AN4190
Application note
Antenna selection guidelines
By Placido De Vita
Introduction
The antenna is a very important component of communication systems. By definition, an
antenna is a device used to transform an RF signal, travelling on a conductor, into an
electromagnetic wave in free space (transmit mode), and to transform an RF
electromagnetic wave into an electrical signal (receive mode).
The choice of antenna is very important for a transmitting - receiving communication
system. The antenna must be able to radiate or receive efficiently so the power supplied is
not wasted.
This application note describes the most important parameters to consider when deciding
what kind of antenna to use in a short range device application.
In the first section of this application note the antenna theory is covered. The main antenna
parameters such as radiation pattern, gain, impedance matching, bandwidth, size and
others are discussed.
In the second part of this document different antenna types are presented.
November 2012
Doc ID 023812 Rev 1
1/29
www.st.com
Contents
AN4190
Contents
1
Antenna theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
Antenna and radiation pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1
1.1.2
1.1.3
1.1.4
Isotropic, directional and omnidirectional patterns . . . . . . . . . . . . . . . . . . 5
Principal patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Field regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Radiation pattern lobes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Radiation density and intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Directivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Antenna gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Antenna efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Antenna bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Antenna polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Input impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Effective isotropic radiated power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2
Antennas for low power applications . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1
Linear antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5
Dipole antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Monopole antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Loop antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Spiral antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Helical antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2
2.3
2.4
Microstrip patch antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Ceramic antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Slot antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3
4
5
Antenna advantages and disadvantages . . . . . . . . . . . . . . . . . . . . . . . 25
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2/29
Doc ID 023812 Rev 1
AN4190
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Coordinate system for antenna analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
x - y plane omnidirectional antenna pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
E and H plane radiation patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Field regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Near and far field regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Vertical, horizontal,3 and elliptic polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Circular polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Half-wave dipole antenna voltage and current distribution . . . . . . . . . . . . . . . . . . . . . . . . . 14
Half-wave dipole antenna model and radiation patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Quarter-wave monopole antenna design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Inverted-L antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Inverted-F antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Spiral antenna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Spiral antenna 3D radiation pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Helical antenna implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Helical antenna axial mode 3D radiation pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Rectangular microstrip antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.8 GHz patch antenna implementation and characteristics . . . . . . . . . . . . . . . . . . . . . . . 23
Ceramic antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Slot antenna implementation and 3D radiation pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Doc ID 023812 Rev 1
3/29
Antenna theory
AN4190
1
Antenna theory
Antenna performance parameters and the language used to describe antennas can be
confusing and sometimes even misleading. While much can be said in general about what
constitutes a good antenna, most designs reflect some sort of compromise or trade-off
between the various desirable attributes because antenna design involves conflicting goals.
Therefore, it is crucial that antenna specifications are reviewed in light of the intended
application. A more complete and accurate understanding of the terminology associated
with antennas allows the most appropriate antenna for a given task to be specified. A great
deal of effort has been made over the years to standardize antenna terminology. The “de
facto” standard is the IEEE Standard Definitions of Terms for Antenna, so in this document
the main antenna parameters as defined in this standard are given.
The purpose of this document is to give a brief and easy description of antenna parameters.
For a complete and rigorous description of antenna behavior, the reader is requested to
refer to the book “Antenna theory: analysis and design” [1].
1.1
Antenna and radiation pattern
An antenna radiation pattern is defined in the IEEE standard as “the spatial distribution of a
quantity which characterizes the electromagnetic field generated by an antenna”. In other
words, an antenna radiation pattern or antenna pattern is defined as a mathematical
function or a graphical representation of the radiation properties of the antenna as a function
of space coordinates, as shown in
Figure 1.
Radiation properties include power flux density,
radiation intensity, field strength, and directivity phase or polarization.
Figure 1.
Coordinate system for antenna analysis
AM14796v1
In most cases, the radiation pattern is determined in the far field region and is represented
as a function of the directional coordinates. That is, in spherical coordinates the distribution
of the quantity over
Θ
and
Φ
for fixed radius (see
Figure 1).
The interpretation of an antenna's radiation pattern can become problematic because of the
three-dimensional nature of the information. The complexity of a three-dimensional plot of a
4/29
Doc ID 023812 Rev 1
AN4190
Antenna theory
radiation pattern can sometimes obfuscate details. In practice, a three-dimensional plot of a
radiation pattern is of limited value in presenting quantitative information. Therefore, two-
dimensional “cuts” of the radiation pattern are often presented. In particular, cuts in the so-
called E and H planes are often presented.
1.1.1
Isotropic, directional and omnidirectional patterns
An
isotropic
radiator is defined as a “hypothetical” lossless antenna having equal radiation in
all directions. Although it is ideal and not physically realizable, it is taken as a reference for
expressing the directive properties of actual antennas.
A
directional
antenna is one having the property of radiating or receiving electromagnetic
waves more effectively in some directions than in others.
An
omnidirectional
antenna is defined as one having an essentially non-directional pattern
in a given plane and a directional pattern in any orthogonal plane. An omnidirectional
pattern is a special type of directional pattern. An example of an antenna with an
omnidirectional radiation pattern in the x - y plane is shown in
Figure 2.
Figure 2.
x - y plane omnidirectional antenna pattern
AM14797v1
1.1.2
Principal patterns
For a linearly polarized antenna, performance is often described in terms of its principal E
and H plane patterns.
The E plane is defined as the plane containing the electric field vector and the direction of
maximum radiation.
The H plane is the plane containing the magnetic field vector and the direction of a
maximum radiation.
The principal E and H planes are orthogonal planes. It is often sufficient to examine only E
and H plane cuts of the three-dimensional radiation pattern. An example of E and H plane
radiation patterns is shown in
Figure 3.
Doc ID 023812 Rev 1
5/29
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
