In this post we are going to learn about Microstrip antenna / printable antenna. It is a type of RF antenna which can be fabricated / printed on a PCB itself.
We will see the following:
General overview of Microstrip antenna:
Microstrip antenna is also called as printed antenna is a recent invention which was designed to incorporate antenna and RF electronics seamlessly on a PCB like illustrated above or on a silicon chip. Microstrip antenna was invented by Robert E Munson (1940 to 2015) who was an antenna engineer at ball aerospace.
His invention made a significant impact on our consumer electronics when it comes to communication via radio frequency. Microstrip antenna eliminated the need for external antenna structure on our electronic devices; the antenna can be designed and fabricated on the PCB itself. Microstrip antennas are usually found in electronic devices whose wireless frequency is in the range of microwave like Wi-Fi, Bluetooth etc.
Since the antenna is on the PCB itself, the antenna is hidden from the users, just imagine our laptops and smartphones still having an old school retractable antenna like old AM/FM radios sets had.
Structure of a Microstrip antenna:
The above illustrated image is of a Microstrip antenna from a satellite TV receiver. Let’s try to understand a simple generalized structure of a Microstrip patch antenna.
A Microstrip antenna consists of the following parts:
1) The metal patch
2) A dielectric substrate
3) A ground plane
Let’s take a brief look at the mentioned parts.
Metal patch: The metal patch consists of length and width ‘L’ and ‘w’ respectively. The length and width is decided by the frequency at which the antenna will be operated. The metal patch is usually an exposed trace made up of copper or sometimes the metal patch will be protected with solder mask.
Dielectric substrate: The dielectric substrate is a non-conductive layer that holds the metal patch. The dielectric material has some dielectric constant and a height that is calculated for the frequency at which the antenna will be operated. The dielectric substrate usually will be the substrate of the PCB. The dielectric constant and height of the substrate plays an important role how effectively the antenna may work.
Ground plane: The ground plane is a conductive metal plate bonded at the opposite side of the dielectric substrate. The ground plane is a continuous conductive metal plate that is usually larger than the metal patch at the opposite side of the dielectric material.
The above three parts are vital components of any Microstrip patch antenna, its shape, size, dielectric constant determines its operating frequency and radiation pattern.
Feeding methods for Microstrip antenna:
Feeding methods are the techniques using which a Microstrip antenna is fed with signal to be transmitted. There are four types of feed methods and they have their own advantages and disadvantages. Below are the four types of feeding methods:
1) Line feed method.
2) Probe feed method.
3) Proximity feed method.
4) Aperture coupled method.
Let’s discuss one by one…..
Line feed method:
In line feed method a narrow conductive line is connected to the metal patch as illustrated below.
Here, we can see the top view of Microstrip antenna which is fed with a narrow conductive strip; the conductive strip is called Microstrip transmission line. The narrow transmission line makes physical contact with RF circuit present on a PCB and also with the patch.
Impedance matching is done by moving the narrow transmission strip in the axis shown beside the transmission line. Impedance matching is very important in any antenna design so that we can transmit maximum power that is being fed to an antenna.
The advantages of line feeding method are: 1) Easy to fabricate 2) It is easy to match the impedance of source and load (antenna) 3) Easy to design overall.
The disadvantages of line feeding method are: 1) Low bandwidth 2) Due to its asymmetrical design cross polarization may occur.
Probe feeding method:
In probe feeding method a coaxial cable is used for feeding the signal. The coaxial cable has an inner conductor and an outer conductor. The inner conductor is connected to patch and the outer conductor is connected to the ground plane.
Impedance matching is done by shifting the inner and outer conductor away from the center of the patch.
The advantages of probe feeding method are: 1) Easy to fabricate 2) It is easy to match the impedance of source and load (antenna) 3) Easy to design overall.
The disadvantages of probe feeding method are: 1) Low band width 2) Due to its asymmetrical design cross polarization may occur.
Proximity fed Microstrip antenna:
Proximity fed method does not make a physical contact with patch or the ground plane. We have illustrated proximity fed method below:
In this method there are two dielectric materials, one of the dielectric materials is bonded with the patch and another dielectric material is bonded with the ground plane. A feed line is sandwiched between the two dielectric material but has no physical / electrical contact with the patch or ground plane. Impedance matching is done by adjusting the length and width of the feed line.
Advantage of proximity fed method: 1) Due to its symmetrical design, low cross polarization will occur. 2) Higher bandwidth capacity compare to other methods 3) Low spurious radiation 4) Easy to design.
Disadvantage of proximity fed method: 1) Difficult to fabricate due to multi-layer design. 2) It is more expensive compare to contact based feed method (first two methods).
Aperture couple method:
In aperture couple method the ground plane is sandwiched between two dielectric materials. On top of a dielectric material a patch is bonded and at bottom side a feed line is bonded. This method is also a contactless one as the feed line does not make any physical or electrical contact with the ground plane or the patch.
Advantages: 1) Low cross polarization 2) Moderate spurious radiation 3) Easy to design.
Disadvantages: 1) It has narrow bandwidth 2) It is the most difficult to fabricate Microstrip antenna.
Conclusion:
Microstrip is a printable antenna which can be etched on a PCB itself; it is easy to design, low cost which can operate on microwave frequencies.
This is just an introduction and I hope this topic would have inspired PCB and antenna designers to take a deeper look on this subject.
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Written by Blogthor
