Introduction
Antennas are a vital component in any wireless system, serving as the interface between the electronic circuitry and open air. With wireless capabilities becoming ubiquitous across consumer, automotive, medical, defense and industrial applications, antenna design has taken on increased importance. This article provides key guidelines and considerations for engineers designing and integrating antennas for printed circuit board (PCB) applications.
Topics covered include:
- Fundamentals of antenna specifications
- Basic antenna theory and types
- Matching network design principles
- Guidelines for integrating antennas on PCBs
- RF layout techniques for antennas
- Considerations for antenna arrays
- Testing and measuring antenna performance
- Real-world antenna integration examples
- Tips for antenna design success
By understanding antenna design principles and following RF PCB layout best practices, engineers can help ensure excellent connectivity, efficiency and reliability for wireless systems.
Antenna Specifications and Parameters
Key parameters characterize the performance of an antenna and influence the selection or design process. Important specifications include:
Frequency Range
- Band of operating frequencies the antenna must cover
- Defined by low, high and center frequencies
- Impacts physical size with lower frequencies requiring larger antennas
Bandwidth
- Range of frequencies over which the antenna meets specifications
- Expressed as absolute bandwidth or percentage of center frequency
- Wider bandwidth allows more variation in operating frequency
Gain
- Measure of increased signal strength radiated in the peak direction
- Expressed in dBi units relative to an isotropic radiator
- Higher gain focuses energy for longer range
Radiation Pattern
- Spatial variation in radiated signal strength from antenna
- Omnidirectional, directional, or combination patterns
- Impacts coverage area shape and range
Input Impedance
- Resistance and reactance presented by antenna at input terminals
- Affects matching network design and transmission line interface
- 50 ohm standard for coaxial cabling
Efficiency
- Ratio of radiated power to input power
- Reduced by resistive and dielectric losses
- Higher efficiency conserves battery power
Polarization
- Direction of oscillating electric field
- Vertical, horizontal, circular common types
- Matched polarity important between transmitter and receiver
With these key parameters in mind, let’s examine some fundamental antenna theory that guides design.
Antenna Theory and Design
Several important theoretical concepts form the core foundation of antenna design:
Resonant Frequency
- Antennas transform wire currents into electromagnetic waves
- Efficient radiation occurs when length matches resonant wavelength
- Optimal length ~ 1/2 wavelength in dielectric medium
Radiation Patterns
- Radiation properties governed by spatial current distribution on antenna
- Different distributions produce different field patterns
- Omnidirectional, directional, monopole patterns
Input Impedance
- Driven by feed point location relative to currents
- Center feed minimizes reactance
- Off-center feed induces loop reactance
Reciprocity
- Antenna properties identical whether transmitting or receiving
- Permits design based on transmission characteristics
Miniaturization
- Physical length reduces with higher dielectric constant materials
- But bandwidth and efficiency may decrease
- Loading techniques can reduce size
With a grasp of antenna theory fundamentals, let’s survey some of the most common antenna types used on PCBs.
Common Antenna Types for PCB Implementation
A wide variety of antenna topologies can be designed and fabricated on printed circuit boards. Some of the most popular options include:
Monopole
- Single straight element mounted perpendicular to ground plane
- Omnidirectional pattern in azimuth
- Quarter-wave length establishes resonant frequency
Dipole
- Two collinear conductive elements of equal length
- Simple bidirectional pattern perpendicular to axis
- Half-wave length sets resonance
Folded Dipole
- Dipole wrapped into compact shape
- Increased impedance from larger radiating surface
- Resonant at length slightly shorter than half-wave
Loop
- Closed loop antenna above ground plane
- Circular polarization and pattern
- Can be made small using loading techniques
Patch Antenna
- Flat rectangular conductive patch on grounded dielectric
- Low-profile construction
- Feeds include microstrip, coaxial, aperture
PIFA
- Planar inverted-F antenna
- L-shaped element with ground connection
- Resonant at quarter-wave length
Slot
- Slot cut into large conductive surface
- Fed to excite slot mode radiation
- Complement of a dipole antenna
This provides an overview of antenna varieties commonly integrated into PCB designs. The choice depends greatly on the radiation pattern, polarization, bandwidth, gain, and efficiency required. Each antenna type has advantages and disadvantages that must be weighed against application needs.
