Electrical enegineering in the field of Antennas and wave propagation and electromagnetism
Electrical Engineering in the field of antennas, wave propagation and electromagnetism
Communication industry has seen tremendous development in the last decade which continues up to date. The developments partly involve bettering transmission and reception of waves that aid in communication. This means that the knowledge of the phenomena of wave propagation, electromagnetism and antenna design and application is a vital requirement for sustainable development in communication technology. This paper analyzes these phenomena and their application in mobile phones. A greater focus is given to Multi-input Multi-output (MIMO) technique and its application in mobile smart phones. Multi-input multi-output antennas are analyzed based on their performance and effects on human body. CST Microwave studio simulation, other handset antennas and SAR effect are described. Ultra wideband antennas are discussed highlighting their advantages and applicability in communication devices and specifically in mobile phone devices. A brief description of frequency selective surfaces is also given, highlighting their applicability in communication.
Mobile phone technology has evolved greatly since the release of the very first commercial cell phone. The cell phone weighed approximately two pounds with long rubber antenna about five inches above a ten-inch body. These dimensions did not, however, match its efficiency as for instance; the battery could only support a thirty minutes talk without a recharge (Zhou 1). The cell was far much expensive compared to today’s developed phones that have a range of features in a small and lighter package. Today’s mobile phone can be equated to a portable personal computer. The cell’s ability to support communication is based on its antenna. Internal antennas are required in the achievement of a simple structure mobile phone. These internal antennas must be multiband and wideband.
It is a requirement that a wireless device should be receiving and sending a multiband of GSM 850 MHz WLAN 2400 MHz, WiMAX 2300 MHz and UMTS 1920MHz and a wideband of 824 MHz to 2690 MHz to function as a computer and phone. UMTS and GSM are used in global connectivity and include 4G LTE, 2G, 3G and 3.5G. WLAN and WiMAX are required in Bluetooth connectivity (Zhou 2).
The design of multiband and wideband antennas can be done using simulation on software. Different designs can be obtained and investigated by varying radiation width and length, inductive shorting strips and coupling and feeding pin positions. These variations lead to variation in scattering parameters and particularly S11 (the return loss). The return loss, far-field patterns, current distributions, antenna efficiency and gain, can be used to investigate the performance of antenna at different frequencies.
The CST microwave studio
The traditional way of designing an antenna array was slow and expensive requiring a large number of prototypes to be investigated. Today, use of electromagnetic simulation and antenna synthesis accelerates the design process, broaden the design investigation and reduce the prototypes required. The CST microwave studio is an electromagnetic design and analysis software that can be applied in a high-frequency range. The software provides a 3D modeling and a strong graphic feedback that simplifies device definition (Knott 1). CST Microwave studio has four simulation techniques; frequency domain solver, Eigen mode solver, transient solver and integral equation solver. The transient solver is highly flexible and is based on finite integration technique. The solver is well applicable in antenna design.
Handset antenna for phones
There are various types of advanced antennas that can be used to solve the wide band effects, interferences and improve power delivery. These types include the following.
This involves the use of the same channel over and over again to carry out various operations simultaneously. The concept of cellularization is employed where a wide geographical area is split into smaller cells. Every operation uses a part of available frequency thus enhancing service provision to subscribers (Azad 16).
This is in the category of directional antenna that has a radiation pattern that is sector-shaped. The concept used in this type is the division of cell areas into sectors using sector antennas. This aids in power focusing over a small area (Azad 16).
One receive one transmit antenna
This the least complex type of antenna. It is a configuration of two antennas, one acting as the transmitter and the other as the receiver. Application of this type of antenna is mostly in wireless fidelity and Bluetooth technologies (Azad 16).
Due to the increasing demand of multisystem handset equipment, it is a vital requirement that a smart phone should support multisystem operations. Today, a multiband operation is widely applied. This means that the smart device must have the ability to operate at not less than four frequency bands. These bands should include PCS (1850-1990 MHz), UMTS (1920-2170 MHz) DCS (1710-1880 MHz) and GSM850/900 (824-960 MHz). In addition, as mentioned above, the internal antenna must be optimum regarding cost, size, weight and profile (Chiu, Chang & Chi 214).
