02-03-2013, 11:02 AM
Overview of Microstrip Antennas
[attachment=52380]
One of the most useful antennas at microwave frequencies
(f > 1 GHz).
It consists of a metal “patch” on top of a grounded dielectric substrate.
The patch may be in a variety of shapes, but rectangular and circular are the most common.
Advantages of Microstrip Antennas
Low profile (can even be “conformal”).
Easy to fabricate (use etching and phototlithography).
Easy to feed (coaxial cable, microstrip line, etc.) .
Easy to use in an array or incorporate with other microstrip circuit elements.
Patterns are somewhat hemispherical, with a moderate directivity (about 6-8 dB is typical).
Disadvantages of Microstrip Antennas
Low bandwidth (but can be improved by a variety of techniques). Bandwidths of a few percent are typical. Bandwidth is roughly proportional to the substrate thickness.
Efficiency may be lower than with other antennas. Efficiency is limited by conductor and dielectric losses*, and by surface-wave loss*.
Basic Principles of Operation
The patch acts approximately as a resonant cavity (short circuit (PEC) walls on top and bottom, open-circuit (PMC) walls on the sides).
In a cavity, only certain modes are allowed to exist, at different resonant frequencies.
If the antenna is excited at a resonance frequency, a strong field is set up inside the cavity, and a strong current on the (bottom) surface of the patch. This produces significant radiation (a good antenna).
Bandwidth: Substrate effects
The bandwidth is directly proportional to substrate thickness h.
However, if h is greater than about 0.05 0 , the probe inductance (for a coaxial feed) becomes large enough so that matching is difficult.
The bandwidth is inversely proportional to r (a foam substrate gives a high bandwidth).
Bandwidth: Typical results
For a typical substrate thickness (h /0 = 0.02), and a typical substrate permittivity (r = 2.2) the bandwidth is about 3%.
By using a thick foam substrate, bandwidth of about 10% can be achieved.
By using special feeding techniques (aperture coupling) and stacked patches, bandwidths of 100% have been achieved.
Radiation Patterns
The E-plane pattern is typically broader than the H-plane pattern.
The truncation of the ground plane will cause edge diffraction, which tends to degrade the pattern by introducing:
Note: Pattern distortion is more severe in the E-plane, due to the angle dependence of the vertical polarization E and the SW pattern. Both vary as cos.
[attachment=52380]
One of the most useful antennas at microwave frequencies
(f > 1 GHz).
It consists of a metal “patch” on top of a grounded dielectric substrate.
The patch may be in a variety of shapes, but rectangular and circular are the most common.
Advantages of Microstrip Antennas
Low profile (can even be “conformal”).
Easy to fabricate (use etching and phototlithography).
Easy to feed (coaxial cable, microstrip line, etc.) .
Easy to use in an array or incorporate with other microstrip circuit elements.
Patterns are somewhat hemispherical, with a moderate directivity (about 6-8 dB is typical).
Disadvantages of Microstrip Antennas
Low bandwidth (but can be improved by a variety of techniques). Bandwidths of a few percent are typical. Bandwidth is roughly proportional to the substrate thickness.
Efficiency may be lower than with other antennas. Efficiency is limited by conductor and dielectric losses*, and by surface-wave loss*.
Basic Principles of Operation
The patch acts approximately as a resonant cavity (short circuit (PEC) walls on top and bottom, open-circuit (PMC) walls on the sides).
In a cavity, only certain modes are allowed to exist, at different resonant frequencies.
If the antenna is excited at a resonance frequency, a strong field is set up inside the cavity, and a strong current on the (bottom) surface of the patch. This produces significant radiation (a good antenna).
Bandwidth: Substrate effects
The bandwidth is directly proportional to substrate thickness h.
However, if h is greater than about 0.05 0 , the probe inductance (for a coaxial feed) becomes large enough so that matching is difficult.
The bandwidth is inversely proportional to r (a foam substrate gives a high bandwidth).
Bandwidth: Typical results
For a typical substrate thickness (h /0 = 0.02), and a typical substrate permittivity (r = 2.2) the bandwidth is about 3%.
By using a thick foam substrate, bandwidth of about 10% can be achieved.
By using special feeding techniques (aperture coupling) and stacked patches, bandwidths of 100% have been achieved.
Radiation Patterns
The E-plane pattern is typically broader than the H-plane pattern.
The truncation of the ground plane will cause edge diffraction, which tends to degrade the pattern by introducing:
Note: Pattern distortion is more severe in the E-plane, due to the angle dependence of the vertical polarization E and the SW pattern. Both vary as cos.