Key Antenna Integration Considerations
Integrating an antenna onto a densely-populated PCB presents challenges. Here are some key considerations when incorporating an antenna:
Impedance Matching
- Any impedance mismatch degrades power transfer and efficiency
- Matching network transforms antenna impedance to 50 ohm transmission line
- L-section and pi-section networks common matching topologies
Isolation From Nearby Components
- Radiated interference can disrupt sensitive circuitry
- Strategic placement away from susceptible devices
- Guard traces, ground planes, shields
- Component shielding cans if needed
Proximity to Ground Planes
- Nearby ground planes impact antenna pattern shape
- Adjust feed point and ground clearance appropriately
- Ensure consistent distance to ground across operating bands
Effects of Nearby Traces
- Adjacent traces change antenna impedance and coupling
- Increased cross-talk and pattern distortion
- Use guard traces, distance, and compact layout
Impact of Components
- Components in antenna field perturb radiation properties
- Change tuning, efficiency, pattern shape
- Only essential devices near antenna
Careful integration combining electrical isolation and mechanical stability is vital to realize optimal antenna performance.
RF PCB Layout Guidelines for Antennas
In addition to antenna-specific integration considerations, following general PCB RF layout best practices helps safeguard performance:
Controlled Impedances
- Match trace impedances to antenna feed point
- Maintain 50 ohm transmission line impedance
- Use impedance calculators and controlled dimension traces
Minimize Stubs
- Stray branches and stubs cause reflections
- Use stubless, point-to-point routing
Continuous Return Path
- Complete uninterrupted ground under antenna and traces
- Eliminate ground gaps which increase radiation loss
Minimize Crossovers
- Signal crossing split ground plane induces noise
- Route antenna feed on same layer as ground under antenna
Solid Ground Plane
- Large uninterrupted ground plane stabilizes radiation pattern
- Provides shielding from interference
Component Placement
- Strategically position components to avoid antenna coupling
- Ensure clearance from antenna fields
Layer Stacking
- Stack power and ground planes between signal layers
- Enclose RF and antenna traces between ground planes
By combining sound antenna design principles with proven RF PCB layout techniques, optimal wireless performance can be realized.
Designing Antenna Arrays
Arraying multiple antennas on a PCB provides benefits like higher gain, electronic beam steering, and pattern diversity. Key array design considerations include:
Gain Enhancement
- More elements increases antenna gain
- But mutual coupling effects must be accounted for
Beam Steering
- Varying phase between elements rotates beam direction
- Permits rapid electronic scan without physical movement
Resonance Control
- Interactions between arrayed elements changes impedance
- May require larger matching network, isolation structures
Pattern Optimization
- Spacing between elements shapes overall pattern
- Wider separation broadens beamwidth
Layout Symmetry
- Equal spacing between identical elements
- Maintains uniform characteristics for phased arrays
Isolation Structures
- Prevent coupling between elements
- Trenches, neutralization lines, parasitic elements
By applying sound array design principles, multiple antennas on a PCB can work cooperatively to produce tailored performance exceeding single elements.
Testing and Measuring Antenna Performance
Evaluating antenna parameters on an assembled PCB requires specialized measurement techniques:
Impedance
- Vector network analyzer scans impedance versus frequency
- Confirms proper matching at desired band
Reflection Coefficient
- Measures power reflected from antenna due to mismatch
- Minimized by antenna-feed impedance alignment
Radiation Pattern
- Conducted in anechoic chamber or open field
- Record signal strength versus spherical or planar angle
Directivity and Gain
- Comparative measurements using reference antenna
- Remove effects of mismatch loss
Efficiency
- Measure input and radiated power to determine losses
- Assess effects of nearby components
Careful antenna testing validates design simulations and ensures all specifications are satisfied prior to release.
