Types of PCB Antennas
There are several common types of PCB antennas used in wireless devices:
Microstrip Patch Antennas
Microstrip patch antennas (also known as planar antennas) are the most popular type of PCB antenna. They consist of a flat rectangular copper patch suspended over a ground plane with a dielectric substrate between them. By controlling the shape and dimensions of the patch, they can be tuned to operate at the desired frequency.
Some benefits of microstrip patch antennas include their low profile, light weight, easy integration with circuits, polarization purity and relatively high gain for a PCB antenna. Their main drawbacks are their narrow bandwidth and larger size compared to other PCB antennas.
Inverted F Antennas
Inverted F antennas (IFA) have a shape resembling an upside down letter F. They are designed with a shorting pin or plate that connects the feed wire to the ground plane, along with a matching network for impedance matching.
IFAs are resonant antennas that function similarly to quarter wave monopole antennas. Their key advantages include small size, low cost, omnidirectional radiation patterns and easy integration within the device PCB.
Monopole Antennas
Monopole antennas consist of a single radiating element, such as a straight wire or helical coil, mounted perpendicular to the ground plane. They function as resonators, with their length approximately one quarter of the wavelength at the operating frequency.
Monopoles provide an omnidirectional radiation pattern perpendicular to the ground plane. They are simple, compact and cost-effective to implement on a PCB.
Dipole Antennas
Dipole antennas consist of two equal length straight elements with a feed gap between them. On a PCB, they can be implemented with the two arms formed out of metal traces with a coupled or direct feed line.
Dipoles have a bidirectional radiation pattern, with gain toward the ends of the two arms. By orienting them properly, they can cover most of the hemisphere above the ground plane. Dipoles are intrinsically narrow band but with tricks like using tapering traces, dual band versions are possible.
Dielectric Resonator Antennas
Dielectric resonator antennas (DRAs) are constructed from a piece of high permittivity ceramic or plastic on top of a ground plane. They resonate at wavelengths much smaller than their physical size due to the high dielectric constant of their substrate.
DRAs have high radiation efficiency and typically wider bandwidth than microstrip antennas. However, dielectric resonator antennas are more complex to design and fabricate.
PCB Antenna Design Considerations
Some important factors to consider when designing a PCB antenna include:
- Operating frequency - The antenna must be tuned to match the desired operating frequency. The antenna's physical length is proportional to the wavelength at this frequency.
- Input impedance - The antenna impedance should match the characteristic impedance of the transmission line feeding it (typically 50 ohms). Mismatch can cause signal reflections.
- Radiation pattern - The desired radiation pattern will depend on the application. Omnidirectional, unidirectional or combinations can be implemented in PCB antennas.
- Gain - More gain enhances range but higher gain antennas have more directional patterns. Enough gain should be provided for robust connectivity.
- Efficiency - Higher radiation efficiency converts more input power into radiated EM waves. Efficiency above 50% is generally acceptable.
- Size - Antenna size is related to wavelength and affects achievable gain. Size constraints must be balanced with desired performance.
- Bandwidth - The impedance and pattern bandwidth should cover the entire operating frequency range. Wider bandwidth allows more variation.
- Polarization - Select vertical, horizontal or circular polarization as needed. Polarization impacts antenna matching and diversity performance.
PCB Antenna Design Process
The PCB antenna development process generally follows these main steps:
1. Define Design Goals
- Specify electrical requirements - operating frequency, impedance, gain, efficiency, polarization, radiation pattern
- Size constraints - maximum length, width and height
- Analysis approach - simulations, prototyping, measurements
- Target applications and use cases
This sets the priorities and tradeoffs to guide the design process.
2. Choose Antenna Topology
- Select antenna topology - microstrip, monopole, IFA, dipole etc.
- Pick construction materials - laminate dielectric, copper thickness
- Consider integration requirements with other PCB components
The antenna type sets boundaries for achievable electrical and physical characteristics.
3. Run Simulations
- Model antenna structure and ground plane in EM simulation software
- Run frequency-domain solver to estimate S-parameters, gain, efficiency
- Optimize dimensions to hit target frequency and impedance
- Tune matching network if needed
- Verify radiation pattern performance
Simulations predict antenna performance before building prototypes.