Great efforts have been put in the multiband internal antenna. Such include the inverted-F antennas and the Planar Inverted-F antennas (PIFA and IFA). The variations of these antennas have gained popularity in the mobile phone industry due to their low cost, miniature size and easy fabrication. Gain performance and bandwidth are improved by the use of system ground plane as part of the antenna. This is in-line with the space economy in mobile devices. However, antenna performance is degraded by the hand-holding of the device and its proximity to the body. There is a spread out of the resonating currents over the ground plane (Chiu, Chang & Chi 214).
Chiu, Chang and Chi designed a loop antenna with multiband attributes that are applicable in mobile smart phones. The antenna was put on printed circuit board. The general design makes use of two rectangular tuning elements that are located near the feed port and used to adjust resonance modes. The main current is restricted in the feed port and loop pattern to minimize current on the ground plane. Degradation of the radiation from the antenna system is minimized since the human hand and head are in the vicinity.
Singh et al also presents a printed multi-input multi-output antenna for use in thin mobile handset. The MIMO antenna consists of a pair of symmetrical antennas on the front with no ground plane on the rear. Antenna simulation is done using CST Microwave studio i.e. Computer Simulation Technology. CST Microwave studio is used to evaluate diversity performances and radiation performance.
The three also investigated how antenna performance is affected by user proximity. The S-parameters, in this case, are investigated. User presence affects the ports’ isolation and operating bands. Impedance matching is improved in low frequency and, as a result, the bandwidth increases. The total radiated power is also studied and how it is affected by user proximity. High total radiated power is likely to significantly improve a handset’s call performance in weak signal areas (Singh et al 88).
There are various techniques used in antennas that include the SIMO technique, MISO technique and the MIMO technique. The single input multiple outputs (SIMO) technique has one transmitter antenna and multiple receiver antennas. The MISO (Multiple inputs single output) technique employs multiple transmitting antennas and single receiving antennae. The configuration can either be an open loop MISO or closed loop MISO (Azad 34).
The multiple input multiple output antenna technique is discussed in detail in this paper as it is the center of concern.
The figure above is a simple representation of the different types of antennas based on the number of receivers and the number of transmitters involved.
MIMO handset antenna
There has been an overwhelming interest in multiple antenna techniques. Multiple antennas have been successfully used to enhance coverage, security, and data transfer rate and radio network performance. Expansive research has helped the multiple antenna techniques to evolve fast. An antenna is the part of a receiving or transmitting system that is designed to receive or radiate electromagnetic waves (Azad 10).
There are several parameters associated with antennas. These include bandwidth, directivity, antenna gain, and antenna efficiency and radiation pattern. Antenna radiation refers to the angular distribution of power dispelled by the antenna. It is simply a graphical representation of relative field strength received and transmitted by an antenna. Antenna efficiency refers to the ratio of total radiated power to total supplied power. Antenna efficiency describes the power an antenna radiates when connected to a transmitter (Azad 11).
A frequency range in which an antenna is effective is known as the bandwidth. The bandwidth describes the performance of an antenna depending on some characteristics and that confirms to specific standards. Directivity is a parameter used to describe the ability of an antenna to focus radiated power in a specific direction. Antenna gain is a combination of directivity and efficiency of an antenna (Azad 10).
Space in a mobile phone handset is limited. This makes the application of M antenna, a requirement for reception and transmission, a challenge. In this case, an MIMO antenna system is proposed. A multi-input multi-output worsens the challenge due to the requirement of an integration of antennas in reception and transmission for the provision of high data rates. This requires decoupling to maintain quality. Solutions have been put forward to counter the challenge of integration in MIMO technology. The solutions are based on antenna components with the magnitude comparable to operation wavelength. The frequency of the solutions is in the high-frequency region where the wavelength is small but sufficient for integration of several antenna components measuring up to a quarter wave length.
The solutions are however insufficient due to the coupling effect that occurs when large numbers of antenna components are arranged close together. This mostly happens especially when operating in regions of low frequency such as Long Term Evolution (LTE700) (Adujar & Anguera 1). Other attempts to provide a solution are the use of less complex geometry and miniaturization with antenna component that is non-resonant in regions of the operation frequency of the wireless device. The solutions are however inadequate in a provision of low coupling and low correlation in antennas.
Although MIMO technology is sufficient in receiving electromagnetic wave signals, antenna components are found not able to provide sufficient MIMO behavior applicable in cellular communication that require transmission of much power in a form of electromagnetic wave signals. Such behavior includes input return loss or gain. The available solutions with adequate reception and transmission of electromagnetic waves can only support a single band.
