Performance Expectations for Reduced-Size Antennas

By Gary Breed Editorial Director

λ very device that L/ = 0.167 and RRad = 5.5 ohms. The feed- This month’s tutorial article transmits and/or point impedance will be 5.5 –jX, where X is a presents a summary of Ereceives radio sig- large capacitance, as high as 1500 ohms in the the advantages and limita- nals needs an antenna. case of thin dipole. This 5.5 –j1500 ohm tions of electrically-small When that device has a impedance must be matched to the system antennas like those used in small size or limited impedance, typically 50 ohms. many wireless devices space for a classic reso- Small loops and monopoles have similarly nant antenna, various low radiation resistance with high reactance. techniques are used to implement reduced- Matching to highly reactive loads is inherent- size antennas. Those techniques may include ly narrow bandwidth, since the magnitude of inductive or capacitive loading, meandered or the reactance changes rapidly with frequency. spiral construction, high dielectric constant Achieving a broader bandwidth match materials to slow down wave propagation, and requires either complex networks or the intro- embedded structures incorporated into the duction of lossy components. packaging. Each of these methods imposes The use of meandered lines, spirals, fractal some type of limitation when compared to patterns effectively distribute the required monopole, dipole, or resonant loop antennas. inductance over the length of the antenna, This tutorial looks at the key limitations of and can result in higher radiation resistance small antennas, with the intention of illus- and lower loss matching networks. However, trating what level of performance can be they require more space to implement. expected from the various design options. Efficiency VSWR Bandwidth In the above example, the initial task of cre- A good impedance match is needed for effi- ating a non-reactive feedpoint requires can- ciently transferring power into, and extracting celling the capacitive reactance with an induc- power from an antenna. Compared to the “nat- tor of 1500 ohms reactance. In practice, such ural” radiation resistance of a monopole (36 an inductor will have a Q no greater than 100, ohms) and a dipole (72 ohms), the radiation usually less, and thus will also have a series resistance of reduced size antennas will be resistance of 15 ohms or more. With the induc- much lower. tor in place, the system will see 20.5 ohms (or As an illustration, the radiation resistance greater) resistive impedance, of which 15 ohms of a small dipole is [1], is loss. Small loops and monopoles have similar problems with losses when attempting to 2 ⎛ πL⎞ match their low radiation resistance. R = 20⎜ ⎟ Rad ⎝ λ ⎠ The analytical work of Harrington, reviewed in [2], shows that efficiency is where L/λ is simply the dipole length in wave- reduced as antennas become smaller, even lengths. Using this equation, a dipole that is with lossless matching. These losses are pri- one-third of normal λ/2 size will have marily resistive losses in the antenna’s con-

60 High Frequency Electronics High Frequency Design SMALL ANTENNAS ducting material, which become Lumped Element Loading—Chip the surface from the primary anten- greater with lower radiation resis- capacitors or inductors may be used na that is installed above. With no tance. In practice, however, most loss to provide the necessary reactance to surface waves, the “ground plane” has will be due to lossy matching compo- create an electrically small antenna greatly reduced interaction with the nents. with a non-reactive feed impedance. antenna, allowing it to perform in a Dielectric Loading—With lower manner approximating its free space Proximity Effects velocity of propagation, antennas performance. This is a narrowband The environment surrounding a constructed on dielectric materials solution, although varactor diodes small antenna will have a large effect are smaller than their free-space can be integrated to provide tuning. on its performance. Small antennas counterparts. Strip or inverted-F Many other structures are possi- used on portable, handheld devices antennas on ceramic substrates are ble. Readers are encouraged to review will be subject to changing orienta- popular for handheld wireless the References and other sources. tions and widely varying proximity to devices. Circular or rectangular conducting and dielectric materials. stacked patch antennas are often References These will all affect the radiation pat- used for circularly-polarized Global 1. D. Miron, Small Antenna tern, and the interactions may affect Positioning System antennas. Design, Newnes, an imprint of the impedance, as well, resulting in Electromagnetic Bandgap (EBG) Elsevier, 2006. less efficient power transfer. These Ground Planes—EBGs are low- 2. J. Volakis, C.-C. Chen, K. changes in polarization and signal height structures comprising many Fujimoto, Small Antennas, McGraw- strength must be accounted for when small-size antenna-like elements Hill 2010. analyzing the total path loss of the with capacitive top-loading surfaces 3. S. R. Best, “The radiation prop- system the portable device will be that approximate a flat substrate. erties of of electrically small folded used with. Because these structures are reso- spherical helix antennas,” IEEE In addition, the operating envi- nant, they present a high impedance Trans. on Antennas and Propagation, ronment may have nearby shielding at their tops. This effectively isolates vol. 52, April 2004, pp. 953-960. and reflecting surfaces, such as vehi- cles, buildings, furnishings, and other objects. These things can block the desired signal path, or magnify the multipath characteristics inherent to portable device communications. Although these larger-scale objects do not interact directly with the antenna, they alter the final radiation pattern.

Some Antenna Examples Small antenna design is a com- promise among gain, efficiency, bandwidth and occupied volume. Some types that address these tradeoffs in different ways are described below. Bent and folded antennas—bent monopoles, including the inverted-F, folded patch antennas, and a wide range of meandered structures may be used. They may be implemented as two- or three-dimensional structures. In general, the highest radiation resistance for a given occupied volume will be achieved with a moderate amount of “meandering.” The four- arm spiral appears to be the most efficient 3-D structure [3].