Real-World Antenna Integration Examples
Here are some examples of antenna integration in different wireless systems:
Smartwatch
- PCB area constraints necessitated compact monopole antenna
- Matched to 50 ohms using L-section network
- Quarter-wave length selected for 2.4GHz Bluetooth band
- Orientation parallel to watch band for omnidirectional pattern
Automotive RADAR PCB
- Four patch antennas arrayed for beam steering
- Microstrip feed lines with tuned isolation trenches
- Integrated into 24GHz RADAR transceiver PCB
- Broadside directional pattern oriented forward
Wearable Medical Sensor
- Meandered inverted-F antenna (MIFA) for size reduction
- Resonant at 915MHz ISM band for sensor data links
- Flexible antenna on flexible PCB conforms to skin
- Ground plane shields antenna from body
These examples illustrate how antenna integration must be tailored to the specific constraints and use cases of each wireless system.
Tips for Successful On-Board Antenna Design
Here are some useful tips to help guide the antenna design and integration process:
- Clearly identify key electrical specs like frequency, bandwidth, gain patterns
- Select antenna topology suited to physical constraints
- Model antenna behavior with electromagnetic simulation
- Design matching network early once antenna selected
- Carefully position antenna considering isolation needs
- Review impact of nearby components like batteries or processors
- Verify final performance through antenna pattern and network analysis measurements
- Build and test prototypes to characterize real-world effects
- Iterate on design based on measured results
By following a structured design flow, RF engineers can overcome the difficulties of on-board antenna integration to achieve wireless connectivity objectives.
Frequently Asked Questions
Here are some common FAQs regarding PCB antenna design:
Q: What are some good antenna options for a small wearable device?
Compact monopoles, loops, helices and meandered antennas are good options. Ensure adequate ground plane clearance for proper radiation.
Q: How early should antenna design be started in the development process?
Ideally during the concept phase so size and placement can inform early PCB layout. Retrofitting antenna design late causes compromises.
Q: What are some techniques to reduce coupling between multiple antennas?
Physical separation, ground trenches, neutralization lines and orienting orthogonally help isolate closely spaced antennas.
Q: How can I estimate the transmission line impedance needed to match my antenna?
Tools like Smith charts allow converting from complex antenna impedance to appropriate real transmission line impedance for a given matching network.
Q: What should I look out for when testing an integrated PCB antenna?
The effects of antenna proximity to the PCB and nearby components. Performance often differs significantly from isolated simulations.
Conclusion
On-board antenna integration presents challenges but following sound design practices enables robust wireless connectivity in the smallest products. As antennas become ubiquitous across virtually all electronics, mastering antenna design and layout flows is an invaluable skill for modern electrical engineers. The guidance provided throughout this article aims to equip PCB designers with the key principles and best practices to unlock the full potential of integrated antenna solutions.
1.Introduction
Antenna PCB design and RF layout are critical in a wireless system that transmits and receives electromagnetic radiation in free space. The wireless range that an end-customer gets out of an RF product with a current-limited power source such as a coin-cell battery depends greatly on the antenna design, the enclosure, and a good PCB layout. It is not uncommon to have a wide variation in RF ranges for designs that use the same silicon and the same power but a different layout and antenna-design practice. This application note describes the best practices, layout guidelines, and an antenna-tuning procedure to get the widest range with a given amount of power. Other important general layout considerations for RF trace, power supply decoupling, via holes, PCB stackup, and antenna and
grounding are also explored. The selection of RF passives such as inductors and capacitors is covered in detail.
Each topic ends with tips or a checklist of design items related to the topic.
Figure 1 shows the critical components of a wireless system, both at the Transmitter (TX) and Receiver (RX).
