What is An FPC Antenna: Everything You Need To Know
- Leonard
- 5 days ago
- 8 min read
Updated: 5 days ago
Introduction
Often, our clients ask us to integrate high-sensitivity connectivity into incredibly compact IoT devices. Our RF engineers have to rack their brains to achieve optimal antenna performance within these limited spaces.
Fortunately, the FPC antenna serves as an innovative yet mature solution to this challenge. Although technologies like LDS and Virtual Antennas are beginning to replace it in some high-end devices, the FPC antenna remains the mainstream choice for compact IoT applications today.
In this post, we will break down the technical features of FPC antennas, discuss key design and integration considerations, and compare them with other mainstream solutions to help you fully understand this critical component.
What is An FPC Antenna
The FPC Antenna, or Flexible Printed Circuit Antenna, is, as the name suggests, an antenna produced using Flexible Printed Circuit technology. Unlike traditional circuit boards that use rigid materials like FR4 glass fiber and cannot be bent, FPC uses flexible materials as the substrate, with Polyimide (PI) being the most common choice.
Physical Structure
From a physical perspective, excluding the adhesive backing, the FPC antenna body resembles a slim "sandwich":
Bottom: Substrate. A flexible film, typically made of PI or LCP, which is insulating and bendable.
Middle: Radiating Element. Conductive copper traces are attached to the substrate. These traces are formed through precise etching processes into specific geometric shapes, responsible for transmitting and receiving RF signals.
Top: Coverlay. A protective layer covering the copper traces to prevent oxidation and physical damage.
Form Factor & Connectivity
In terms of system architecture, FPC antennas are usually classified as "Off-board" components. Unlike chip antennas soldered directly onto the mainboard or onboard antennas that are part of the PCB, the FPC antenna is more like an independent module. It connects to the mainboard via a cable and a connector.
A standard FPC antenna assembly typically consists of three parts:
FPC Body: The radiating part of the antenna. The back is usually equipped with adhesive, allowing it to be stuck to the inner wall of the device casing like a sticker for fixation.
RF Cable: Usually a micro coaxial cable used to transmit RF signals.
Connector: The end of the cable is typically crimped with a micro RF connector (such as IPEX/U.FL/MHF terminals) to snap onto the RF receptacle on the mainboard.
This material and structure allow the FPC antenna to adapt to various narrow or non-planar device spaces, offering designers more freedom.
Materials and Manufacturing
Materials of FPC Antenna
The substrate of an FPC antenna not only ensures its primary characteristic of flexibility but also affects the antenna's performance, especially at high frequencies. The two mainstream substrates are PI (Polyimide) and LCP (Liquid Crystal Polymer).
PI: Its advantages are mature processing and low cost. However, it has a relatively high dielectric constant and high loss, making it more suitable for frequencies below 6 GHz.
LCP: It features a lower dielectric constant (approx. 3.0) and a loss tangent of 0.002-0.005. It can be applied to 5GHz and even millimeter-wave bands. If applied to the 5GHz Wi-Fi band, the loss of an LCP FPC antenna is 70% lower than that of a PI FPC antenna. Additionally, LCP has better heat resistance and lower moisture absorption. Therefore, high-end devices using FPC antennas usually choose LCP substrates; for instance, the iPhone has adopted LCP material for its integrated antennas.
MPI (Modified Polyimide): Developed to address the drawbacks of LCP—high cost, complex processing, and limited supply—MPI sits as a middle ground between standard PI and LCP. By optimizing the material formula of PI to lower its dielectric constant, MPI achieves high-frequency performance nearly comparable to LCP. Crucially, it does so with significantly lower manufacturing costs and complexity. As a result, MPI has begun to replace LCP in many specific application scenarios.
Manufacturing
The manufacturing process of FPC antennas is similar to that of standard flexible circuits but includes specific steps to meet RF transmission requirements. The main workflow includes:
Circuit Generation/Etching: Using photolithography and chemical etching on copper-clad laminates to remove excess copper and retain the designed antenna radiator pattern.
Coverlay Lamination: FPC does not use the solder mask ink found on rigid boards. Instead, it uses a type of insulating film with adhesive called Coverlay. This is laminated over the copper traces under high temperature and pressure for insulation and protection.