4. Fabricate and Measure Prototypes
- Produce PCB antenna prototypes for testing
- Take return loss measurements to check impedance matching
- Measure radiation patterns, gain, efficiency in anechoic chamber
- Iterate on design if needed to refine performance
Testing prototypes validates simulation results and identifies areas for improvement.
5. Model Performance in System
- Integrate antenna model with rest of wireless system/device
- Simulate total performance including other components
- Verify that bandwidth, gain and efficiency requirements are met
- Make any last design tweaks
This evaluates real-world performance with impedance mismatches, near-field effects etc.
6. Move to Production
Once the antenna design meets all electrical and mechanical requirements, it can be handed off for manufacturing and integration with the rest of the product. Maintaining consistency between any design changes and the original antenna model is critical.
Matching Network Design
Designing matching networks is an important skill for optimizing antenna performance. Matching networks transform the antenna's impedance to the characteristic impedance of the transmission line (typically 50 ohms).
Lumped Element Matching
Discrete inductors and capacitors can be used to build matching networks. Simple L-match and T-match networks are common matching circuit topologies.
Quarter Wave Transformer
Sections of transmission line can be used to perform impedance transformation based on the formula:
Zq = √(Z1*Z2)
Where Zq is the quarter wave line impedance, Z1 is the antenna impedance and Z2 is the transmission line impedance.
###Stub Matching
Open or short circuited stub sections connected at the antenna feed point can be tuned to cancel out reactances and match to the feed line impedance.
Feeding Techniques
There are several approaches to connect and provide an input signal to the antenna from the PCB.
Microstrip Feed Line
A common approach is to use a microstrip transmission line routed from the RF source to the antenna terminals. The line width determines the characteristic impedance.
Coaxial Cable
For testing or modular antennas, coaxial cables like SMA can be soldered or otherwise attached to the antenna feed points.
Aperture / Slot Coupling
The feed signal is coupled to the antenna through a slot or aperture in the ground plane underneath it. Allows indirect feeding without connectors.
Proximity Coupling
Instead of direct ohmic contact, the feed line is placed in proximity of the antenna to allow coupling of energy through EM fields.
Analysis and Measurements
Important antenna parameters that should be analyzed and measured include:
- Return loss / S11 - Indicates impedance matching at the input. Should be below -10 dB.
- Radiation pattern - Provides antenna directionality which depends on the antenna type.
- Gain - Amount of power radiated in the peak direction. High gain boosts range.
- Efficiency - Ratio of radiated power to input power. Affected by conductor and dielectric losses.
- Directivity - Concentration of radiation in certain direction. Driven by physical structure.
- Bandwidth - Range of frequencies where antenna performs well. Wider is better for multifunctionality.
Both simulations and laboratory measurements are used to evaluate PCB antenna designs against these metrics.
Common Simulation Tools
- ANSYS HFSS - industry standard 3D EM field simulator
- CST Studio - multipurpose EM analysis software
- FEKO - comprehensive method of moments (MoM) solver
Typical Measurement Setup
- Vector Network Analyzer - for impedance and S-parameter measurements
- Anechoic chamber - electrically isolated environment to measure far-field patterns and gain
- Signal generator, spectrum analyzer - further antenna testing and characterization
PCB Antenna Design Examples
WiFi 802.11n 2.4 GHz PCB Dipole Antenna
- Target frequency - 2.45 GHz
- Dipole arm length - L = c/(2f√εr) = 30 mm
- PCB substrate - FR4 εr = 4.4, h = 1.6 mm
- Feed - 50 ohm microstrip line
- Directivity - 1.9 dBi
This basic dipole antenna provides omnidirectional coverage for 2.4 GHz WiFi networking.
GPS L1 Band Microstrip Patch Antenna
- Frequency - 1.575 GHz
- Square patch dimensions - W = L = 40mm
- Substrate - Rogers RO3003, h = 3.5 mm
- Microstrip feed line - 50 ohms, inset from edge
- Measured gain - 4 dBi
Microstrip patch optimized for right-hand circular polarization and peak gain at GPS L1 band center frequency.
Multiband 4G/LTE PCB Monopole Antenna
- Pentaband 4G smartphone antenna
- Monopole with folded meander line arms
- Length tuned for 700/850/1700/1900/2100 MHz LTE bands
- Matching network for 50 ohm feed line
- 55% efficiency across all bands
Compact monopole antenna covering major 4G frequency bands with a single radiating element.