Antenna Selection for Wireless Communication Systems
By Kimmo Koskiniemi, Engineering Group Manager, Pulse Antenna Division

To successfully implement a reliable, robust, and high-performing wireless system, it is necessary to thoroughly consider the antenna selection and review the requirements of the system in the very early design stages of the hand-held device. This will avoid numerous iterations and unnecessary system redesigns. For the designer, selecting an antenna supplier early provides access to detailed antenna implementation guidelines regarding footprints, mechanical aspects, and contact issues. On the business side, it is advisable to pick a solution that meets your cost target or that provides a good balance between performance and cost.

Mechanical Layout
The mechanical layout of the device’s design will determine the size and type of antenna needed. The typical mechanical properties of an antenna are the form factor (external or internal), construction, size requirement, mounting requirement, type of connection, aesthetic, and other mechanical considerations along with the mechanical durability and reliability of the antenna component. Material selection and mechanical design of both the device and the antenna have direct impact on manufacturability. Competent selection leads to cost effective, easy-to-manufacture solutions, especially for very high product volumes. It is significant to note that a bigger antenna does not correlate with better antenna performance. This is demonstrated by small, ceramic-chip antennas that can outperform much larger antennas when implemented following design guidelines.

Most hand-held portable devices will need an internal antenna, instead of an external one, for purely aesthetic reasons. In fact, more than one antenna will be needed depending on the applications and functions of the device. The typical construction of the internal antennas are stamped metal, printed circuit board (PCB), flexible printed circuit (FPC) on plastic carrier, meander line, low temperature co-fired ceramic (LTCC), ceramic, quadrifilar, and patch antenna. The size and mechanical constraints of the device, as well as the electrical requirements, also determine the type of construction and technology to use. Most of these internal antennas can be mounted through a SMD process, SMT process, pogo pins, spring contact, mini U.FL/I-Pex connector, or direct solder. When long feed cables are used, it is important to factor in signal loss from the cable, which can be several decibels. This has a big impact on the system’s total link budget. Manufacturing considerations should be taken into account in antenna selection.

Electrical Properties
The other major criteria to consider in selecting an antenna are the electrical properties. These consist of operating frequency, bandwidth, maximum gain, average gain, efficiency, return loss or voltage standing wave ratio (VSWR), polarization, directivity, side and back lobe levels, front to back ratio, radiating patterns, impedance, and power rating of the antenna.

The operating frequency is determined by the type of application. For example, WiFi 802.11 b/g, ZigBee, and Bluetooth use the same 2.4GHz ISM band that has a bandwidth of approximately 80MHz (2.4GHz~2.48GHz), while a commercial GPS system uses the L1 1.575GHz band with a bandwidth of 2MHz (1575.42MHz +/-1MHz). Depending on the region of the world, a GSM system uses the 850MHz/1900MHz or 900MHz/1800MHz bands. 3G systems use a variety of bands which also depend on region; for example, a 1.9-2.1GHz spectrum is used in the Europe WCDMA system. A quad-band or penta-band cellular band antenna is often used for worldwide roaming and interoperability as it combines 4-band GSM and W-CDMA 2100 to receive and transmit signals in all cellular bands. Other applications include, but are not limited to, WiMAX, UWB, ISM900, ISM5/5.8GHz, DVB-H, MediaFLO, DECT, RFID, VHF, UHF, AM, and FM. Multi-antenna systems, like diversity and Multi In Multi Out (MIMO), are used in applications where enhanced data rates are needed. With multiple antennas, it is crucial to design and characterize the whole antenna system, including antenna-to-antenna isolation and correlation coefficient testing.

Range and Performance
The maximum (max) gain, average gain, and efficiency determine the range and performance of the antenna. The higher these numbers, the better the range and performance of the antenna. The antenna should have sufficient return loss or VSWR in the operating frequency range. Typically, a -10dB return loss (2.0:1.0 VSWR or better) is a respectable number. Consider the max gain with efficiency or average gain in order to get a better picture of the overall performance of the antenna. Be careful not to fall into the trap of looking at just one of these properties and jumping to a conclusion. Maximum or peak gain is a good gauge in evaluating directive antennas, but can be a misleading term if used as the primary criteria to determine general antenna performance. Often, in more complex devices, the gain is lower, so there is implementation loss when the antenna is placed and connected to the actual device. This is because high peak gain always means some level of directivity and may result in antenna gain that is much lower in some other direction due to nulls in the radiation pattern. Selecting an antenna that has a margin over the recommended decibels ensures that it will meet system requirements in a real-life application.