Surface Finish: Exposed pads (where the feed line will be soldered) undergo ENIG (Electroless Nickel Immersion Gold) treatment to prevent copper oxidation and ensure good solderability.
Punching/Cutting: Using precision molds or laser cutting to shape the FPC sheet into individual antennas.
It is worth noting that in the circuit etching phase, precision is vital. The width of the copper traces directly affects antenna performance. We have encountered cases where a line width difference of just 0.1mm caused a significant drop in antenna sensitivity. Often, what passes as a standard quality check for a regular FPC is not good enough for an FPC antenna. Therefore, manufacturing tolerances for FPC antennas must be strictly controlled.
Benefits of FPC Antenna
Why are FPC antennas so popular in the IoT and consumer electronics sectors? This is due to the excellent balance they strike between physical form factor and RF performance:
High Design Flexibility: This is the trump card of FPC antennas. They can be bent and twisted to perfectly fit inside round, curved, or irregular device casings. This allows product designers to utilize "dead space" inside the device without compromising the external shape.
Excellent RF Performance: Compared to space-constrained ceramic chip antennas, FPC antennas usually possess a larger radiating area, often providing higher efficiency and bandwidth. Additionally, being omnidirectional, their signal coverage is generally superior to PCB onboard antennas.
Faster Time-to-Market: With PCB onboard antennas, if RF testing fails, the entire mainboard often needs redesigning, which is costly and time-consuming. FPC antennas are independent components; tuning the frequency or gain only requires modifying the FPC pattern or swapping the antenna model without altering the mainboard.
Space Saving: Being "Off-board," they do not occupy valuable mainboard real estate, allowing engineers to place more functional components on the PCB.
Challenges
Assembly Consistency
FPC antennas usually require manual placement. If assembly line workers deviate in placement (e.g., shifting 1mm left or right) or if air bubbles form due to uneven sticking, it can cause the antenna's center frequency to shift (detuning), affecting the final yield.
MIMO Antenna Coupling in Compact Designs
In Wi-Fi 6/6E and 5G IoT devices, the adoption of MIMO or multi-antenna diversity technologies is increasing. However, compact device designs make antenna mutual coupling a significant challenge. When two FPC antennas are placed too closely together, mutual coupling is prone to occur, leading to reduced total efficiency and increased correlation, thereby degrading MIMO performance.
Common solutions include:
Orthogonal Polarization Layout: Using vertical and horizontal polarizations to reduce coupling.
Neutralization Lines: Adding an antiphase coupling path between antennas (effective when spacing is ≥ λ/4).
EBG Structures: Introducing Electromagnetic Band Gap materials to suppress surface wave propagation.
Software Diversity: Switching antennas based on RSSI to select the optimal signal path.
Of course, the ideal approach remains maintaining an antenna spacing of ≥ λ/4 (approx. 3 cm for 2.4 GHz); when space is constrained, engineers must rely on algorithms and layout optimization for remediation.
FPC Antenna Design & Integration Considerations
To maximize performance, the following must be considered:
How to Place FPC Antennas
Position is key to performance.
Keep-out Zone: There must be sufficient clearance around the antenna radiator. Strictly avoid sticking FPC antennas directly onto metal surfaces (unless it is a specifically designed anti-metal NFC antenna), and do not cover batteries, speakers, or shields. Metal acts like a wall that reflects or absorbs RF signals, rendering the antenna ineffective.
Edge Placement: Try to place the antenna at the edge or corner of the device casing, which usually results in a better radiation pattern.
Adhesive
Secure attachment ensures stable signals.
Choose the Right Adhesive: Even minor lifting can cause frequency shifts. It is recommended to use heat-resistant, industrial-grade 3M double-sided tape (such as 3M 467).
Surface Preparation: Before sticking, ensure the inner wall of the casing is clean, dry, and oil-free. For low-surface-energy plastics (like PP), a primer may be needed.
Curvature
Bend with care.
Avoid Creasing: Although FPC is flexible, it cannot be folded like paper. Dead folds (creases) can break the copper traces, causing an open circuit.
