• Antenne integrate a microstriscia

With the rapid diffusion of wireless system, of the miniaturization and with the increasing demand of low cost radio frequency systems, has become necessary to find some applications that are able to meet present requirements. For Wireless LAN applications (WLAN), especially in closed places, microstrip antennas, coupled directly to the Radio Frequency electronics, are a good choice. The simplicity of realization, the low cost, the flexibility of use and the reduced dimensions make perfect for chip integration microstrip antenna. The physical laws establish that the critical size for an antenna at resonance is ëg/2. With some expedients, such as the augmentation of the dielectric constant and the use of a grounding we can further reduce the dimensions of the antenna. Nevertheless, the application of these techniques degrades efficiency and bandwidth of the antenna, so we can advise it for uses in low power/short range. As reference we will use a square microstrip antenna designed on a GaAs substrate, with a dielectric constant of 12,9. Working with a frequency of about 5.6 Ghz and reducing further the dimensions of the antenna, shorting one edge of the square patch with a contact through the substrate we can obtain dimensions of about 4,5mmx4,5mm. The miniaturization to which we have subjected the antenna modifies its radiation efficiency to about 3,2%. These are acceptable data for wireless communication in a short distance, like, for example, a WLAN network realized for linking the personal computers of an office. For improving the reception of the signal from a microstrip antenna can be convenient to utilize an appropriate Low Noise Amplifier (LNA). On the market there are low cost and low performance transistors based on Si and transistors based on GaAs with better performances but decidedly more expensive. A possible solution for this problem, always having a look at the low cost object, can be find in the utilization of the SiGe BiCMOS technology, which offer excellent performances at high frequency using the SiGe heterojunction bipolar transistors (SiGe HBT) instead of the Si homojunction bipolar transistors (JBT).