1. Introduction
Many emerging techniques [1-5] are parts of wireless telecommunication systems. These systems are characterized by their most essential elements which are antennas. The use of the proper antenna leads to effective propagation of electromagnetic energy from the transmitter to the receiver through the wireless channel. Planar antennas present significant advantages over conventional antennas such as monopoles or helical antennas: low cost, low profile and reduced backward radiation towards user’s head [6].
The Planar Inverted F Antenna (PIFA) is widely used in the mobile phone market due to its low profile, small size, and built-in structure. Other major advantages are easy fabrication, low manufacturing cost, and simple structure. It has narrow bandwidth in contradiction to UWB antennas [7], and presents moderate to high gain in vertical and horizontal polarization. The PIFA consists of a ground plane, a top plate element, a feed wire feeding the resonating top plate, and a shorting pin that is connecting its radiating patch to the antenna’s ground plane [8-10].
We can modify a PIFA antenna to operate over multiple frequency bands using various methods. For example, we can use parasitic antenna elements and/or slitting the antenna element to form different paths for surface currents [11,12]. Modified PIFA structures can cover the frequency range of six telecommunication standards [13].
We can tune the operating frequency of a PIFA either mechanically or electrically. The electrically reconfigurable tuning method is relevant to changing the electrical length of the antenna. This can be conducted using a short circuit connection with an external tuning circuit [14,15]. Other tuning methods, that have been reported, include the use of an adjustable reactive component between the PIFA patch [16,17] and the ground plane, or switched tuning stubs, which have been applied in both single-band and dual-band PIFAs [18].
In this paper the feeding point position, the height of the antenna, the shorting strip position, the short capacitor place have all been studied in detail. The influence of the short capacitor value to the resonant frequency is simulated and experimentally measured in a proposed structure antenna ideally used for GSM band.
This paper is split into four sections. In the second section details are given about the antenna structure and design process. The corresponding simulated and measured results for return loss and radiation patterns are presented and discussed in the third section. Finally, conclusions are given in the fourth section.
2. Structure Design
Figure 1 shows the geometry of the antenna. The features of the design include offset short strip and capacitive loads which are illustrated. The ground plane and the top patch layer lie over a dielectric layer with electric permittivity (ερ = 4.4) and 1.5 millimeters thickness. The top patch layer and the ground plane layer have a space described by the variable h. The width (Wg) of ground plane is 26.4 mm and the length (Lg) is 66 mm. The dimensions of patch layer are (W × L) = 26.4 mm × 14.5 mm.
The RF signal is fed to the top patch layer through an SMA female panel mount connector. The SMA connector’s dimensions are equal to commercial standards. The width of strip feed is Wf and is Lf mm away from the angle noticed by the number 1 as showing in Figure 2. The top patch and ground plane are shorted together by a shorting strip that has a width of Ws. The feed strip and shorting strip are spaced apart Lb mm away. For AC coupling we have placed two capacitors C1 and C2 to the feed and shorting strips.
The varactor is placed on a PCB which is Lbc mm away from the corner noticed with No. 1 (Figure 1). The PCB is vertically to the top and to the ground plane and is constructed by the same material as the top and ground plane. The dimensions of this PCB are: (Lc × h). The varactor is Luc mm away from the corner No. 2 (Figure
Figure 2. Schematic details with feed and short strips.
3). Also, we have placed three other electronics components (L1, L2 and C3) on this PCB.
The variables which describe the geometry of the antenna are presented in Table 1.
Varicap Driven Circuit
Varactor SMV1232 [19] is used as a variable capacitance. The main characteristic of the varactors is that the capacitance is inversely proportional to the square root of
Figure 3. Topology of the varactor circuit.
the applied voltage. Figure 4 depicts the circuit that is used to the studied PIFA. To ensure DC insulation between top patch and ground plane in feed and short strip, we placed one capacitor to each strip (Figure 5). The values of the inductances and capacitance are shown in Table 2.
Figure 6 shows the constructed antenna with the DC feed cable attached to the varactor driven circuit.