Bending Radius: Even for curved installations, maintain a smooth arc. The bending radius should never be less than 3mm (or strictly refer to the datasheet). Excessive bending changes the physical length and impedance of the copper, shifting the center frequency higher.
Cable Routing
The cable is part of the RF link.
Avoid Interference: The RF coaxial cable should be routed away from power lines, data lines (like HDMI, USB 3.0), and DC-DC converters, which are strong noise sources.
Fix the Cable: Loose cables can generate parasitic coupling. It is best to use clips or tape to secure the cable to the chassis or mainboard to prevent movement.
FPC vs PCB vs LDS Antenna: What is the Difference
Figure 3: PCb vs FPC vs LDS Antenna
MIOT delivers a complete antenna portfolio and customized
design services to enable all connectivity solutions.
Feature | PCB Antenna (PCB Trace) | FPC Antenna | LDS Antenna (Laser Direct Structuring) |
Definition | Copper trace directly printed on a rigid PCB | Separate flexible circuit board + cable | Circuit directly patterned on plastic/metal housing via laser |
Cost | Low (almost free) | Medium (materials + assembly cost) | High (mold + laser process expensive) |
Space Usage | Occupies PCB area, requires large clearance | Does not occupy PCB, can be attached inside casing | Utilizes plastic housing/surface, extremely high space efficiency |
Design Flexibility | Low (limited by PCB shape) | High (flexible, can be bent and placed freely) | Very high (can form any 3D shape) |
Performance | Moderate (affected by PCB ground plane) | Excellent (good omnidirectional coverage, high efficiency) | Outstanding (very high precision, consistent performance) |
Typical Applications | Mouse, low-end routers, toys | IoT devices, tablets, wearables | Flagship smartphones, high-end wearable devices |
Summary: If cost is critical and board space is ample, choose PCB antennas. If you seek a balance between space utilization and performance, FPC antennas are the top choice. Consider expensive LDS only for flagship products where space is extremely compressed.
Applications of FPC Antenna
FPC antennas, due to their thin profile, flexibility, and ease of integration, can accommodate a wide range of design constraints and development requirements. They are perfect for the following situations:
Production and Development Stage
Early-stage or low-volume products that require rapid iteration and design flexibility.
Physical Space Constraints
Devices with limited main PCB or enclosure space, where minimizing RF routing and antenna footprint is critical.
Design Integration Requirements
Projects that add wireless functionality to an existing design require minimal impact on the original layout.
Mechanical and Form-Factor Constraints
Products with irregular shapes or curved structures, where antennas must be bendable or conformal to the enclosure.
Rapid Prototyping and Iteration
New product development phases where antennas must be easy to tune and modify, reducing the cost and risk of design changes.
Lightweight Design Requirements
Wearable or portable devices where a low-profile, lightweight antenna contributes to overall weight reduction.
FAQ
Q1: Can FPC antennas be stuck onto metal casings?
A: Standard FPC antennas cannot. Sticking them directly on metal will cause a short circuit or severe detuning. If placement on metal is necessary, you must use a special "anti-metal antenna" or add a ferrite sheet between the antenna and the metal for isolation.
Q2: Can FPC antennas be bent arbitrarily?
A: No. When bending is necessary, avoid the following:
Avoid 180° dead folds (creases).
Avoid bending the printed area more than 90°.
Avoid bending exposed conductor parts.
Avoid bending the area near the gold fingers (insertion area).
Q3: What is the lifespan of an FPC antenna?
A: The FPC antenna itself is very durable; aging primarily affects the adhesive and connectors. In a normal environment, the lifespan can exceed 5-10 years.
Conclusion
The FPC antenna, with its unique flexible structure, excellent RF performance, and moderate cost, has become the bridge connecting physically constrained spaces with the wireless digital world.
The FPC antenna remains the best balance point in the "Performance-Cost-Size" triangle for the current IoT and small-to-medium smart device market. For engineers, understanding the material properties of FPC antennas and following correct placement and integration guidelines are key to ensuring the quality of the product's wireless connection. We hope this guide provides a valuable reference for your RF selection in your next product.
