PCB Antenna Design: Types, Principles, and Best Practices

In the world surrounded with wireless communication, the demand for compact, inexpensive and reliable antennas has never been higher. Antennas are the application of electromagnetic waves. There are many theories and practicals available on antennas, but only few of them get recognised. For wireless data transfer the one solution can be usage of Printed Circuit Board (PCB) antennas. Using PCB antennas represents a convenient solution for integrating them directly into electronic devices, eliminating the need for bulky external antennas. No matter if your antenna is placed as a printed element or pulled off the shelf, it’s important for PCB designers to know how antennas work at a deep level. In this article, we’ll present some of the main concepts behind antennas so that engineers can see how to design, select, and place antennas.

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.

PCB Antenna Basic Theory:

Antenna are reciprocal devices, either they emit electromagnetic waves when provided with a voltage and current or receive electromagnetic waves and convert them into a voltage and current. 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 signal, it becomes an antenna. This condition is called “resonance”, as the electrical energy fed to the antenna is radiated into free space, the conductor has a length λ/2, where λ is the wave length of the electric signal. The antenna is fed by an antenna feed that has an impedance of, say, 50 Ω, and transmits to the free space, which has an independence of 377 Ω2. Thus, the antenna geometry has two most important considerations:

1. Antenna length

2. Antenna feed

Antennas can be designed to pick up an electric field or magnetic field, depending on its geometry. The simplest antennas are a single wire of specific length. Magnetic antennas use a loop of wire, and the transmitter/receiver element in the system acts like a load that completes a circuit that contains the loop antenna.

Antenna Types:

Any conductor of length λ/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 λ/4 over a ground plane. This is generally fed by a 50-Ω 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.

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. There are guidelines 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.

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.

Antenna Parameters:

Return Loss: The return loss of an antenna signifies how well the antenna is matched to the 50-Ω transmission line (TL). The TL characteristic impedance is typically 50 Ω, although it could be a different value. The industry standard for commercial antennas and testing equipment is 50-Ω impedance, so it is most convenient to use this value. Return loss indicates how much of the incident power is reflected by the antenna due to mismatch. An ideal antenna, when perfectly matched, will radiate the entire energy without any reflection.

If the return loss is infinite, the antenna is said to be perfectly matched to the TL. In most cases, a return loss ≥ 10 dB is considered sufficient. A return loss of 10 dB signifies that 90% of the incident power goes into the antenna for radiation.

Bandwidth: Bandwidth indicates the frequency response of an antenna. It signifies how well the antenna is matched to the 50-Ω transmission line over the entire band of interest, that is, between 2.40 GHz and 2.48 GHz for BLE applications.

Radiation Efficiency: A portion of the non-reflected power gets dissipated as heat or as thermal loss in the antenna. Thermal loss is due to the dielectric loss in the FR4 substrate and the conductor loss in the copper trace. This information is characterized as radiation efficiency. A radiation efficiency of 100 percent indicates that all non-reflected power is radiated to free space. For a small-form-factor PCB, the heat loss is minimal.

Radiation Pattern: Radiation pattern indicates the directional property of radiation, that is, which directions have more radiation and which have less. This information helps to orient the antenna properly in an application.

Gain: Gain indicates the radiation in the direction of interest compared to the isotropic antenna, which radiates uniformly in all directions. This is expressed in terms of dBi—how strong the radiation field is compared to an ideal isotropic antenna.

Physical Design of an Antenna:

The goal of an antenna designer, both on a PCB or when integrated as an external component, is to define the geometry of the antenna so that some particular operating goals are met:

• High radiation efficiency
• Low loss along the feedline and front end
• Directionality (radiation pattern)
• Feedline matching
• Sufficient bandwidth

The physical design of an antenna and its placement in a PCB will determine all of the above operating characteristics. Field solvers used for antenna designs can be used in two ways: to determine the voltage and current distribution in the antenna body in the frequency domain (using method of moments or boundary element method), or to determine the electromagnetic field radiated around the antenna.

Once the electromagnetic field around the antenna is determined, software can be used to determine the radiation pattern and radiation efficiency. Field solvers for antennas will generally operate in the frequency domain, so these values can be plotted as a function of frequency. This then lets a designer determine the antenna’s emission bandwidth.

Meandered Inverted-F Antenna (MIFA):

The MIFA is a widely used antenna in human interface devices (HIDs) due to its compact size and efficient performance. Cypress has developed a robust MIFA that offers excellent performance in a small form factor.

• Size: 7.2 mm × 11.1 mm (284 mils × 437 mils)
• Suitable for: Wireless mouse, keyboard, presenter, and other HID applications
• Design: The antenna trace width is 20 mils throughout. The primary parameter that may change, depending on the PCB stack spacing, is the width (W) of the RF trace (transmission line).

Antenna Tuning and Matching:

Antenna tuning ensures that the return loss is greater than 10 dB for the antenna when measured from the chip output towards the antenna within the desired frequency band. This process is crucial for efficient power transfer, both in transmission and reception. Know more about impedance matching through our recent blog on this topic.

• Transmission Mode: A return loss greater than 10 dB ensures that 90% of the power output from the chip is transmitted to the antenna.
• Receive Mode: The same tuning procedure should be followed when looking into the radio to ensure the impedance is 50 Ω. This guarantees that 90% of the received power is transferred to the radio.

Since both antenna tuning and radio tuning involve optimizing impedance matching, they are collectively referred to as antenna tuning. Power transfer is maximized when the output impedance of the radio is the complex conjugate of the antenna impedance. In most antenna tuning processes, this is achieved by:

1. Transforming the antenna impedance to 50 Ω
2. Matching the balun to 50 Ω

Design Considerations for PCB Antennas:

When working on the PCB antenna design, there are a few main factors that should be considered to reach optimal performance with least space required:

1. Frequency Band: The operating frequency of the wireless system is used to determine the dimensions and configuration of PCB antennas. Design equations and simulation tools can be used to optimize the antenna dimensions for specific frequency bands.

2. Antenna Geometry: The geometry of the PCB antenna including shape, size and layout, directly influences the radiation pattern, efficiency and impedance. Careful design considerations are necessary to achieve the best performance.

3. Ground Plane: A continuous and well connected PCB ground plane is almost necessary in every application and especially when it comes to PCB antennas. In this case, the ground plane acts as a reference point and it helps to minimize the radiation losses.

4. Impedance Matching: To achieve the maximum radiation performances with minimal signal reflections and losses, matching the impedance of PCB antenna to impedance of internal circuitry is crucial. Matching networks, stub tuning and other impedance matching components can be used to achieve the optimal matching of the impedance. Having an antenna with unmatched impedance can create unwanted losses and signal deviations.

Performance Testing and Optimization:

Once the PCB antenna design is finished, it’s essential to perform testing and required optimization, to ensure compliance with desired specifications and standards. Various testing methodologies such as:

• S-parameter measurements
• Radiation pattern measurements
• Impedance matching analysis

Conclusion:

In conclusion, the design of PCB antennas requires careful consideration of various factors, including frequency band, antenna geometry, ground plane, and impedance matching. These are the critical parts of electronics and electromagnetics, if not designed correctly the theory fails and the antenna doesn’t work at all. By following best practices in combination with usage of advanced design and testing techniques, users can develop efficient, reliable and low cost PCB antennas for a whole range of wireless communication products.