Most of the wireless systems have 50 Ohm impedance and the antenna should match this as closely as possible for minimum mismatch/loss in the system. Take into account polarization type (vertical, horizontal, or circular) of the antenna and the radiating patterns (in XZ, ZY, and XY planes) in order to fully characterize the antenna. Most hand-held, portable devices need a linearly polarized antenna with omni-directional radiating patterns for 360 degree omni-directional coverage. True omni-directional radiating patterns exist only in theory, because in most cases, the device mechanics affect the antenna patterns, causing nulls and directivity in the patterns. In many cases, the best way to determine true antenna performance is the total 3D radiated efficiency of the antenna as it indicates how much of the antenna’s energy can be transferred to radio waves and how much is lost due to mismatch and radiation losses. Especially in small portable or hand-held devices, 3D efficiency is a much better parameter to compare than maximum gain because, in actual use, the device can be in any orientation that’s possible. In addition, because in use, the max gain peak beam can be directed towards the user’s body, the gain is lost due to body attenuation.

Size and Performance Challenges
The ability to solve some of the most demanding design issues in small, hand-held devices with multiple radio applications will narrow the antenna selection and the type of technology needed. Some of these challenges are isolation, minimum (min) gain and efficiency, hand/body effect, and specific absorption rate (SAR) requirements. Isolation becomes an issue when multiple antennas are located close together in a small hand-held device. Antennas that operate in the same or in a close frequency range couple among themselves and cause a reduction in performance. The min gain and efficiency requirement becomes an issue in small devices due to proximity to other components such as the LCD, metal shields, battery and other electronic components. The lower frequency applications, such as 850MHz and 900MHz bands, are challenging due to the longer wavelength in the lower operating frequencies. Hand/body effect requirements need consideration due to frequency detuning when the device is held close to human body tissue. The resonance frequency will be shifted out of band and cause a high attenuation in the signal due to impedance mismatch and degradation in antenna performance. Moreover, the tissue near it absorbs the radiated energy from the antenna and prevents the signal from propagating to the open air. Likewise, SAR requirements should be considered due to regulatory requirements.

Material Considerations
The materials selected and the actual radiator RF design also contribute to the final electrical performance of the antenna. The materials used need to have low loss (tangent d) and good conductivity. Care needs to be taken when choosing a ceramic antenna. While there are many different types of ceramics, such as low temperature co-fired ceramic (LTCC) and ceramic monopole, performance varies and results might differ when compared to the proprietary ceramic designs of individual antenna companies. Consistency of design and performance specifications are critical, so it is beneficial to work with an antenna company that has strong intellectual property, research, and engineering capabilities. Even with miniature ceramic antennas, a 70%~80% efficiency, 1dBi~2dBi max gain, and better than -3dBi average gain can be achieved if the right antenna is selected and design is implemented.

Testing
All antenna products should be fully field tested to ensure quality and reliability. These tests include mechanical and reliability testing, such as mechanical and temperature shock, vibration, and extreme temperature testing, as well as testing the pre-designed mechanical interface prior to assembly. Electrical characterization should be performed with plastic covers, a phone shell, and a phantom head/hand as though a person were actually using it.

Antenna paramets measurements are very involved and require state-of-the-art measurement equipment such as anechoic chambers, testers, and sophisticated software for data analysis. Low- and high-frequency antennas require different types of test methods due to different signal wavelengths. Special test cases, like very low AM frequencies, require open air test ranges. One needs to know and understand how antennas are tested to truly appreciate the meaning of the performance numbers. It is not as straightforward as comparing antennas based on the specifications on the datasheets. For example, sometimes the test setup used and the test board size are not clearly noted, which can have a large impact on real antenna performance, especially in small devices with internal antennas where the actual device chassis and mechanics need to be understood as part of the antenna structure.

Applying Antenna Selection Criteria
An example of how to apply these criteria is in the selection of antennas for an advanced hand-held device that has a penta-band GSM/3G (850/900/1800/1900/2100), GPS, WiFi 802.11 a/b/g/n, Bluetooth, WiMAX, UWB, and DVB-H antennas. Diversity antennas might also be incorporated for added performance on received diversity. These antennas need to be small in size, yet big in performance. A penta-band GSM antenna with a size of 40 x 8 x 6mm can be applied in a compact, high-performance structure, while a high-performing ceramic PIFA antenna can be used for GPS (10x3.2x2mm), WiFi (10x3.2x1.5mm), Bluetooth (3.2x1.6x1.1mm), WiMAX (3.2x1.6x1.1mm), UWB (13.4x10.2x0.8mm), and DVB-H (45x6.6x5mm) functions. Due to the high isolation property and distributed near field radiation of these special ceramic antennas, SAR and body/hand effect can be reduced to a minimum and excellent isolation can be accomplished for these antennas, even in a small, hand-held device.

Summary
In any wireless communication system, the antenna plays a major role in the reliability and performance of the system. Today’s small hand-held devices challenge antenna designers with demands for ultra-thin, compact sizes and high-performance devices that have the ability to meet multiple standards. Selecting the antenna that best meets the required electrical and mechanical criteria is critical. Planning for the antenna and determining the best one to use during the design phase will save time and money and go a long way towards ensuring it will function optimally.

Pulse Microwave
www.pulseeng.com
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