Designing for Antenna Isolation in Your Wireless System
Anyone who has taken apart an old cell phone or designs IoT devices knows multiple communication capabilities are present in these designs, each requiring different antennas. The RF designer should already take precautions for interconnect isolation, but antenna isolation is just as important when modeling and designing wireless systems.
The most basic antenna isolation technique simply requires placing antennas farther apart and designing the matching network to provide some level of filtration away from the desired operating frequencies. On a real PCB for a wireless device with multiple communication protocols, the solution requires going further and considering the stackup, as well as engineering some electromagnetic bandgap structures for suppressing interference.
Types of Antenna Isolation
Antenna isolation measures should be implemented when multiple antennas are present on the same board. The simplest form of isolation is to separate different antennas into different portions of the board as the radiation emitted from reflectorless antennas will naturally fall off with distance. This is followed by carefully tuning the antenna matching network to prevent excess gain. Isolation is reciprocal, i.e., it is a function of both antenna gains and transmittance between the two elements. A low isolation value between two antennas means the antennas pick up each other’s radiation.
When we say “types” of antenna isolation, we’re really referring to how electromagnetic radiation from one antenna gets received by another antenna. When a real board is put into its enclosure, the environment for radiation can become quite complex. Isolation needs to be designed to suppress the following sources of interference:
- Direct radiation: This involves simply reducing the strength of radiation sent from one antenna and received by another antenna. This is a function of directionality, polarization sensitivity, and any shielding elements.
- Enclosure resonances: Emitted radiation can excite resonances inside an enclosure, which then causes interference between different board sections due to reflections and multipath propagation. Enclosure resonances appear as small spikes in the radiation pattern.
- Waveguide mode excitation: Propagating parallel-plane waveguide modes can be excited when an antenna is excited and radiates at certain frequencies. This problem is not the result of a mis-planned return path; instead, this is an effect that occurs due to radiation from an antenna. Similarly, surface waves can be excited by a radiating antenna, particularly planar antennas, which can then be guided to a different board section thanks to the refractive index contrast between the substrate refractive index and air.
- Noise coupling: Noise from one section can propagate into another section as EMI. The issue with EMI between antennas is partially solved with smart floorplanning.
Antenna isolation is a measure of how easily one antenna will pick up radiation from another antenna, which is quantified in terms of S12 between the two antenna elements. Typical isolation goals are set to at least +20 dB, depending on the product, and isolation can be measured with a vector network analyzer. Antennas that share a reference plane, such as antennas on a smartphone, can have low isolation due to currents excited in the ground plane, which will decrease the efficiency of both antennas.
Isolation Against Direct Radiation
When dealing with highly directional antennas, such as phased arrays, there is little more to do than to carefully place antennas such that the main lobes and sidelobes are not pointed directly at each other. Similarly, when dealing with two polarized antennas, the two antennas simply need to be oriented such that the antennas are electrically orthogonal to each other. However, this is not practical in many advanced mobile/IoT products.
In the case where radiation is unpolarized or weakly polarized, and the antennas are close to each other, the gain of the two antennas and the matching networks must be precisely tuned to provide the right level of isolation. LC matching networks with a series or shunt resistors can provide sufficient matching to a feed microstrip at the relevant antenna frequencies; the isolation provided by matching networks may be sufficient when the two antenna frequencies are quite different. However, with high-power radiators and with sufficiently closely-spaced antennas, additional measures may be needed to increase the level of isolation.
Electromagnetic Bandgap (EBG) Structures for Isolation
Even if you’ve never heard of an electromagnetic bandgap (EBG) structure, you’ve probably heard of via fences. A via fence is likely the simplest type of EBG structure you’ll encounter in most RF designs, but variations on via fence structures can be designed to provide wideband isolation between antenna arrays. These structures can be used to address two of the four isolation points listed above: surface wave suppression and waveguide mode suppression.
Conceptually, these structures can be analyzed electrostatically or using a circuit model; both aspects provide an understanding of how these structures aid isolation. In terms of a circuit model, these structures can be analyzed as LC bandstop filters, producing high impedance at the resonance frequency for the structure. Placing multiple EBG structures in parallel (i.e., in multiple layers) or in series (i.e., next to each other on the same layer), allows the resonance and bandwidth to be precisely tuned to desired values. Moreover, stacking in parallel effectively forms a higher order filter and narrows the structure’s bandwidth.
Although EBG structures take up more board space than a via fence, they can be designed to provide much higher isolation. In addition to providing antenna isolation through surface wave and waveguide mode suppression, EBG structures also help suppress simultaneous switching noise (SSN) in a PDN. This makes them quite useful for analog components running at a single frequency or a small number of frequencies, but they are not so useful for digital PDNs. This is because, like digital signals, SSN in a digital PDN occurs over a broad bandwidth. Take a look at this IEEE article for more information on EBG structures.
The design and analysis tools in Altium Designer® can help you design a matching network, analyze circuit models for EBGs, or layout your boards for sufficient isolation. The layout tools are ideal for designing EBGs in your board, and its simulation tools can help you tune your matching networks and analyze circuit models for your isolation structures. Altium Designer also includes an integrated set of tools for building schematics, managing components, and preparing deliverables for your manufacturer.