Figure 1. Typical Short-Range Wireless Syst
A well-designed antenna ensures optimum operating distance of the wireless product. The more power it can transmit
from the radio, the larger the distance it can cover for a given packet error rate (PER) and receiver sensitivity.
Similarly, a well-tuned radio at the receiver side can work with minimal radiation incident at the antenna. The RF
layout together with the radio matching network needs to be properly designed to ensure that most of the power from
the radio reaches the antenna and vice versa
2.Antenna Basics
An antenna is basically a conductor exposed in space. If the length of the conductor is a certain ratio or multiple of
the wavelength of the signal1, it becomes an antenna. This condition is called resonance‖, as the electrical energy
fed to antenna is radiated into free space.
Figure 2. Dipole Antenna Basic
feeds the antenna at its center point by a transmission line known as ―antenna feed. At this length, the voltage and
current standing waves are formed across the length of the conductor, as shown in Figure 2.
The electrical energy input to the antenna is radiated in the form of electromagnetic radiation of that frequency to free
space. The antenna is fed by an antenna feed that has an PCB impedance of, say, 50 Ω, and transmits to the free space,
which has an impendence of 377 Ω2
.
Thus, the antenna geometry has two most important considerations:
1. Antenna length
2. Antenna feed
The /2-length antenna shown in Figure 2 is called a dipole antenna. However, most antennas in printed circuit
boards achieve the same performance by having a /4-length conductor in a particular way. See Figure 3.
By having a ground at some distance below the conductor, an image is created of the same length (/4). When
combined, these legs work like a dipole antenna. This type of antenna is called the quarter-wave (/4) monopole
antenna. Most antennas on the PCB are implemented as quarter-wave antennas on a copper ground plane. Note that
the signal is now fed single-ended and that the ground plane acts as the return path.
Figure 3. Quarter-Wave Antenna
For a quarter-wave antenna that is used in most PCBs, the important considerations are:
1. Antenna length
2. Antenna feed
3. Shape and size of the ground plane and the return path
3 Antenna Types
As described in the previous section, any conductor of length /4 exposed in free space, over a ground plane with a
proper feed can be an effective antenna. Depending on the wavelength, the antenna can be as long as the FM
antenna of a car or a tiny trace on a beacon. For 2.4-GHz applications, most PCB antennas fall into the following
types:
1. Wire Antenna: This is a piece of wire extending over the PCB in free space with its length matched to /4 over a
ground plane. This is generally fed by a 50-Ω
4 transmission line. The wire antenna gives the best performance and RF range because of its dimensions and three-dimensional exposure. The wire can be a straight wire, helix,or loop. This is a three-dimensional (3D) structure, with the antenna over a height of 4-5 mm over the PCB plane,protruding into space.
Figure 4: Wire Antenna
2. PCB Antenna: This is a trace drawn on the PCB. This can be a straight trace, inverted F-type trace, meandered
trace, circular trace, or a curve with wiggles depending on the antenna type and space constraints. In a PCB
antenna, the antenna becomes a two-dimensional (2D) structure in the same plane of the PCB; see Figure 5.There are guidelines
5 that must be followed as the 3D antenna exposed in free space is brought to the PCB plane as a 2D PCB trace. A PCB antenna requires more PCB area, has a lower efficiency than the wire antenna,but is cheaper. It has easy manufacturability and has the wireless range acceptable for a BLE application.
Figure 5. PCB Antena
3. Chip Antenna: This is an antenna in a small form-factor IC that has a conductor packed inside. This is useful
when there is limited space to print a PCB antenna or support a 3D wire antenna. Refer to Figure 6 for a
Bluetooth module containing a chip antenna. The size of the antenna and the module in comparison with a one cent is coin is given below.
Figure 6. Cypress EZ BLE Module (10 mm × 10 mm) with Chip Antenna
Next Part We will take about how to Choosing an Antenna.
