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Handset Device Antennas - Research and Technology

With experienced engineers worldwide, Laird Technologies maintains state-of-the-art research facilities in North America, Europe and Asia.

Laird Technologies utilizes only the best high tech R&D equipment and software including, three Satimo near field chambers, three Dasy-3 SARs, a range of mechanical and environmental test equipment, and CAD, Solid Works, IDEAS and Pro E software.

With more than 150 patents issued and 250 plus patents pending, Laird Technologies draws on a powerful intellectual property portfolio to deliver precisely configured handset antenna solutions to its customers.

Laird Technologies invests heavily in R&D, conducting multiple handset antenna research projects. Our research group works closely with our customer projects groups and customers to ensure new development products meet customer expectations. Committed to bringing functional new technology to our customers, we concurrently work with RF, mechanical design, and production methods to reduce size and cost, and improve RF functionality.

Market Trends

The handset market is perpetually evolving. We have seen the shift from external retractable antennas to stubby antennas, arriving ultimately at internal antennas. Today's trends include:

• The prevailing implementation of internal antennas over external antennas. CDMA system types are still mainly external antenna applications.
• Integration of antennas into other components resulting in cost reduction and improved functionality.
• Development of smaller antennas, which still meet demanding performance goals.
• Enhancement of mobile phone functionality.

Challenges in Terminal Antenna Development

Terminal antenna design is an art that is greatly dependent on the characteristics of the mounting platform. This platform dependency presents a substantial challenge to new terminal antenna development. The variation in shape and construction of handset devices, including bar and foldable mobile phones, dictates unique behavior in every new terminal. In addition to designing antennas and matching networks that optimize performance for a given platform, Laird Technologies applies extensive terminal design knowledge to make optimal use of the antenna solution. Another aspect of effective terminal antenna design is the antenna's sensitivity to the surrounding environment. A terminal must function in a variety of situations, whether in a chest pocket, on a table, or of course close to the head. Great attention to detail and experience is required during the design process to maintain optimal performance in all defined user scenarios.

In most cases, a terminal antenna is a unique design applying one of our well-proven production processes. Laird Technologies designs antennas for mass production with production processes that conform to high quality demands. We have an outstanding competence in these areas as well as in the measurement of antennas in production.

RF Antenna Characterization for Terminals

When characterizing an antenna for usage on a wireless terminal, it is important to consider three main characteristics:

• How well the antenna fits to transceiver impedance - otherwise referred to as impedance mismatch. For a given impedance, compatibility is quantified by a number of different elements, such as Antenna Impedance, Reflection Coefficient, Return Loss, and Voltage Standing Wave Ratio (VSWR). We use a network analyzer to measure these elements, and a Smith Chart to provide a visualization of impedance mismatch.
• How well the antenna radiates, usually quantified by Gain, Mean Effective Gain (MEG) or Efficiency. We measure these elements in our state of the art near or far field measurement chambers and simulation tools.
• How well the antenna interacts with other objects. A wireless terminal in use is most often close to an object that will distort its operation. It is vital to address these effects in the design work. Apart from distorted operation, a terminal must also comply with safety standards regarding electromagnetic power absorption in the human body. We quantify power absorption by measuring the Specific Absorption Rate (SAR) in our state of the art SAR measurement chambers.

A great depth of experience is necessary to correctly characterize a terminal antenna. This is true whether measurements or simulations are used. Laird Technologies has cultivated a unique capability in this field. We perform accurate antenna measurements in USA, Sweden and China, and are compliant to relevant standards.

In our development work, we extensively use Electro Magnetic Simulation tools, expediting new idea testing without the impediment of building physical prototypes. Using this technology, we can extract all parameters that characterize an antenna, and visualize currents on the antenna and chassis, enabling the exciting possibility of design improvement. In just a few hours, a parameter scan can reveal vital information on how a design is actually working.

Laird Technologies uses a number of programs, and is highly proficient in these programs. The programs rely on Finite Difference Time Domain (FDTD), Finite Element Method (FEM) and Method of Moments (MoM).

Antennas Used on Terminals

Electrical Dipoles
Electrical dipole antennas are very popular within the antenna community. They typically have a size that is approximately half a wavelength. A dipole at this length is purely resistive and has an antenna resistance of 73 ohms. At 900 MHz a half-wavelength is 0,15 m and at 1.8 GHz 0,075 m.

Due to these rather large sizes, half wavelength antennas are seldom used at frequencies below 1 GHz for handheld terminals. However, quarter wavelength dipole antennas are popular. They form a first half of the half wavelength antenna. The terminal's PCB provides the other half.

When a dipole is shorter than a half wavelength, it becomes increasingly capacitive with decreasing antenna resistance. The radiated field from an electrical dipole is linear polarized.

Small Loops
The radiation behavior of a small loop, in free space only, is comparable to a small electrical dipole. An electrical dipole operated within close proximity to a user exhibits decreased performance. However, when the loop is in operation close to a user, it makes constructive use of the user's body. Another difference between small loops and short dipoles is that the impedance is reactive with a small resistive part. These antennas are widely used in pagers, with limited mobile phone application, so far.

Helical Antennas
Helical antennas are historically the most widely used terminal antenna type. Electrically, they appear to be a number of short dipoles with small loops positioned in-between. The ingenious effect is that the capacitive behavior of a short dipole is balanced by the inductive behavior of a small loop. With the diameters used by terminal antennas, the short dipoles will dominate the radiated fields. One of the greatest disadvantages of helical antennas is that it is complicated to achieve good multiband performance.

Sometimes two or four helices are intertwined. These antennas are called bifilar and quadrifilar antennas and are popular within space/satellite applications.

Helical antennas are often combined with a retractable quarter wavelength whip, thus making it possible to have both small size and improved performance by using a whip when needed.

Meander Antennas
These antennas comprise one or more metallic traces that meanders back and forth in the antenna. They are closely related to helical antennas in that the inductive behavior of the meander is balancing the capacitive properties characteristic of a short dipole. However, the trace is not making loops. Instead, it should be interpreted as small sections of transmission lines with short dipoles positioned in-between. In addition, in this case the radiated field is dominated by the electrical dipoles. Compared to helical antennas, meander antennas offer a greater possibility to design antennas with multiband performance.

Laird Technologies also uses this technology in conjunction with foldable handsets. In which case, a flat meander antenna is secured to the top of the lower foldable section.

Planar Internal Antennas
Due to the drive toward fitting antennas inside the terminals, planar antennas have grown increasingly popular. A classical planar antenna is the patch antenna. It is often a rectangular metallic film mounted above a ground plane. However, a patch antenna must be about a half wavelength in size, which for most terminal applications is not permissible. One popular method to reduce size is to use dielectrics with a high dielectricity constant. This adds weight and reduces the antenna bandwidth. For terminal applications, almost only GPS applications currently use these antennas. Another way to reduce size is to cleverly incorporate grounding. By doing this, the added inductance to the capacitive planar antenna shifts antenna resonance to a lower frequency. Known as Planar Inverted F Antennas (PIFA), the design of this group of antennas normally includes some kind of slot, thus adding electrical length to the antenna.

When designing antennas at frequencies above 1.7 GHz, parasitic elements are popular. Consisting of a grounded strip that runs parallel to the antenna, the effect of these parasitic elements is upper band widening.