Y. Belhadef, N. Boukli-Hacene, M. Abri

Mobile and wireless communications continue to drive developments in both services and systems. To cover a multitude of standards, an ideal antenna would be compact and cover numerous cellular communications bands and modulation formats. Yet any reduction in antenna size results in loss of bandwidth and efficiency. Recent advances in miniature planar invertedF (PIFA) and wire-plate (thread-plate) antennas help achieve practical compromises in these tradeoffs. By using SuperNEC software, it has been possible to model and simulate the performance of these antennas in three dimensions (3D) and to create relatively wideband antennas. To ensure correct operation, very wide bandwidths were obtained by means of a single resonance for every standard.

To create a PIFA structure for the GSM and DCS standards, an antenna was designed with a 1.2-mm-diameter coaxial probe feeding the resonator (Fig. 1). The antenna was simulated at 900 and 1800 MHz.1-3 The peak of the returnloss coefficient is equal to 25 dB (Fig. 2a). Figure 2b shows the input impedance locus. The change of the antenna main parametersnamely the feed position (6 mm starting fromthe plan of short circuit) and the reducing of the radiation element height (5 instead of 7 mm) leads to an ideal adaptation to the resonant frequency with a peak of 46.5 dB for the return-loss coefficient.

A measure of the antenna's efficiency was obtained by measuring its voltage standing wave ratio (VSWR) for a required frequency. The bandwidth corresponds to the frequency band on which the antenna is adapted to the feed. A VSWR lower than 2.0:1 assures a good match, as it corresponds to a reflexion coefficient below 9.54 dB. A bandwidth of about 41 percent is considered a wide bandwidth for these antennas.

A polar radiation pattern in three dimensions (3D) is presented (Fig. 2c). For the E plane ( = 0 deg.), the radiation pattern is constituted of two lobes. The former lobe has a large aperture while the latter has a small aperture. This means that the pattern is omnidirectional in a half-plane. For the H plane ( = 90 deg.) and the azimutal plane ( = 90 deg.), the pattern is omnidirectional.

The previous structure was simulated at 900 MHz per the GSM standard. The antenna's operation was then tested at a second frequency located around 1800 MHz, which corresponds to the DCS 1800 standard. This second test was carried out by simply changing the wire length feed starting from the antenna structure and adapting the antenna to the higher frequency.

The simulation results clearly indicate that the return loss has a minimal value of about 42 dB at resonance frequency, which indicates that the antenna is perfect (Figs. 3a and 3b). The real part of the input impedance of the antenna is equal to 51 while the imaginary part is 0 at the frequency of interest, showing a good impedance match. The calculated bandwidth is about 36 percenta fairly wide bandwidth.

Figure 3c presents the polar radiation pattern in 3D at a resonant frequency of 1800 MHz. On the reference plane = 0 deg., the radiation pattern comprises four lobes with slightly similar apertures. On the plane ( = 90 deg.), omnidirectional behavior is observed. For = 90 deg., two lobes are seen according to the directions = 0 deg. and = 180 deg.

The wire-plate antenna for the 1800-MHz DCS standard constitutes four short-circuit wires distributed on the horizontal and vertical radiation element medians. The shortcircuit wires situated on the horizontal median are 13 mm from the center. Those placed orthogonally are 7 mm from the center. Moreover, the radiation element (28 x 28 mm) and the ground plane (150 x 120 mm) are simply separated by an air layer as a dielectric (Fig. 4).4-6

According to ref. 4, this DCS antenna design employs four short-circuit vertical and horizontal switches to cover the DCS 1800 or UMTS band. The switch commutation is carried out in a two-by-two procedure. When the two vertical switches are in the ON position (closed) and the two horizontal switches are in the OFF position (open), the DCS 1800 band has been selected. The opposite configuration selects UMTS operation. In this study, the four switches were used at the same time (i.e., four short-circuit wires between the radiation element and the ground plane so that the structure operated to 1800 MHz; Figs. 5a and 5b). The structure simulated by SuperNEC is therefore different from the one in ref. 4.

This antenna's height was reduced to 9 mm between the radiation element and the ground plane by operating the antenna at the resonant frequency (Figs. 5a and 5b). The antenna wire-plate simulation by the SuperNEC simulator at 1800 MHz (Fig. 4) shows that this structure is well matched. The return loss presents a peak lower than 60 dB at 1800 MHz, which means that the antenna input return loss is 0. At the resonant frequency, the input impedance locus is located at the Smith chart's center. It corresponds to a real part of 50 . The bandwidth calculation provides a value of about 48 percent. The three-dimensional radiation pattern is presented in Fig. 5c. The antenna radiation is globally omnidirectionnal.

In short, PIFA and wireplate antennas show great promise for integration in mobile telephones. The results obtained with SuperNEC software for several designs show wide bandwidths for relatively small antennas.

References

1. Pascal Ciais, Antennes multistandards pour communications mobiles, Thse d'Electronique de Universit de Nice-Sophia Antipolis, 2004.

2. P. Ciais, R. Staraj, G. Kossiavas, C. Luxey, "Design of an internal quadband antenna for mobile phones," IEEE Microwave and Wireless Components Letters, Vol. 14, No. 4, April 2004, pp. 148-150.

3. P. Ciais, R. Staraj, G. Kossiavas, C. Luxey, "Compact internal multiband antenna for mobile phone and WLAN standards," Electronics Letters, Vol. 40, No. 15, July 2004, pp. 920-921.

4. P. Panai, R. Staraj, G. Kossiavas, G. Jacquemod, Reconfiguration dynamique d'une antenne, LEAT, Universit de Nice-Sophia Antipolis, France.

5. C. Delaveaud, P. Leveque, and B. Jecko, "New kind of microstrip antenna: The monopolar wire-patch Antenna," Electronics Letters, Vol. 30, 1994, pp. 1-2.

6. G.M. Reliez, J.B. Muldavin, "RF MEMS Switches and switch circuits," IEEE Microwave magazine, December 2001, p. 59-71.