AN91445
Antenna Design Guide
Author: Tapan Pattnayak
Associated Part Family: CY8C4XX7-BL, CY8C4XX8-BL, CYBL10X6X, CYBL10X7X
Related Application Notes: None
To get the latest version of this application note and the associated Gerber file, please visit
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AN91445 explains antenna design in simple terms and recommends two Cypress-tested PCB antennas that can be
implemented at a very low cost for use with the Bluetooth Low Energy (BLE) solutions that are part of Cypress’s
®
PRoC™ and PSoC families. The PRoC BLE and PSoC 4 BLE 2.4-GHz radio must be carefully matched to its antenna
for optimum performance. It concludes by showing how to tune the antenna in the final product.
Contents
Introduction ...............................................................1
Antenna Basics.........................................................2
Antenna Types .........................................................4
Choosing an Antenna ...............................................5
Antenna Parameters.................................................5
Antennas for Cypress PRoC/PSoC BLE ..................8
Cypress-Proprietary PCB Antennas .........................8
7.1
Meandered Inverted-F Antenna (MIFA) ...........8
7.2
Antenna Feed Consideration ......................... 10
7.3
Antenna Length Considerations ..................... 12
7.4
Inverted-F Antenna (IFA) ............................... 13
8
Chip Antennas ........................................................ 16
9
Wire Antennas ........................................................ 18
1
2
3
4
5
6
7
10
11
Antenna Comparison .............................................. 19
Effect of Enclosure and Ground Plane on
Antenna Performance ............................................ 20
11.1
Effect of Ground Plane .................................. 20
11.2
Effect of Enclosure ........................................ 20
12 Guidelines for Enclosure and Ground Plane .......... 21
13 Antenna Tuning ...................................................... 22
13.1
Tuning Procedure .......................................... 23
14 Summary ................................................................ 29
15 Appendix A: Checklist ............................................ 30
16 Appendix B: References ......................................... 31
Worldwide Sales and Design Support ............................. 33
1
Introduction
An antenna is a critical component 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 using the same silicon and the same power but 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. This is an important consideration
for BLE system which has to operate from a tiny power source. Below Figure shows the critical components of a
wireless system both at the Transmitter (TX) and Receiver (RX).
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Document No. 001-91445 Rev. *B
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Antenna Design Guide
Figure 1. Typical Short-Range Wireless System
A well-designed antenna increases the 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 in the receiver side can work with minimal radiation incident at the antenna.
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
1
the wavelength of the signal , 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
1
See “harmonic antenna operation”
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Document No. 001-91445 Rev. *B
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Antenna Design Guide
In
Figure 2,
the conductor has a length
/2,
where
is the wave length of the electric signal. The signal generator
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,
2
which has an impendence of 377 Ω .
Thus, the antenna geometry has two most important considerations:
1.
2.
Antenna length
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
3
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.
2.
3.
Antenna length
Antenna feed
Shape and size of the ground plane and the return path
2
Impedance of Free Space if there is no material nearby
We will see the effect of this return path later. This is a very important aspect in PCB layout of the antenna and the antenna feed.
3
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Document No. 001-91445 Rev. *B
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Antenna Design Guide
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
4
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.
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 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 Antenna
5
4
5
The feed is generally of 50 ohm in most RF PCB catering to low-power wireless applications. However, other impedance values are possible.
Please refer to the section on MIFA and IFA on page 7
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Document No. 001-91445 Rev. *B
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Antenna Design Guide
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
4
Choosing an Antenna
The selection of an antenna depends on the application, the available board size, cost, RF range, and directivity.
Bluetooth Low energy (BLE) applications such as a wireless mouse requires an RF range of only 10 feet and a data
rate of a few kbps. However, for a remote control application with voice recognition, an antenna should have a range
around 20 ft in an indoor setup and a data rate of 64 kbps.
For wireless audio applications or indoor positioning, antenna diversity is required. For antenna diversity, two
antennas are placed orthogonally on the same PCB such that at least one of them is always receiving some radiation
while the other may be shadowed by reflection and multi-path-fading. This is required where real-time audio data is
transmitted and a high throughput without packet loss is required.
5
Antenna Parameters
The following section gives some key antenna performance parameters.
Return loss:
The return loss of an antenna signifies how well the antenna is matched to the 50- transmission
line (TL), shown as a signal feed in
Figure 7.
The TL impedance is typically 50
,
although it could be a different
value. The industry standard for commercial antennas and testing equipment is 50- 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 (Equation 1).
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, as shown in
Figure 7.
S
11
is the
negative of return loss expressed in decibels. As a rule of thumb, a return loss ≥ 10 dB (equivalently, S
11
≤ –10
dB) is considered sufficient.
Table 1
relates the return loss (dB) to the power reflected from the antenna (percent).
A return loss of 10 dB signifies that the 90% of the incident power goes into the antenna for radiation.
Equation 1
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Document No. 001-91445 Rev. *B
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