There’s more than one reason why you shouldn’t retrofit an antenna. But in practice, there are situations where it can be hard to avoid. It could be because a device has failed certification. It could be due to a change in manufacturing requirements. Or, it could be simply because the antenna was only considered late during the design process.
In each of these cases, retrofitting an antenna represents a unique challenge. Plenty of design decisions have already been made, meaning there are more constraints for the designer to integrate an antenna. If you do have to retrofit, then a flexible printed circuit (FPC) antenna may be a better option – but if you do have the luxury of time, circuit board space and some RF know-how, then you may be able to work with an SMD.
The cost of retrofitting
Antennas are one of the few components that must be carefully designed into the product to get the best performance. This means that they have recommended keep-out areas, require a ground plane area to radiate and require additional circuitry (such as transmission lines) where their physical design determines performance. In an existing design, there is far less flexibility to integrate these components, antennas and circuitry, which may restrict the levels of performance that can be achieved. Fortunately, however, there are quite a few solutions for cases such as these, like FPC antennas and case-mounted antennas.
Consider the antenna first, where possible
To get efficient operation of an antenna, it needs to be positioned away from interference such as high speed processors. There is also the matter of the antenna's radiation pattern, which will render it more suited to a certain location on the circuit board.
By considering the antenna early on in the design process, you have greater flexibility to find an antenna without the constraints of having a set PCB size, housing materials/dimensions and component layout. With this flexibility, there are a wider variety of antennas available, which will enable you to find an antenna that fits most with the early concept decisions of your product. The antenna is one of the few components that can improve their performance depending on how they are designed-in. This means they can be less straightforward to integrate according to best practices, when compared to other components in a design such as integrated circuits (ICs).
Likewise, if there is no suitable off-the-shelf antenna that suits a given design, it will give the project team time to approach antenna manufacturers in the case that a custom antenna solution may be required – which it often is.
Off-PCB antenna solutions
For some applications, an off-PCB antenna solution may offer a more convenient route for retrofitting an antenna. These include flexible printed circuit board antennas (FPCs) and case-mounted antennas, which can even work on housing materials as challenging as metal.
In retrofitted applications where there is little opportunity to change the design of the product housing, a flexible printed circuit board (FPC) antenna could offer a quick solution. These antennas can be bent flexibly to conform to a product housing, while moving the ground plane to an external circuit board to keep PCB space free on the host.
Case-mounted antennas for metal surfaces
In designs where the product housing contains metallic elements or composite materials, an embedded antenna (whether FPC or a surface mount device [SMD]) can fail to perform well. This may require the device to use an antenna located outside of the enclosure. One such solution may be a case-mounted antenna, such as a REFLECTOR. These antennas use a ferrite-like layer to isolate the radiating antenna from noise generated by materials underneath them – such as a metallic case. This ensures that they can perform as if they were in free space, without needing to be located as such.
The later an antenna is integrated within the device, the riskier and more challenging the task could become. Antennas require a physically optimised environment (in terms of keep-out space, clearance from other components, placement, circuit board size) in order to operate well. As devices continue to become smaller, ‘retrofitting’ an antenna onto a circuit board becomes an even greater challenge.
Where possible, we would always recommend researching and selecting an antenna as early as possible in the design-development cycle. This will give you the greatest flexibility needed to integrate an antenna smoothly and without performance trade-offs.
In the case that you do need to retrofit an antenna, you need to take into account these physical requirements – then assess whether you have sufficient time and space to integrated an embedded antenna, or whether you require an antenna that offers easier integration, such as a flexible printed circuit board or a case-mounted antenna.