Improve mobile handset antenna performance with new tuning techniques

Today's mobile phones need to not only support cellular frequencies, but also non-cellular features such as those used for mobile TV, Bluetooth, WLAN, and location-based services. As the real estate available for the antenna shrinks with each handset generation, antennas are wrapped around camera and keyboard circuitry and re-pathed, which causes them to lose efficiency. Some of this lost performance can be recovered with antenna tuning, which is when the system uses dynamic impedance tuning techniques to optimize the antenna performance for both the frequency of operation and environmental conditions. However, the challenge is that any successful tuning scheme needs to be low-loss, highly linear, able to handle very high RF signal levels, and consume low power.

Antenna Tuning Architectures
An open-loop antenna tuning system is often used when a passive antenna can no longer meet performance requirements, such as when bandwidth requirements increase, handset designs becomes more complex, and there is less space for antennas. In an open-loop system, the tunable element fine-tunes the performance of the antenna at set frequency bands and modes of operation (Figure 1), taking into account static information such as transmit/receive frequencies, modulation schemes, or use case. Open-loop systems do not measure the operation of the antenna in real time, however, so they cannot take into account environmental conditions.

Environmental conditions are very important in a mobile device, and they change regularly as users walk, drive, or move their fingers. To address these conditions, antenna designers can use adaptive closed-loop antenna tuning; in this case, a mismatch sensor provides consistent feedback by tracking the antenna's operation.

The mismatch sensor compares the VSWR (the amplitude of the power that is reflected back to the antenna) to the transmit power, and it makes an adjustment to the impedance tuning circuitry. The tuning algorithm forces the tunable elements to constantly track and adjust to the optimal setting (Figure 1) in all use cases.

Tuning Challenges
Theory is helpful, but the biggest roadblock to implementing adaptive antenna tuning in cellular handsets has been the absence of a high-performance, electronically-tunable reactive component that is low loss and has an adequately wide tuning ratio. In terms of "high performance," the most challenging component requirements are power handling and linearity. For example, GSM antennas typically must handle transmit power up to +33dBm, but under mismatch conditions, the tunable component actually needs to handle RF signal levels measuring a surprising 30Vpk or +40dBm.

The quest for the best tuning material has resulted in a fair amount of research over the years. For instance, some researchers have worked with micro-electromechanical systems (MEMS) and ferroelectric materials technologies (such as barium strontium titanate; BST) in order to implement tunable antennas and filters. These techniques show promise, but they still face significant technological and manufacturing hurdles. In order to adequately address the need for antenna tuning, designers need a technique that already supports high-volume manufacturing, and, preferably, uses proven technology.

Antenna Complexities
Antennas are complex devices, and antennas embedded in mobile handsets are no exception. Since the RF transceiver of the mobile handset is designed for 50Ω impedance, the handset antenna would, ideally, also demonstrate 50Ω impedance across the entire frequency band. In reality, this rarely occurs because the laws of electromagnetics dictate that small handsets have an inherently narrow antenna bandwidth, poor matching, and low radiation efficiency.

Antennas, then, are usually designed at non-50Ω impedance across the whole band, with a typical VSWR between 2:1 or 3:1 for multi-band antennas. The antenna's impedance is also affected by other factors, such as how the phone is being held (the so-called "head and hand effect"). The subscriber's body also causes absorption of the power, further limiting the antenna's radiation efficiency. Generally speaking, handset antennas operate at VSWR of better than 3:1, but if, for instance, the subscriber rests a finger on top of the antenna radiator, this mismatch could increase up to a 9:1. Considering that all devices in the signal chain were designed to operate at VSWR of 1:1, this could be problematic. Figure 2 traces the impact of the "hand effect," or the detuning of the antenna when a hand is near the antenna radiator. This effect changes the resonant frequency of the antenna, causing it to be badly mismatched at its intended operating frequency.