in a diversity configuration, which will provide a stronger transmission than one antenna alone. In other cases, a device needs more than one wireless connection operating on different frequencies and design should allow for some separation between these, to allow them to operate independently in “co-existence”.
With good design, you can achieve high performance and reliability for all of the antennas on the board. The designer’s goal is to place the antennas in a way that achieves isolation between all of the antennas and allows them all to operate correctly together.
Every antenna has its own radiation pattern, which will be shown on the manufacturer’s datasheet, as the antenna would radiate in perfect conditions, for example in an anechoic chamber.
Radiation patterns are usually shown as a 3D pattern and as a two-dimensional cross section of that pattern. The antenna will radiate around an axis that runs along the length of the antenna, and the electrical energy from the antenna is reflected most strongly in a direction that is perpendicular to that axis. This is the polarisation of the antenna.
The radiation pattern for an antenna is normally measured in the centre of the frequency band where the antenna is to be used.
Figure 1. 3D antenna pattern for an LTE antenna at 1990 MHz. (Source: Antenova)
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Figure 2. Antenna pattern for the same LTE antenna at 1990 MHz, cross section. (Source: Antenova)
Diversity is an antenna technique where two antennas are used together on the same frequency to create a more reliable wireless connection. It is a great solution for mobile wireless devices that are moving around in free space, as the two antennas working together in tandem are more likely to provide a reliable link as the device is moves around.
With diversity, the first antenna is the main antenna and the second antenna is the diversity antenna. The two antennas can share the same ground plane, and they are placed in different spatial zones, this reduces “coupling” – a phenomenon where the two antennas reflect similar signals.
In a diversity configuration, both antennas transmit simultaneously, but pointing in different directions. The receiver will then take the stronger of the two signals, which will improve the reliability of the signal received. The pair of antennas can be placed on the opposite corners or opposite sides of the PCB.
When the antennas are used in a diversity configuration, they are placed to reduce coupling – i.e., the two antennas are placed so that their radiation patterns are completely different to each other.
The polarisation of an antenna is parallel to the long axis of the antenna, and the antenna radiates its energy in perpendicular to this axis, with zones of null power at each end of the antenna.
For the best cross correlation, the pair of diversity antennas should be placed at cross polarisation, different polarisation in space and in polarity. The receiver will take the strongest signal from one or the other of the antennas. This achieves a stronger reception than can be achieved with one antenna alone. In practice, this usually means that the two antennas will be placed at 90° to each other, which achieves different polarisation, and sends more reliable signals to the receiver as the devices moves around.
Reducing coupling effects
Antenna-to-antenna coupling is a natural occurrence in a compact device. This decreases the radiation pattern and increases the cross-channel interference suffered by each antenna. It also alters the input characteristics of the antenna. You can minimize coupling effects by etching via on the ground plane.
Distance between antennas
The distance between the two diversity antennas should be at least one quarter of a wavelength, and the distance between them will affect the signal.
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Figure 3: Example –The diagram shows the Isolation vs. Distance from the Main antenna to the Diversity antenna. 40mm, 25mm and 20mm are shown for comparison. This example shows Antenova’s Integra antenna SR4L049. (Source: Antenova)
Isolating antennas at different frequencies
Antennas that operate on different frequencies should be placed so that they will not interfere with each other. In antenna terms, they need to be electrically isolated from each other. The goal is to place each antenna so that it achieves high performance, however the isolation may reduce the radiating power of the antenna. The value for isolation is measured as the S21 co-efficient, with a network analyser.
There are a number of ways to improve the isolation between the antennas. Simply placing the antennas further apart, with consideration for their radiation patterns, will create some separation, which will help.
Placing the antennas so that there is isolation between each pair of antennas will help them to radiate independently, but with some reduction in the transmit power of each one.
The next option is to use a filter to reduce the efficiency of one antenna in frequency that the opposite antenna requires.
Figure 4. Beam steering is an antenna technique to improve isolation and cross correlation in diversity applications. (Source: Antenova)
ECC (Envelope Correlation Coefficient)
To verify the isolation capability of a MIMO antenna system, the ECC is an important performance criterion in MIMO antenna systems to study. The ECC can be calculated based on the S‐parameters or the far‐field characteristics of the antenna system. ECC based on far‐field parameters considers the direction of the radiated beam of each antenna in the MIMO antenna system, while ECC based on S‐parameters considers the port characteristics of the two antennas.
ECC based on far‐field properties is considered more accurate in an isolation analysis, although it is more difficult because of the need to measure the radiation patterns of the antenna. An ECC value less than 0.5 is generally considered acceptable for a MIMO antenna system.
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Radiated vs. Return loss
Figure 5: The image shows the radiation plots for two antennas in parallel and at right angles, and how the second configuration shows a more consistent signal. (Source: Antenova)
Plenty of mobile devices use multiple wireless connections. They could be any combination of 4G/LTE, MIMO/WLAN, Bluetooth or GNSS positioning antennas, and there can easily be five or six antennas within one design – for example diversity antennas for LTE/4G to provide the cellular link, a Wi-Fi antenna for Bluetooth, and a GNSS antenna for positioning. In this case, the aim is for all of the antennas to operate in side by side, but without interfering with each other, in the multi-systems environment. If co-existence is not considered, it may well be the case that a strong LTE/4G signal will block the signal from a smaller antenna, such as a tiny Wi-Fi antenna.
Designing a PCB with more than one antenna is not straightforward. This article highlights some of factors that affect the behavior of antennas and their RF signals and which should be considered in the layout of the PCB. To be sure that the components are all working correctly together, the PCB should be sent for over- the-air-testing in an anechoic chamber. It will highlight any modifications that are needed to the design and show how the antennas are behaving together and predict the real-world performance for the device.
This article was originally published on Embedded.
Geoff Schulteis is Senior Antenna Applications Engineer with Antenova Ltd. Geoff has more than 20 years of experience in designing, integrating and testing antennas, and he currently heads technical support for Antenova’s customers in North America. He is an antenna engineering professional with more than 20 years’ experience designing, integrating and testing antenna systems for consumer products from R&D through manufacturing and commercial deployment.