An antenna is an electrical component that is needed to transmit and receive electromagnetic energy from the space surrounding it in order to establish a wireless connection between two?or more devices. The variety of devices using wireless communication is enormous, for example mobile phones, base stations, and wireless local area network connections (WLAN). Because of the wide variety of antenna-using devices, multiple types of antennas are needed and available. The antenna's performance, in general, is characterized by some basic terms, such as antenna efficiency and gain. In this article, these basic antenna characteristics and their definitions are introduced.
Antennas have many different attributes and the type of antenna that’s needed depends on many characteristics. One of the main attributes is the operating frequency or frequencies. Some machine-to-machine (M2M) antennas might work on only one ISM band while modern mobile phones or laptop dongles might have almost ten operating bands that all need to be implemented in one device. The operating frequency selection for certain antennas in part determines the material that can be used to produce the antenna. Materials include flex, ceramic, steel plate,?FR-4, or some wire material.
The type of antenna element is another important attribute. To implement several frequency bands in one antenna element, planar inverted-F antennas (PIFAs) are commonly used, but monopoles with parasitic elements are well known, too. These basic antenna structures, together with traditional inverted F-antennas (IFAs), can also be used in ceramic antennas. With newer wireless technologies, the selection of operating frequencies is widening and allocated frequency bands are getting lower which creates demand for more antenna area and volume on the device at a time when consumers want things smaller, especially with handheld and portable devices. It is the antenna that takes the brunt of the burden for size constraint. Therefore, physical size has taken on increased importance in antenna design.
Other attributes involve antenna performance, which is evaluated in terms of resonance bandwidth. The antenna naturally needs to cover all intended frequency ranges with low enough antenna feed port impedance matching and high enough radiation performance. Although resonance bandwidth can be initially verified with a network analyzer (impedance matching), a more meaningful evaluation is done in an anechoic chamber using a passive or active device to get an accurate antenna efficiency measurement.
Passive antenna measurements are executed for passive devices during the R&D process to give some guidelines for the progress of the antenna’s development. It is very useful information during the antenna R&D process because it guides the designers towards a final antenna solution. A passive device can be some kind of a mock-up model and is not required to be operational since the idea in this is to compare different antenna solutions to each other. The final device performance evaluation, however, needs to be made with a fully functional active device. This reveals if there is some interference caused by the device itself on the antenna's operation. Any problems naturally need to be eliminated. In active measurements one measures the total radiated power (TRP) and total isotropic sensitivity (TIS) of the device. These figures combine the antenna performance with the device's radio capability. TRP value indicates the power level that the device's radio can deliver through the antenna to the surrounding space. TIS indicates the radio's capability to sense incoming signals with a low power level. These two important parameters are specified by mobile operators, and their requirements have to be fulfilled prior?to the designed device being brought to market. By making sure that the antenna performs with great enough efficiency throughout the entire operating band or bands with an active device, ensures that the antenna and the device's entire radio is of good quality.
Figure 1: Antenna measurements being performed in anechoic chamber.