31-07-2014, 03:06 PM
The Basics of Patch Antennas
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Introduction
This article introduces some of the basic concepts of patch antennas. The main focus will be on
explaining the general properties of patch antennas by using the simple rectangular probefed
patch. It will cover topics including: principles of operation, impedance matching, radiation
pat ern and related aspects, bandwidth, and efficiency
Properties of a Basic Microstrip Patch
A microstrip or patch antenna is a lowprofile antenna that has a number of advantages over
other antennas it is lightweight, inexpensive, and easy to integrate with accompanying
electronics. While the antenna can be 3D in structure (wrapped around an object, for example),
the elements are usual y flat; hence their other name, planar antennas. Note that a planar antenna
is not always a patch antenna.
The fol owing drawing shows a patch antenna in its basic form: a flat plate over a ground plane
(usually a PC board). The center conductor of a coax serves as the feed probe to couple
electromagnetic energy in and/or out of the patch. The electric field distribution of a rectangular
patch excited in its fundamental mode is also indicated.
Fundamental Specifications of Patch Antennas
Radiation Pat ern
The patch's radiation at the fringing fields results in a certain farfield radiation pat ern. This
radiation pat ern shows that the antenna radiates more power in a certain direction than another
direction. The antenna is said to have certain directivity. This is commonly expressed in dB.
An estimation of the expected directivity of a patch can be derived with ease. The fringing fields
at the radiating edges can be viewed as two radiating slots placed above a groundplane. Assuming al radiation occurs in one half of the hemisphere, this results in a 3 dB directivity.
This case is often described as a perfect fronttoback ratio; al radiation towards the front and no
radiation towards the back. This fronttoback ratio is highly dependent on groundplane size and
shape in practical cases. Another 3 dB can be added since there are 2 slots. The slots are typically
taken to have a length equal to the impedance width (length according to the yaxis) of the patch
and a width equal to the substrate height. Such a slot typical y has a gain of about 2 to 3 dB (cfr.
simple dipole). This results in a total gain of 8 to 9 dB.
The rectangular patch excited in its fundamental mode has a maximum directivity in the
direction perpendicular to the patch (broadside). The directivity decreases when moving away
from broadside towards lower elevations. The 3 dB beamwidth (or angular width) is twice the
angle with respect to the angle of the maximum directivity, where this directivity has rol ed off 3
dB with respect to the maximum directivity. An example of a radiation pat ern can be found
below
Antenna Gain
Antenna gain is defined as antenna directivity times a factor representing the radiation efficiency. This efficiency is defined as the ratio of the radiated power (Pr) to the input power (Pi). The
input power is transformed into radiated power and surface wave power while a smal portion is
dissipated due to conductor and dielectric losses of the materials used. Surface waves are guided
waves captured within the substrate and partial y radiated and reflected back at the substrate
edges. Surface waves are more easily excited when materials with higher dielectric constants
and/or thicker materials are used. Surface waves are not excited when air dielectric is used. Several techniques to prevent or eliminate surface waves exist, but this is beyond the scope of
this article.
Antenna gain can also be specified using the total efficiency instead of the radiation efficiency
only. This total efficiency is a combination of the radiation efficiency and efficiency linked to the
impedance matching of the antenna
Bandwidth
Another important parameter of any antenna is the bandwidth it covers. Only impedance
bandwidth is specified most of the time. However, it is important to realize that several
definitions of bandwidth exist impedance bandwidth, directivity bandwidth, polarization
bandwidth, and efficiency bandwidth. Directivity and efficiency are often combined as gain
bandwidth.
Impedance bandwidth/return loss bandwidth
This is the frequency range wherein the structure has a usable bandwidth compared to a certain
impedance, usually 50 Ω.
The impedance bandwidth depends on a large number of parameters related to the patch antenna
element itself (e.g., quality factor) and the type of feed used. The plot below shows the return
loss of a patch antenna and indicates the return loss bandwidth at the desired S11/VSWR (S11
wanted/VSWR wanted). The bandwidth is typical y limited to a few percent. This is the major
disadvantage of basic patch antennas. Several techniques to improve the bandwidth exist, but
these are beyond the scope of this article.
Conclusions
In this article, the basic properties of linear and circular polarized patch antennas have been
covered. We defined a basic set of specifications that al ow the user to understand and write a set
of requirements for a specific application. Besides the ones covered here, many more design
options and different implementations of patch antennas are available. Coverage of these
alternatives is beyond the scope of this article, but they should be considered during the
specification and development phases of the antenna.