Literature shows that significant effort has been applied in trying to reduce mutual correlation and coupling. The most applied technique is based on spacing antennas at a distance more than 0.5 lambdas. This is aimed at achieving high isolation from one port to another. The technique is well suited for high-frequency operation but the limited space in a current handset makes its application a challenge in low-frequency region.
Another technique is a provision of polarizations. The technique involves the provision of differing radiation patterns and making the antenna components operate at 2.6GHz Long Term Evolution band and orienting them orthogonally to each other. However, perpendicularity is less achievable in regions of low frequency below 1GHz since the ground plane plays the main radiator. In this case, balanced antennas that prevent currents appearance on the ground plane is seen as a better alternative. Through this, a high evaluation value in ports is achieved but not in regions below 1GHz (Adujar & Anguera 1).
Other notable attempts to reduce coupling is by the provision of modifications on the ground plane. These include an addition of quarter wavelength slots or stubs, integrating decoupling networks, hybrid couplers or neutralization lines in the handset to increase isolation. Dielectric antennas have also been mentioned in literature as a way of enhancing isolation and correlation. Dielectric antennas are capable of near-field confinement. The major shortcomings with the dielectric antenna are complexity, cost and losses involved (Adujar & Anguera 2).
The international journal of applied engineering research discusses the various design considerations necessary in the design of Multi-input Multi-output systems. The major considerations are:
Antenna arrays configuration
The array topology is an important aspect to consider when designing an MIMO antenna array. The topology selected should maximize capacity and reduce symbol error rates. Spacing is applied in spatial diversity to separate the components and raise the channel number the transmitter and receiver. However, the technique is not applicable in small areas. A reduction in the area reduces the distance between elements causing a phenomenon known as coupling. This reduces the capacity of the channel.
Polarization diversity is a preference, where the components in the array are fed with signals that are polarized differently. Orthogonal polarization can then be applied to minimize correlation in the signals.
This is a system that connects a set of antennas to an array system. Improved capacity and reduced probability of symbol error can be obtained from a well-selected sub-array. Selection can be soft selection or hard selection. The hard selection has a part of the antenna being active while soft selection has all antennas being active. An FFT-based soft selection is found to be highly efficient in the presence of mutual coupling.
These have the ability to attain diversity gain in the absence of multiple antennas. Reconfigurable antennas can display pattern diversity to their structures via changes in electrical length. The antennas have gained use in MIMO systems where they are used to achieve pattern diversity. MEM actuators are used to vary the spiral antenna arm length in various radiation patterns of equal frequency resulting to left-hand circular polarization and right-hand circular polarization (LHCP and RHCP). The circular configuration maximizes link capacity and minimizes spatial correlation. Studies have proved reconfigurable antennas to be of great advantage to the Multi-input multi-output technique. This has been proved through simulation, and experimental results and researchers are yet to determine the pattern reconfigurability that would be most responsive and advantageous in specified environment (International journal of applied engineering research 87).
These are the most preferred antennas for application in a mobile unit for a multi-input multi-output system. Their preference is based on their ease of fabrication and low cost. The micro-strip antennas are however, disadvantaged by their low bandwidth. This, however, has attracted attention and engineers have proposed several techniques of improving the bandwidth. These techniques include decreasing the electric permittivity value of substrate or increasing its thickness (International journal of applied engineering research 88).
Another way is by the reduction of the area of the ground plane. A reduced ground plane area results in a decreased capacitance between the patch and the ground plane, which in turn results to an increased bandwidth. In addition, the bandwidth could also be increased by the use of a pair of slots that are L-shaped in the ground plane. Modern designers have adopted varying shapes of patch antenna such as H-shaped, E-shaped, U-shaped, and spiral. The shaped have different aims such as improving impedance bandwidth, reducing cross polarization, achieving good isolation and improving gain (International journal of applied engineering research 88).
This is a major concern in the design of MIMO systems that is as a result of less spacing between components. The performance of a multi-input multi-output system can be described using mutual coupling that is a measurable parameter. Electromagnetic field interactions between antenna elements can be analyzed using calculated mutual coupling. A high mutual coupling is undesirable as it reduces antenna efficiency due to consequential high correlation coefficients.
