Integration suggestions for FPC antennas
What you will learn:
- SMD antennas and FPC antennas have different effects on PCB design.
- Design tips for FPC antennas.
This article examines the impact on the design and integration of FPC antennas (Flexible Printed Circuit). The FPC is an alternative to a chip antenna and can be an interesting choice for certain embedded designs.
FPC antennas are thin, typically only 0.15mm, and have a pull tab and adhesive area to attach them in various configurations in a small electronic device (Fig. 1). They come complete with a cable and plug to attach to the bottom or case of a manufacturer’s electronic product. An FPC is light, probably less than 0.5 g.
Advantage: No concerns about the ground level
The FPC works differently than an SMD chip antenna (Surface Mount Device). An SMD antenna that uses the ground plane on which it is mounted for radiation works on the principle of reciprocity. A dipole antenna uses two radiators, the length of each radiator being related to the wavelength of the frequency used by the antenna. Embedded antennas have a radiator in their bulk and use an area on the circuit board as a counterweight – commonly referred to as the ground plane.
In the case of an SMD antenna, the length of the ground plane is directly related to the wavelength of the antenna. The correct length of the ground plane must be specified in the design so that the antenna can operate and operate with good efficiency.
FPC antennas differ in this regard – they don’t need a ground plane to radiate. As such, they allow the designer more freedom to arrange the components in the circuit within the design.
However, it is important to note that the coaxial cable becomes part of the antenna. Therefore, the cable routing should be designed to keep this part of the antenna away from other components that may cause noise and interference.
Since the ground plane for the FPC antenna is not a constructive consideration, the size of the host circuit board does not matter, as is the case with SMD or chip antennas. The placement of the FPC antenna still has to follow a few basic rules, however, as most antennas are sensitive to their surroundings.
The antenna radiates in six spatial directions (Fig. 2). Ideally, at least three to five of these directions should be clear of obstruction for the antenna to operate effectively. For the other directions with obstacles in the radiation paths, there should still be a minimum distance that is specified in the data sheet of the antenna manufacturer.
The plastic housing for the device is not an obstacle unless the material used to make the housing is glass-filled or metallic paint has been applied. Metal objects and components such as data cables and printed circuit boards (PCBs) in close proximity to the FPC antenna are usually the culprits blocking the signals.
Orientation of the FPC
The shape of the device determines the ideal orientation of the FPC with respect to the host circuit board. The position of the antenna in relation to the circuit board depends on the proximity of the floor (Fig. 3).
Regardless of the option chosen, the distance (d) becomes a critical dimension in the design. This is stated in the data sheet of the antenna manufacturer.
Device integration example
Figure 4 shows an integration example for an FPC antenna. The device has the main components in an outer housing. The FPC was attached to the inside of the plastic case of the device, with the cable routed along the circuit board to avoid disturbing other components. In this case the FPC was placed on the same plane relative to the PCB. Due to the slim design of the device, this was the optimal location for the antenna.
The antenna’s I-PEX cable connects it to the design. All transmission lines in the product are designed for a characteristic impedance of 50 Ω and should be kept as short as possible. All other parts of the RF system, such as the transceiver and power amplifier, should also be designed for an impedance of 50 Ω.
We recommend using a commercially available RF design package to create the transmission line layout, taking into account the printed circuit board thickness, the copper thickness and the dielectric constant. The program calculates the recommended width for the transmission line and the corresponding distances between the reference ground plane on either side of the antenna feed track to maintain the 50 Ω system impedance.
Is an FPC Antenna Good for Your Design?
SMD antennas may be the most obvious choice for a small circuit board, but FPC antennas are often used in applications where there is not enough space for an SMD antenna.
Standard FPC antennas are not recommended for the smallest devices, but there are many applications in which FPCs can work well. They can be a particularly useful choice for structures where the length and space of the ground plane are limited.
Remember that FPCs are not compatible with placement machines. However, they are a good choice for small-volume manufacturing where equipment is hand-assembled.