Design of Microstrip Patch Antenna with Polarization Diversity for Wireless Applications. Great isolation between different polarizations, several designs patch antenna giving dual polarized radiation at as single. The circular patch is having a four triangular slot on it. Be divided into four basic catagories: 2. Microstrip Patch Antenna. A microstrip patch antenna (MPA) consists of a conducting patch of any planar or nonplanar geometry on one side of a dielectric substrate with a ground plane on other side. It is a popular printed resonant antenna for narrow-band microwave wireless links that require semi.

In telecommunication, a microstrip antenna (also known as a printed antenna) usually means an antenna fabricated using microstrip techniques on a printed circuit board (PCB).^[1] It is a kind of internal antenna. They are mostly used at microwavefrequencies. An individual microstrip antenna consists of a patch of metal foil of various shapes (a patch antenna) on the surface of a PCB (printed circuit board), with a metal foil ground plane 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 transmitter or receiver through foil microstriptransmission lines. The radio frequency 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 printed circuit 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 microwave integrated circuits to the antenna itself to make active antennas.

Patch antenna[edit]

The most common type of microstrip antenna is the patch antenna. Antennas using patches as constitutive elements in an array are also possible. A patch antenna is a narrowband, wide-beam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate, such as a printed circuit board, with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane. 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 bandwidth. 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 mobile radio communications devices.

Advantages[edit]

Microstrip antennas are relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. They are usually employed at UHF and higher frequencies because the size of the antenna is directly tied to the wavelength at the resonant frequency. A single patch antenna provides a maximum directive gain of around 6-9 dBi. 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 phased array of antennas with dynamic beamforming ability.^[2]

An advantage inherent to patch antennas is the ability to have polarization 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.^[3] This unique property allows patch antennas to be used in many types of communications links that may have varied requirements.

Rectangular patch[edit]

The most commonly employed microstrip antenna is a rectangular patch which looks like a truncated microstrip 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 wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant 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.

Specifications[edit]

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 Q factor 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.^[4] 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[edit]

Another type of patch antenna is the planar inverted-F antenna (PIFA).The PIFA is common in cellular phones (mobile phones) with built-in antennas.^[5][6]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.^[7]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.^[8]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 folded inverted conformal antenna (FICA)^[9] has some advantages with respect to the PIFA, because it allows a better volume reuse.

References[edit]

1. ^Lee, Kai Fong,; Luk, Kwai Man (2011). Microstrip Patch Antennas. World Scientific. pp. 8–12. ISBN184816453X.
2. ^'Welcome to antennas 101'by Louis E. Frenzel, 'Electronic Design' 2008
3. ^Bancroft, R. Microstrip and Printed Antenna Design Noble Publishing 2004, chapter 2-3
4. ^Lo, Y.T., Solomon D. andRichards, W.F. 'Theory and Experiment on Microstrip Antennas,' IEEE Transactions on Antennas and Propagation, AP-27, 1979 pp. 137-149.
5. ^'PIFA - The Planar Inverted-F Antenna'.
6. ^Iulian Rosu.'PIFA – Planar Inverted F Antenna'.
7. ^Taga, T. Tsunekawa, K. and Saski, A., 'Antennas for Detachable Mobile Radio Units,' Review of the ECL, NTT, Japan, Vol. 35, No.1, January 1987, pp. 59-65.
8. ^'Inverted-F Antenna (IFA)'at antenna-theory.com
9. ^Di Nallo, C.; Faraone, A., 'Multiband internal antenna for mobile phones,' Electronics Letters , vol.41, no.9, pp. 514-515, 28 April 2005

External links[edit]

• Microstrip Antennas antenna-theory.com
• Microstrip Antenna Tutorial EM Talk

Polarization reconfigurable microstrip patch antenna is presented for 2.4–2.5 GHz wireless sensor network and WLAN applications. Dual feed degenerate mode patch is used as the starting point for circular polarization and L-shaped islands which can be connected or disconnected to the main patch via RF switches are placed around the patch. When L-shape structure is connected, the patch radiates in linear polarization modes with either vertical or horizontal polarization depending on the feed being used. When RF switches are not biased, the antenna is in circular polarization mode. Full wave simulations and measurements were carried out to validate the design.

Polarization reconfigurable antennas are becoming increasingly popular due to ever increasing wireless applications, especially at 2.4 GHz ISM band. Different polarizations on the same antenna offer key advantages in multipath fading environments and provide means for multiple-input multiple-output (MIMO) systems. Although polarization agile antenna concept has been around for quite some time [1], switching complexities and symmetry in radiation patterns still present challenges in the design.