4G Handset Antenna Multiband Frequency with MIMO capability
The increasing number of the antenna in small mobile devices with a constant volume leads to antenna inefficient which consequently leads to low coverage and data rates. A possible solution to this challenge is the use of reconfigurable antennas that cover part of the used band. Recent developments have seen increased feasibility in the use of this technique. Such developments include the Micro Electro Mechanical Systems technology and Complementary Metal Oxide Semiconductor technology (Ilvonen, Valkonen, Holopainen & Viikari 233).
Ilvonen et al discusses the design of a 4G antenna with reconfigurable frequency and MIMO capability. They introduce a way of investigating geometry suitability of antenna for frequency tuning. Their study establishes a correlation between tuning circuit losses and antenna input impedance reactance behavior. A value less than 0.5 of envelope correlation coefficient is necessary for good multiplexing efficiency and diversity.
The envelope correlation coefficient can be calculated assuming isotropy and uniformity, from radiation patterns. Multiplexing efficiency=μ1μ2(1-ρ) Where μ1 and μ2 are total efficiencies of the antenna components 1 and 2 and multiplexing efficiency defines Signal to Noise Ratio loss (SNR) for ideal MIMO antenna. Low SNR calls for an application of diversity techniques.
The long term evolution (LTE-A) band is used in antenna design. The band is split into two sub-bands, a low band of 698-960 MHz and a high band of 1430-2690 MHz. The low band requires a bandwidth of 262MHz that calls for a large volume of an antenna. However, in this paper, the volume of an antenna can be reduced by frequency tuning. This also compensates for losses that result from user-related impedance detuning. Non-self-resonant components are feasible in this case since they have low inherent selectivity. This gives the resonant components prominence in frequency tunability over self-resonant components (Ilvonen et al).
The geometry of the antenna is printed on a 120 * 60 * 1.5 mm PCB ground plane. The board is provided with a copper coating on the sides and each antenna has a reserved area of 390mm^2. Between antennas, a USB port space is reserved. A complete antenna structure is then placed in a polycarbonate enclosure. The enclosure should have an external dimension of 122 *62 *7 mm. electromagnetic simulations are carried out using CIST Microwave studio.In a single-input single-output system, the major determinant of antenna performance in low band is the antenna component size, frame and antenna location (Ilvonen et al). On the other hand, the feedings location has a significant effect on a multi-input multi-output system. The impedance behavior is also affected by the shape of the antenna component influencing the component losses and tuning range.
This paper seeks to obtain a solution to the main challenge in frequency reconfigurable antennas. The achievement of an acceptable efficiency at low frequencies and at the same time maintaining adequate range of tuning is the major challenge. There should be adequate frequency tuning range to cover a wide range of long term evolution bands (LTE-A). Three generic structures of an antenna are analyzed with different geometries and strong coupling. Two feeding locations are analyzed. Envelope correlation is calculated when antenna components are in resonance and the best performance obtained when antenna feeds are far placed from each other.
The optimal DTC value in each frequency is used to calculate the total loss. It is found that mismatch losses in high band are significantly high. Total loss performance is mostly affected by antenna geometry and less significantly by feeds location. These establishments lead to the next step of designing the prototype (Reddy 14).
Specific absorption rate effect
As the mobile phone industry continues to expand with expansive technological development, public concerns on health implications of mobile phone usage increases. The human head is the part that is mostly affected by electromagnetic radiations radiated from handsets. There has been an increased use of mobile phones across all ages including children that raise a concern on the degree and nature of absorption of Electromagnetic waves as related to morphology and age (Dein & Amr 1).
Dein & Amr investigated the effect of such radiations based on antenna component parameters such as directivity, total efficiency and radiation efficiency. The specific absorption rate (SAR) is the best method for determination of the effect of electromagnetic exposure in near fields of radio frequency sources. The specific absorption rate in a human head at any point is given by SAR= σE22p where E represents the electrical field peak amplitude in human head σ represents tissue conductivity and p represents tissue density.
The above study was carried out through modeling of the human head and varying the sizes as a percentage of the adult head taken to be a 100%. The head models constituted a shell filled with a liquid with brain properties. The study concluded that spatial peak specific absorption effect at a given point on the human head of different sizes varies with head size at different frequencies. The terminal antenna parameters as designed by the designer are also affected by the head size. The effect is, however, challenging and cannot be eliminated since it is an electromagnetic attribute (Dein & Amr 5).
