Design and Functionality of an Antenna Tuner Using Phase-Locked Loop as the Controller

Reference#: P02850

In 2010 the significant impact of changes in antenna impedance made national headlines with the media coverage of the "iPhone Death Grip." iPhone 4 users discovered that when holding the phone a certain way, their hand bridged the gap between the separate cellular and WiFi antennas that make up the band of metal around the edge of the phone, essentially turning the two antennas into one long antenna, changing their impedance, and causing an almost complete loss of signal strength. In response to the media frenzy surrounding this issue, Apple's CEO Steve Jobs held a special event to demonstrate that other brands of cell phones have similar problems, stating that "everyone has this problem and it needs to be solved."

The "iPhone Death Grip" is just one well-publicized example of the need for dynamic antenna impedance matching. Other situations, such as bringing electromagnetic material within close proximity of a radio frequency (RF) device, can change antenna impedance. No matter the cause, impedance changes reduce RF signal strength, and this in turn causes dropped calls, reduces data transmission rates, creates the appearance of poor carrier coverage, and reduces the battery life of wireless mobile devices.

The ideal solution is to enable RF devices to dynamically adjust antenna impedance in response to changing conditions. Research and development in this area has resulted in a number of techniques for adaptive impedance matching. However, most techniques that are based on relatively simple, low-cost circuitry only enable devices to switch between a few preset impedance values, improving power transfer though not necessarily maximizing it. More sophisticated solutions that continuously vary antenna impedance to maximize power transfer rely on costly, power-sapping microprocessors. What is needed is a low-cost, low-power, dynamic antenna impedance matching solution that maximizes system performance under all conditions.

Researchers at The John Hopkins Applied Physics Laboratory (APL) have developed a patent-pending RF impedance matching network controller that uses a phase-locked loop (PLL) as a dynamic tuner of the impedance between an RF transmitter/receiver and an antenna. The circuit design relies on basic electrical components rather than a microprocessor yet offers continuously variable impedance matching to ensure maximum power transfer at all times.

Prototype testing has demonstrated that the design delivers impedance locking times of <1 ms while reducing transmitted power fluctuations and improving radiated power. It delivers high performance in terms of its power use, response time, simplicity, and cost over existing solutions available on the market.

Dr. G. R. Jacobovitz
Phone: (443) 778-9899