The simplest method to achieve multipolarization is to switch perturbation segments of the antenna in an effort to change the antenna's electrical characteristics [2–16]. Usually, PIN diodes or RF MEMS was utilized as switch. Changing circular polarization states of a circularly polarized microstrip antenna can be achieved by employing two separate feeds (one for each polarization) and by simply switching the feeds. However, linear polarization states, in that configuration, would require a hybrid coupler and another set of RF switches, which was not practical. Another drawback of this configuration is to use multiple switches on the feed line which increases the noise figure of the system by at least the insertion loss of the switches used. Among slot coupled or slot reconfigurable shaped antennas, U-shaped slot antenna was studied for multiple polarizations [17–19]. However, symmetry in beam and electrical characteristics was relatively poor. Another extensively studied reconfigurable antenna shape was E-shaped antenna [3]. These antennas were proposed as wideband polarization agile structures. However, the size of the antenna was much larger than their resonant mode counterparts, and achieving four polarizations was problematic. For instance, in [3], linear polarization in only one state was achieved, and the frequency band was divided into two segments to cover the entire ISM band which makes the antenna impractical for many wireless applications. Varactor diodes loaded antennas were also proposed for frequency agile antennas [20–23]. Limited bandwidth and complex voltage biasing circuitry are prominent limitations of these antennas.

In our study, we took a well-known degenerate mode microstrip patch antenna for circular polarization states and loaded this structure with L-shape parasitic units which could be connected to the degenerate mode by RF switches. When parasitic unit is connected to the main patch, the antenna operates in linear polarization mode. Depending on which feed point is selected, either one of the linear polarization states can be invoked. We also employed chip inductors as RF choke instead of quarter-wave match sections which inevitably distort radiation pattern. Simulation and measurements indicate all four polarizations using only two RF switches.

The antenna structure is shown in Figure 1. It is composed of degenerate mode square patch with corners cut and L-shape parasitic elements around the cut corners. RF switches are placed on cut corners to the center of L-shape element. When diodes are OFF, L-shape structure acts parasitic to the main antenna, and when diodes are ON, it is strongly coupled to the main patch. The distance between the parasitic element and the degenerate patch is critical to obtain proper circular and linear polarizations. Feed points determine the sense of polarization, that is, right hand or left hand circular, when diodes are OFF. Likewise, when diodes are ON, vertical or horizontal polarization can be obtained from the feed port. Polarization states for the switch positions and feed ports are summarized in Table 1.

Figure 1 Reconfigurable antenna structure.

Microstrip Patch Antenna Design Pdf

Layout of the biasing circuitry is very important and often the cause for asymmetry in radiation patterns. Quarter wavelength chokes have been implemented in past designs, but we observed that it distorted (even slightly tilt) the main radiation pattern. Hence, we used RF chip inductors as RF chokes. The ground return for the RF choke is implemented via small DC cable around the patch as opposed to via, although it's been simulated with via. In that sense, our design is different than earlier reconfigurable designs. The DC current through the diode is limited from DC supply with a series resistor.

Current distributions on the patch antenna when it is fed from port 1 or port 2 are shown in Figures 2 and 3, respectively. The symmetry in current distributions is mainly due to preserved symmetry in the antenna design. When diodes are ON, the current distribution on L-shape is stronger and enforces the current distribution on the main patch for linear polarization; that is, 90° phase shift between the alternate corners is not observed.

Figure 2 Current distribution on the patch (port 1 is fed, and port 2 is loaded with 50 Ω), (a) when diodes are ON and the (b) when diodes are OFF.

Figure 3 Current Distribution on the patch (port 2 is fed, and port 1 is loaded with 50 Ω), (a) when diodes are ON and (b) when diodes are OFF.

Microstrip Patch Antenna Ppt

3. Simulations and Measurements of the Reconfigurable Antenna

Simulations of the antenna are carried out in FEKO, 3D electromagnetics solver. For realization, we used a substrate from Taconic RF65 (${\epsilon }_r=6.5$, $\tan \delta =0.002$). Substrate thickness is 3.17 mm. We used BAR64 PIN diodes from Infineon Technologies and 0603 SMD components for RF choke inductors from Coilcraft. The prototype is shown in Figure 4. Simulation and measurements of the antenna when diodes are OFF and ON are shown in Figures 5 and 6, respectively.

Figure 4 Prototype of the reconfigurable antenna.

Figure 5 Measured and simulated input reflection coefficients when diodes are OFF.

Figure 6 Measured and simulated input reflection coefficient when diodes are ON.

Although measurements exhibit slight shift in frequency, the match is under −10 dB throughout 2.4–2.5 GHz.

The reconfigurable antenna gains are shown in Figures 7 and 8. They well match with those of standalone patch antenna modes. Cross-polarization levels could be further improved with slight modifications in the L-shape island geometry.

Figure 7 Antenna gain when diodes are OFF.

Figure 8 Antenna gain when diodes are ON.

Radiation patterns of the reconfigurable antenna fed from port 1 are illustrated in Figure 9. There is no beam asymmetry or tilt in the patterns due to symmetry in feed structure.

Figure 9 Gain patterns of the antenna at 2.45 GHz.

A reconfigurable microstrip patch antenna design for WSN applications has been presented. The biasing of the diodes and isolation of RF from DC path are achieved using discrete SMD components. The structure preserves its symmetry for circular and linear polarization modes. All the four polarizations can be achieved by simply switching the feeds and the diodes on the top metallic part. Antenna gain of the reconfigurable antenna is very comparable to its standalone degenerate mode patch for circular polarization and single mode for linear polarization counterparts. The proposed configuration is simple and easy to implement for polarization diversity antennas.

The authors thank EMSS FEKO, GmbH, Germany, for providing extended software license.

References

Microstrip Antenna Pdf