Zhao et al investigated a dual element multi-input multi-output antenna SAR performance in smart phones. Flat phantoms and SAM head phantoms were utilized in the study of SAR properties of antennas. They focused on investigating the effect of the length of the ground plane on the value of specific absorption rate. Results established that the frame mode greatly influences the multi-input multi-output antenna specific absorption rate. A strong frame mode results to a low peak value of specific absorption rate and a more uniform distribution. This results in a high efficiency of antennas.
Nevertheless, the specific absorption rate hot spot location is shifted to the ground plane center leading to a reduction in distance between consecutive antenna hot spots. SPLSR value rises. On the bandwidth aspect, the ground free structure and the semi-ground-free structure are employed for a better bandwidth. This is, however, risky to the user as it is likely to radiate directly to the user’s head (Zhao et al 3278).
Ultra wideband antennas
Ultra wideband systems provide an alternative to meet the ever increasing demand for high bandwidth, low power requirement and minimized fading from multipath. Ultra wideband systems transmit short duration pulses, unlike other systems that transmit sinusoidal waves. Ultra wideband systems, therefore, require highly efficient and accurate ultra wideband antennas for the transmission of these pulses (Zhao Kun 3).
Experimental results have shown the applicability of ultra wideband antennas in mobile phone devices. Like in other antennas, the design of ultra-wideband antennas requires specific considerations. It is important to ensure that the adopted design has no probability of causing spreading of the pulse during transmission. In addition efficiency of the antenna in radiating electromagnetic waves should be guaranteed since power input in an ultra-wideband antenna is quite low. The fractional bandwidth of an ultra-wideband antenna should be greater than 20 percent to provide sufficient broadband to cater for bandwidth requirements (Clarke, Karunaratne & Schrader 21).
Frequency selective surfaces
These are flat complex materials that are designed to be absorbing, redirecting and reflective in some frequency bands while they are transparent in others. Frequency selectivity is facilitated by absorption or redirecting of energy. Absorptive nature of devices is achieved by incorporation of lossy material or by resonant structures. On the other hand, redirecting is facilitated by diffraction and interference. Grating or guided-mode resonance helps in this.
Examples of frequency selective surfaces include the Salisbury screen that was used in military cars, the circuit analogy absorber. Frequency selective surface could be used in devices that require frequency selectivity such as smart phone. This is because of their applicability in both narrow and broadband. In addition, the frequency selective surfaces require tightly packed small components because the bandwidth and resonant frequency are dependent on size and shape and not on array spacing.
Health effects of smart MIMO phones on human beings
Transmitters operating in high frequencies make use of curtain antennas while those operating in low frequency use monopoles as antennas. Transmitter power in low bands is high leading to strong electric fields. Currently, the mobile phone development has reached the fourth generation. The fourth generation mobile phones are Long Term Evolution (LTE). The generation is characterized by fast data transmission rates amounting to 50 Mbps uplink and 100Mbps downlink at the base station from the mobile unit.
Different types of fields that are associated with mobile phone devices have been found to be a threat to human health, especially where long term exposure is involved. Electromagnetic Field exposure has been predicted to relate to childhood leukemia. Other threats associated with high electromagnetic fields are acoustic neuroma and glioma in heavy users of mobile phones with MIMO systems. However, the mechanisms involved in these threats are yet to be established (Scenihr 56).
The phenomena of wave propagation, electromagnetism and antennas are the bases of communication in mobile phone devices. An antenna receives and transmits electromagnetic waves facilitating communication. This implies that the selection and design of antennas to be used in the mobile phone for specific operation characteristics has to be done with a well-informed mind to enhance efficiency. Information on antenna properties and ways of enhancing the properties is a vital requirement.
There is a wide range of antennas that can be applied in mobile phone handsets. These include but not limited to multi-input multi-output antenna, single input single output antenna, single input multi output antenna, and multi-input single output antenna and ultra wide band antennas. The choice of application of each of these antennas is based on the required operation performance of the device. A description and representation of the various types of the antenna based on some transmitters and receivers is also given.
Multi input multi output system can be implemented with MIMO antennas or ultra wideband antennas that provide a large bandwidth, high data rates, reduced mutual coupling and low power usage. Several techniques are applied to reduce coupling effects in MIMO antennas. These include a use of polarization, spacing antenna at 0.5 lambdas to achieve high ports isolation, and modifying the ground plane (e.g. by integrating decoupling networks).
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