Fringing Field In Microstrip Patch Antenna Gain
Microstrip Antennas: The Patch Antenna Microstrip (Patch) Antennas 1. In this section, we'll discuss the microstrip antenna, which is also commonly referred to as the patch antenna. Note: I'll use the terms microstrip antenna and patch antenna interchangeably.
The rectangular patch antenna is analyzed, and what is learned here will be applied to understanding. (Home) Rectangular Microstrip Antenna Introduction to Patch Antennas Microstrip or patch antennas are becoming increasingly useful because they can be printed directly onto a circuit board. Microstrip antennas are becoming very widespread within the mobile phone market. Patch antennas are low cost, have a low profile and are easily fabricated. Consider the microstrip antenna shown in Figure 1, fed by a microstrip transmission line. The patch antenna, microstrip transmission line and ground plane are made of high conductivity metal (typically copper). The patch is of length L, width W, and sitting on top of a substrate (some dielectric circuit board) of thickness h with.
The thickness of the ground plane or of the microstrip is not critically important. Typically the height h is much smaller than the wavelength of operation, but should not be much smaller than 0.025 of a wavelength (1/40th of a wavelength) or the will be degraded.
(a) Top View of Patch Antenna (b) Side View of Microstrip Antenna Figure 1. Geometry of Microstrip (Patch) Antenna. The frequency of operation of the patch antenna of Figure 1 is determined by the length L. The center frequency will be approximately given by: The above equation says that the microstrip antenna should have a length equal to one half of a wavelength within the dielectric (substrate) medium. The width W of the microstrip antenna controls the input impedance. Larger widths also can increase the bandwidth.
For a square patch antenna fed in the manner above, the input impedance will be on the order of 300 Ohms. By increasing the width, the impedance can be reduced. However, to decrease the input impedance to 50 Ohms often requires a very wide patch antenna, which takes up a lot of valuable space. The width further controls the.
The normalized radiation pattern is approximately given by: In the above, k is the free-space, given. The magnitude of the fields, given by: The fields of the microstrip antenna are plotted in Figure 2 for W= L=0.5. Normalized Radiation Pattern for Microstrip (Patch) Antenna.
The directivity of patch antennas is approximately 5-7 dB. The fields are linearly polarized, and in the horizontal direction when viewing the microstrip antenna as in Figure 1a (we'll see why in the next section).
Next we'll consider more aspects involved in Patch (Microstrip) antennas. Fringing Fields for Microstrip Antennas Consider a square patch antenna fed at the end as before in Figure 1a. Assume the substrate is air (or styrofoam, with a permittivity equal to 1), and that L= W=1.5 meters, so that the patch is to resonate at 100 MHz. The height h is taken to be 3 cm. Note that microstrips are usually made for higher frequencies, so that they are much smaller in practice.
When matched to a 200 Ohm load, the magnitude of is shown in Figure 3. Magnitude of S11 versus Frequency for Square Patch Antenna.
Some noteworthy observations are apparent from Figure 3. First, the bandwidth of the patch antenna is very small. Rectangular patch antennas are notoriously narrowband; the bandwidth of rectangular microstrip antennas are typically 3%.
Secondly, the microstrip antenna was designed to operate at 100 MHz, but it is at approximately 96 MHz. This shift is due to fringing fields around the antenna, which makes the patch seem longer.
Microstrip Patch Antenna Calculator
Hence, when designing a patch antenna it is typically trimmed by 2-4% to achieve resonance at the desired frequency. The fringing fields around the antenna can help explain why the microstrip antenna radiates. Consider the side view of a patch antenna, shown in Figure 4. Note that since the current at the end of the patch is zero (open circuit end), the current is maximum at the center of the half-wave patch and (theoretically) zero at the beginning of the patch. This low current value at the feed explains in part why the impedance is high when fed at the end (we'll address this again later). Since the patch antenna can be viewed as an open circuited transmission line, the voltage reflection coefficient will be 1 (see the for more information).
When this occurs, the voltage and current are out of phase. Hence, at the end of the patch the voltage is at a maximum (say +V volts).
At the start of the patch antenna (a half-wavelength away), the voltage must be at minimum (-V Volts). Hence, the fields underneath the patch will resemble that of Figure 4, which roughly displays the fringing of the fields around the edges. Side view of patch antenna with E-fields shown underneath. It is the fringing fields that are responsible for the radiation. Note that the fringing fields near the surface of the patch antenna are both in the +y direction.
Microstrip Antenna Theory
Hence, the fringing E-fields on the edge of the microstrip antenna add up in phase and produce the radiation of the microstrip antenna. This paragraph is critical to understanding the patch antenna.
The current adds up in phase on the patch antenna as well; however, an equal current but with opposite direction is on the ground plane, which cancels the radiation. This also explains why the microstrip antenna radiates but the microstrip transmission line does not. The microstrip antenna's radiation arises from the fringing fields, which are due to the advantageous voltage distribution; hence the radiation arises due to the voltage and not the current. The patch antenna is therefore a 'voltage radiator', as opposed to the, which radiate because the currents add up in phase and are therefore 'current radiators'. As a side note, the smaller is, the more 'bowed' the fringing fields become; they extend farther away from the patch. Therefore, using a smaller permittivity for the substrate yields better radiation.
In contrast, when making a microstrip transmission line (where no power is to be radiated), a high value of is desired, so that the fields are more tightly contained (less fringing), resulting in less radiation. This is one of the trade-offs in patch antenna design. There have been research papers written were distinct dielectrics (different permittivities) are used under the patch antenna and transmission line sections, to circumvent this issue. Next, we'll look at alternative methods of feeding the microstrip antenna (connecting the antenna to the receiver or transmitter). Next: Top: This page on microstrip antennas and patch antennas is copyrighted.
