Novel PIFA with polarization and frequency agility

Emmanuel Dreina* (1) (2), Michel Pons(1), Tan-Phu Vuong(2) and Smaïl Tedjini(2)

(1) France Telecom R&D, 28 Chemin du vieux chêne, 38243 Meylan – France (2) LCIS/INPG, ESISAR, 50 rue de Laffemas, 26902 Valence - France

Introduction

One of the major problems in a wireless environment is multipath fading caused by multiple reflections due to local scatterers surrounding the mobile. At the mobile, one way to combat multipath fading on the downlink is to implement antenna diversity techniques which consist of receiving several signals transmitted over independent paths. The most commonly used schemes are space diversity and radiation pattern diversity because of their structural simplicity. However, they are not always compatible with the spatial constraints imposed by the mobile handset. An alternative antenna diversity technique, which is more suitable, is the polarization diversity. In fact, polarization diversity allows to have more compact antenna systems and to compensate for polarization mismatch due to random handset orientation. Polarization diversity is gaining importance in modern wireless communication system. Thus, several studies [1] show that this technique provides the best diversity gain in multipath environments, such as indoor or urban dense environments, where the polarization of the radio wave may change significantly during the propagation. Several antenna architectures offering polarization diversity have been proposed, among which are works on microstrip antennas with polarization agility such as [2], [3]. It is well now that PIFAs (Planar Inverted-F Antenna) are more compact and have lower directivity and larger bandwidth than classical microstrip antenna. We hence propose a compact PIFA with polarization and frequency agility in order to mitigate multipath fading. The antenna presents an innovative structure that can cover two bands at 2.4 and 3.5 GHz (Wifi and WiMax) according to both orthogonal linear polarizations.

Antenna topology

The schematic of the proposed antenna structure is given in Fig. 1. The antenna is constructed on a square substrate (FoamClad – Arlon) with a relative permittivity of 1.1 and a total thickness h of 6.75 mm and 42 mm length. The square ground plane has the same length as the substrate. On the other face of the substrate, there are a square patch with length L1 = 8 mm and two other identical rectangular patches with dimensions L1 x L2 with L2 = 12 mm. These two rectangular patches are disposed near orthogonal edges of the square patch. In order to obtain a PIFA structure we need to create a shorting plate between the ground plane and the radiating patch. This structure has two shorting plates which consist of a group of 9 vias leading to a thin rectangular patch. These shorting plates are disposed near the two other orthogonal edges of the square patch. The four patches around the centre square patch are at a distance of 1 mm of the edge of the square patch. The RF feed probe is located on the diagonal line of the square patch. With this position, the matching is the same for both linear polarizations. The polarization agility is obtained by connecting to the square patch, either the shorting plate 1 to obtain a vertically polarized PIFA, or the shorting plate 2 to obtain a horizontally polarized PIFA.

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The choice of the resonant frequency is controlled by connecting or not to the square patch the rectangular patch in front of the connected shorting plate. In fact, if the rectangular patch is connected to the square patch, the electric length of the radiating part is longer, so the PIFA has a lower resonant frequency.

The electronic control of the agile antenna states is accomplished by means of 4 groups of 3 pin diodes mounted between the square patch and the four rectangular patches. Anodes of groups 2 and 3, and cathodes of groups 1 and 4 are on the square patch. The pin diodes (MA4GP907) were simulated as lumped element capacitor of 25 fF for the open state and as lumped element resistor of 3 Ω in the shorted state in order to take the non ideal behavior of the diodes into account. By shortening one or two of these groups of diodes using dc bias circuits, a specific polarization and frequency can be excited for this antenna. There is one dc bias on each rectangular patch and another on the square patch through the feed probe. This structure allows controlling each group of pin diodes in order to create an agile antenna with 4 different states. Table 1 presents, for each of the antenna's state, the potentials of the 3 patches and the state of each group of pin diodes.

