A novel compact wideband triband antenna for mobile terminals based on PIFA element is proposed. The antenna operates at the following frequency bands: Wireless-LAN 802.11 b, g, a and WiMAX 3.5 GHz. The antenna was studied by means of numerical simulations as well as the ground plane dimensions and user's hand effects. The overall size of the radiating element which is
Nowadays, modern mobile handsets are becoming more and more complex devices. Indeed, the modern handsets should be miniature in size and provide access to different communication standards throughout a broadband in order to obtain high data rates without increasing the transmitted power. In fact, due to the increase of wireless standards, there is need of multiband antennas implementation on the terminals.
In this paper, we present a tri-band antenna for the desired Wireless-LAN and WiMAX applications. The antenna must be integrated in such a way to take up a very small space on the printed circuit board. Due to these requirements which are multiband system and reduced size, a planar PIFA (printed inverted F antenna) is an attractive candidate [
The PIFA structure guarantees the quarter wavelength resonance frequency and then an operating frequency band. The PIFA is fed by a strip while another is used to ground it. It is undoubtedly true that PIFA offers higher-order resonance frequencies. It is not only hard to control the higher-order frequency resonance but even worse. Both the radiation performance and return loss are inefficient to be used at these higher frequencies. The key to obtain additional resonance frequencies should be the PIFA’s structure modification [
For the third wide frequency band (4.9–6 GHz), an additional resonance frequency appeared when a meander line is added to the overall structure.
The tri-band structure, shown in Figure
Top and side views of the tri-band antenna.
The geometry of the resultant multiband PIFA is shown in Figure
Triband radiating element with an SMA connector (dimensions are in mm).
The simulations have shown that this radiating element can be excited at three resonance frequencies. The first resonance frequency is mainly governed by the PIFA dimensions; the second frequency is mainly due to the J-shaped open-end arm while the last corresponds to the meander line length. Beside these design parameters and the antenna height, the ground plane dimensions influence the input matching. The proposed tri-band antenna is entirely manufactured through a 0.1 mm thickness copper sheet and by folding the ground and feed arms of the PIFA.
The antenna prototype is shown in Figures
Photos of the tri-band antenna showing the ground and feeding strips.
Photos of the tri-band antenna with a 10 cm
Simulated and measured reflection coefficient of the tri-band antenna.
Now, the PCB ground plane length and width are varied to investigate the influence of the ground plane current distribution on the antenna characteristics. Figure
Simulated reflection coefficient for different PCB dimensions.
One is led to conclude that the resonance frequencies depend on the current distribution on the ground plane and then on its dimensions leading to set these dimensions from the beginning. It is clear that these three resonance frequencies can be found by optimising the antenna design parameters whatever the PCB dimensions and then the antenna application.
Several parameters can affect the three resonance frequencies, in particular the antenna height. Figure
Simulated reflection coefficient for different antenna heights.
It is obvious that the antenna height has an important effect on the quality factor and bandwidths. As the antenna height is decreased, the quality factor increases and the bandwidths become narrower. It should be noted that the antenna height has a slight effect on the resonance frequencies.
In fact, these results are particularly interesting. They show a possible height decrease to 7 mm or less but to the detriment of the bandwidth.
Also, it is well worth noting that the return loss characteristics, in particular its minimum, do not change if the PIFA radiating element is moved to the left or right upper corner of the PCB ground plane in order to accommodate the RF and signal processing modules.
The radiation characteristics of the proposed antenna are measured in an anechoic chamber. The measured realized gain is shown in Figure
Simulated and measured maximum gain of the tri-band antenna.
The radiation patterns of the proposed antenna at the three resonance frequencies, 2.44 GHz, 3.5 GHz, and 5 GHz are illustrated in Figures
Measured (solid line) and simulated (dashed line) radiation patterns at 2.44 GHz: (a)
Measured (solid line) and simulated (dashed line) radiation patterns at 3.5 GHz: (a)
Measured (solid line) and simulated (dashed line) radiation patterns at 5 GHz: (a)
This antenna does not offer the same radiation pattern at the three resonance frequencies.
There is a good agreement between the measured and simulated results. The feeding cable is going behind the antenna during the measurements process and had no effect on the radiation patterns. Good omnidirectional radiation patterns at different frequencies were obtained in
Also, it should never be forgotten that the efficiency is an important parameter for small antennas. The measured and simulated efficiencies of the proposed antenna are shown in Table
Measured and simulated efficiency of the proposed antenna.
Frequency (GHz) | 2.44 | 3.5 | 5.5 |
---|---|---|---|
Simulated total efficiency | 91% | 94% | 99% |
Measured total efficiency | 75% | 90% | 95% |
It is worth stating at this point that the efficiencies are obtained by dividing the measured directivity and the measured realized gain. This latter allows for ohmic losses, mismatch losses, connector losses, edge power losses, and external parasitic resonances. As for the directivity, it can be calculated by integrating the radiation patterns measured in a sufficient number of cutting planes [
So, there is a good agreement between the measured and simulated efficiencies except for the first band. This discrepancy at the first operating band is due to the difference in values between the measured and simulated reflection.
It is undoubtedly true that the user’s hand will affect the antenna performances. In this section, a simple hand model, shown in Figure
General view of the tri-band antenna with the proposed hand model.
Figure
User’s hand effect on the reflection coefficient of the proposed antenna.
It would appear, then, that the hand and its position affect the resonance frequencies. In fact, the two lower frequencies are decreased but in actual fact the tri-band antenna still operates at the three bands for any realistic position of the hand for
Hand effect on the efficiency of the proposed antenna.
Frequency (GHz) | 2.44 | 3.5 | 5.5 |
---|---|---|---|
Simulated efficiency without hand | 91% | 94% | 99% |
Measured efficiency without hand | 75% | 90% | 95% |
Simulated efficiency with hand | 41% | 54% | 60% |
It is obvious to everyone that the hand changes the radiation pattern of a handheld antenna due to its high permittivity. Figure
3D simulated radiation patterns at 3.5 GHz when the terminal is used in hand.
A novel multiband antenna based on PIFA radiating element for multistandard terminals is proposed. An antenna prototype has been fabricated and tested. The good agreement obtained between experimental and numerical simulations results has validated the design procedure and the utility of such multistandard antenna. It has a
Even if the antenna is held by a hand at any real position, it still covers the three bands for