No portion can be reproduced except by permission from the author. Copyright 2011-2016, antenna-theory.com. Patch antennas, microstrip antennas.
Diagram of the feed structure of a microstrip antenna array. In, a microstrip antenna (also known as a printed antenna) usually means an fabricated using techniques on a (PCB).
They are mostly used. An individual microstrip antenna consists of a patch of metal foil of various shapes (a ) on the surface of a PCB, with a metal foil on the other side of the board. Most microstrip antennas consist of multiple patches in a two-dimensional array. The antenna is usually connected to the or through foil. The current is applied (or in receiving antennas the received signal is produced) between the antenna and ground plane.
Microstrip antennas have become very popular in recent decades due to their thin planar profile which can be incorporated into the surfaces of consumer products, aircraft and missiles; their ease of fabrication using techniques; the ease of integrating the antenna on the same board with the rest of the circuit, and the possibility of adding active devices such as to the antenna itself to make. Main article: The most common type of microstrip antenna is the. Antennas using patches as constitutive elements in an array are also possible. A patch antenna is a narrowband, wide- antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating substrate, such as a, with a continuous metal layer bonded to the opposite side of the substrate which forms a. Common microstrip antenna shapes are square, rectangular, circular and elliptical, but any continuous shape is possible. Some patch antennas do not use a dielectric substrate and instead are made of a metal patch mounted above a ground plane using dielectric spacers; the resulting structure is less rugged but has a wider.
Because such antennas have a very low profile, are mechanically rugged and can be shaped to conform to the curving skin of a vehicle, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into communications devices. Advantages Microstrip antennas are relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. They are usually employed at and higher frequencies because the size of the antenna is directly tied to the at the. A single patch antenna provides a maximum directive gain of around 6-9.
It is relatively easy to print an array of patches on a single (large) substrate using lithographic techniques. Patch arrays can provide much higher gains than a single patch at little additional cost; matching and phase adjustment can be performed with printed microstrip feed structures, again in the same operations that form the radiating patches. The ability to create high gain arrays in a low-profile antenna is one reason that patch arrays are common on airplanes and in other military applications. Such an array of patch antennas is an easy way to make a of antennas with dynamic ability. Catia v5 service pack installation. An advantage inherent to patch antennas is the ability to have diversity.
Patch antennas can easily be designed to have vertical, horizontal, right hand circular (RHCP) or left hand circular (LHCP) polarizations, using multiple feed points, or a single feedpoint with asymmetric patch structures. This unique property allows patch antennas to be used in many types of communications links that may have varied requirements. Rectangular patch The most commonly employed microstrip antenna is a rectangular patch which looks like a truncated transmission line. It is approximately of one-half wavelength long. When air is used as the dielectric substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space.
As the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric 'fringing fields' which increase the electrical length of the antenna slightly. An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss.
Microstrip Antenna Design
Specifications The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth. As the dielectric constant of the substrate increases, the antenna bandwidth decreases which increases the of the antenna and therefore decreases the impedance bandwidth. This relationship did not immediately follow when using the transmission line model of the antenna, but is apparent when using the cavity model which was introduced in the late 1970s by Lo et al. The radiation from a rectangular microstrip antenna may be understood as a pair of equivalent slots. These slots act as an array and have the highest directivity when the antenna has an air dielectric and decreases as the antenna is loaded by material with increasing relative dielectric constant. The half-wave rectangular microstrip antenna has a virtual shorting plane along its center.
This may be replaced with a physical shorting plane to create a quarter-wavelength microstrip antenna. This is sometimes called a half-patch.
The antenna only has a single radiation edge (equivalent slot) which lowers the directivity/gain of the antenna. The impedance bandwidth is slightly lower than a half-wavelength full patch as the coupling between radiating edges has been eliminated. Other types Another type of patch antenna is the (PIFA). The PIFA is common in cellular phones (mobile phones) with built-in antennas. The antenna is resonant at a quarter-wavelength (thus reducing the required space needed on the phone), and also typically has good SAR properties.
This antenna resembles an inverted F, which explains the PIFA name. The PIFA is popular because it has a low profile and an omnidirectional pattern. These antennas are derived from a quarter-wave half-patch antenna. The shorting plane of the half-patch is reduced in length which decreases the resonance frequency. Often PIFA antennas have multiple branches to resonate at the various cellular bands. On some phones, grounded parasitic elements are used to enhance the radiation bandwidth characteristics. The (FICA) has some advantages with respect to the PIFA, because it allows a better volume reuse.
Nowadays, a new version of microstrip antenna with partially etched ground plane has come up. It is also popularly known as printed monopole antennas. They have very broad bandwidth unlike mircostrip antennas. References.
Lee, Kai Fong,; Luk, Kwai Man (2011). World Scientific. by Louis E. Frenzel, 'Electronic Design' 2008. Bancroft, R. Microstrip and Printed Antenna Design Noble Publishing 2004, chapter 2-3. Lo, Y.T., Solomon D.
And Richards, W.F. 'Theory and Experiment on Microstrip Antennas,' IEEE Transactions on Antennas and Propagation, AP-27, 1979 pp. Iulian Rosu. Tsunekawa, K. And Saski, A., 'Antennas for Detachable Mobile Radio Units,' Review of the ECL, NTT, Japan, Vol. 35, No.1, January 1987, pp.
at antenna-theory.com. Di Nallo, C.; Faraone, A., 'Multiband internal antenna for mobile phones,' Electronics Letters, vol.41, no.9, pp. 514-515, 28 April 2005. M. Gogoi, 'Printed Monopole Antenna with Tapered Feed Line, Feed Region and Patch for Super Wideband Applications,' IET Microwaves, Antennas and Propagation, vol.
39-45, January 2014 External links. antenna-theory.com.
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