Antenna states State of groups of pin diodes Potentials of the patches Polarization Frequency

1 2 3 4 1 2 3 V H 2.45 3.53 ON ON OFF OFF - U - 2U - 2U X X ON OFF OFF OFF - U 0 - 2U X X OFF OFF ON ON U 2U 2U X X OFF OFF OFF ON U 2U 0 X X

Square patch 1

Group 4

Group 1

Shorting plate 1

Ground plane

Feed probe

Rectangular patch 2

Rectangular patch 3

Shorting plate 2

Group 3

Group 2

L1 L2

h

Fig 1: Geometry of the PIFA with polarization and frequency agility

Table 1: Potentials of the patches and state of groups of pin diodes for each antenna state

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a)

b) Fig. 3: Simulated radiation pattern in dBV/m and

polarisation ellipses at 3.53 GHz, a) shorting plate 1 connected, b) shorting plate 2 connected

Fig. 2: Antenna structure in Cartesian and spherical bases

Z

Y

X

Theta

Phi

The antenna allows easy integration of active elements due to its planar architecture. It can be fabricated by using standard printed circuit techniques thanks to the use of a foam substrate. The dimensions of the PIFA are designed for a 50 Ω impedance match at 2.45 and 3.53 GHz. The EM simulation of the structure was done with Microwave Studio from CST.

Polarization agility The polarization and radiation pattern at 3.53 GHz as a function of azimuth, Phi, and elevation, Theta, (Fig.2) are given in Fig. 3.a and 3.b. They illustrate respectively the vertically polarized PIFA (shorting plate 1 connected) and the horizontally polarized PIFA (shorting plate 2 connected). The polarization agility of the antenna is clearly visible where the antenna gain is strongest, i.e. when 130° < Phi < 230° and 60° < Theta < 120°. This structure allows changing the radiation characteristics between two orthogonal linear polarization states.

Consequently, conventional switching diversity schemes can be applied and this structure can be easily integrated in a mobile phone. This antenna can provide a diversity gain of about 7 dB in common urban or indoor environments [1]. When the antenna works at 2.45 GHz, the same polarization switching is observed. The antenna directivity evolves little when the state of polarization is changed.

Frequency agility

As explained above, this agile PIFA is able to work at two different frequency bands. The antenna is designed in such a way that if the rectangular patch in front of the connected

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/ dB

shorting plate is connected, the resonant frequency is 2.45 GHz and if it is not connected, the resonant frequency is 3.53 GHz. The centre frequencies and bandwidths do not change with the polarization state because the structure has an axis of symmetry. Return losses of the PIFA for the high and low resonant frequencies are shown in Fig. 4. When the rectangular patch is connected to the square patch the bandwidth is 120 MHz at -10 dB (2.38 to 2.5 GHz), the total efficiency is 0.76 and the maximum gain is about 2.9 dBi. When the rectangular patch is not connected, the bandwidth is 300 MHz at -10 dB (3.39 to 3.69 GHz), the total efficiency is 0.9 and the maximum gain is about 4.2 dBi. The efficiency difference can easily be explained by the fact that when groups of pin diodes 1 or 2 are in the shorted state, they represent an ohmic resistance, so they induce additional losses. This antenna structure can be proposed for wireless systems where two distinct bandwidths need to be covered. It is easy to modify the two resonant frequencies by changing the lengths L1 and L2.

Conclusion A compact PIFA with polarization and frequency agility was presented. It was shown that using groups of pin diodes the polarization can be switched between two orthogonal linear polarizations. This antenna is devoted to RF front-end of wireless systems using polarization diversity at the reception. Switching between two frequencies can also be achieved. The antenna presented can cover the entire 2.4 GHz ISM band or can be used for WiMax applications at 3.5 GHz.

References: [1] Dietrich C.B., Jr., Dietze K., Nealy J.R., Stutzman W.L., " Spatial, polarization and

pattern diversity for wireless handheld terminals", IEEE Trans. Antennas Propaga., vol. 49, no 9, pp. 1271-1281, Sept. 2001

[2] Schaubert D., Farrar F., Sindoris A., Hayes S.,"Microstrip antennas with frequency agility and polarization diversity", IEEE Trans. Antennas Propaga. Vol. 29, no 1, pp. 118-123, Jan. 1981

[3] Fries M. K., Grani M., Vahldieck R., "A reconfigurable slot antenna with switchable polarization", IEEE Microwave Wireless Compon. Lett., vol. 13, pp. 490-492, Nov. 2003.

Frequency / GHz Fig 4: Return loss of the PIFA, rectangular patch not connected ( ) and connected ( )

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