Antenna Design and RF Layout Rules ( Part I )

1 Introduction

Antenna Design and RF Layout

Antenna Design and RF Layout

Antenna 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

Typical Short-Range Wireless Syst

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

Dipole Antenna Basic

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 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

 

Quarter-Wave Antenna

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

Wire Antenna

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 Anten

 

PCB Anten

PCB Anten

 

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 onecent is coin is given below.
Figure 6. Cypress EZ BLE Module (10 mm × 10 mm) with Chip Antenna

 

Cypress EZ BLE Module (10 mm × 10 mm) with Chip Antenna

Cypress EZ BLE Module (10 mm × 10 mm) with Chip Antenna

 

Next Part We will take about how to Choosing an Antenna.

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