Antennas for wireless applications must handle more bands than ever before, given the growing number of cellular and other wireless standards contained in handsets and base stations. In support of measurements to be performed on cellular base stations, the authors have implemented a printed-circuit dipole antenna on a two-layer RO4350B laminate from Rogers Corp. The compact printed antenna covers cellular bands at 900, 1800, and 2100 MHz and is ideal for use with a low-cost portable electric (E) field meter for testing the E-field intensity of cellular base stations.

Measuring the E-field around base stations is of crucial interest for technicians or other employees working in close proximity to cellular base-station antennas. By covering three bands within a single design, the need for multiple separate antennas is eliminated, saving weight and cost for the test equipment. The novel antenna contributes to a much lower-cost portable E-field meter than any other commercially available E-field probing solution. Both simulated and measured results will be shown for the triband printed dipole antenna design.

The need for small-form-factor, low-cost printed-circuit antennas grows rapidly with the increase of functions embedded within wireless systems, such as handsets that must support Bluetooth, cellular operation, and Global Positioning System (GPS) functions within a single unit. Several different types of antennas are available, for example, to cover multiple cellular communications bands within a single unit. They are typically fabricated on ceramic substrate materials in order to shrink the occupied area of the antennas, given the high dielectric constant of the ceramic substrate materials.1 In this work, the authors propose a low-cost printed antenna on a typical RO4350B laminate material from Rogers Corp.2, which is supposed to occupy the same size and area as a ceramic substrate for the RF and digital electronics of a portable E-field meter under construction. The RO4350B laminate is a glass-reinforced hydrocarbon/ceramic material that can be processed with the same methods used for low-cost printed circuits based on FR-4 substrate materials. It features a dielectric constant of 3.48 at 10 GHz and is commonly used for high-frequency power-amplifier designs. The RO4350B laminate material features a low z-axis coefficient of thermal expansion (CTE) to ensure high stability in multilayer circuits interconnected by means of plated through holes (PTHs).

The table provides typical electrical characteristics for the triband printed dipole antenna, while Fig. 1 presents the physical layout of the antenna with typical dimensions provided. The red and blue colors shown in Fig. 1 represent the top and bottom layers of the printed-circuit design, respectively.

The antenna is a modified type inverted F antenna design.3 As for wideband/multi-band antennas of this class, the return loss is of main importance. In order to evaluate the return-loss performance of the antenna, simulated performance levels predicted by a three-dimensional (3D) electromagnetic simulation program (Fig. 2) were compared with experimental measured results from a first prototype antenna fabricated on the RO4350B material (Fig. 3). There was a clear difference (especially at 2.1 GHz) between the simulation and experimental results that can be easily explained due to a nonideal testing environment and the assumptions applied to the simulations that the antenna was operating in an open-field environment.

The computer EM simulations revealed a great deal about the antenna's return-loss performance. Since antennas for such applications are always installed inside a box with conductive cubes, which mimic the presence of nearby radio-frequency (RF) integrated circuits (RFICs), the return loss can only be further improved by defining a specific placement of the antenna inside the box (i.e., by defining specific placement of the antenna on the substrate). The efficiency of the printed antenna is directly inversely proportional to the return loss. So, in order to achieve the highest possible efficiency with a printed antenna in this type of application, it is critical to place the antenna properly inside the structure, taking into account all nearby conductive materials from the product's RF and digital electronics.

The simulations showed the antenna's radiation patterns to be omnidirectional (Fig. 4), since the simulations assumed no ground reflectors for the test setup. In an actual application, however, nearby reflectors will exist in the form of surrounding electronic devices fabricated or mounted on the antenna's printed circuit board. Thus, it is expected that the antenna's measured elevation pattern will be almost one-half the value of the simulated elevation pattern (in deg.). Similarly, the measured gain is expected to be about 2 to 3 dB higher than values predicted by the EM simulation.

The printed dipole antenna should find application in cellular telephones, a consumer market, as well as for an electric field probe in low-cost portable E-field meters. In order that such antennas can measure the electric field strength correctly, a calibration procedure must be followed for the first industrial prototype.

In summary, the low-cost triband printed dipole antenna design shows great promise for applications requiring a compact antenna solution for multiple-frequency-band coverage. The simulated and measured results for the antenna are in good agreement, with good frequency stability versus temperature expected as a result of the RO4350B laminate material. The antenna is suitable for both in cellular telephone applications as well as in electric field sensors.

References

1. "Ceramic antennas target handset and wireless data," III-Vs: The Advanced Semiconductors Magazine, Vol. 19, No. 4, May 2006.

2. Rogers Corporation, RT/duroid laminates, www.rogerscorp.com.

3. B. Kim, J. Park and H. Choi, "Tapered type PIFA design for mobile phones at 1800 MHz," Vehicular Technology Conference, Vol. 2, pp. 1012-1014, April 2005.

4. S. Vangelis Angelopoulos, Yorgos E. Stratakos , Nikolaos K. Uzunoglu, and Dimitra Kaklamani, "Multiband miniature coplanar waveguide antennas for GPRS-802.11b and 802.11b-802.11a wireless applications," WCNC 2003, New Orleans, LA, March 2003.