Some additional antenna evaluation metrics, such as standing wave ratio (SWR) value, isolation between antenna elements, and antenna impedance matching to feed line, are characteristics that are verified with a network analyzer. In some cases gain (directivity) and antenna polarization are important attributes. Metrics for these can be determined in an anechoic chamber.
For multi-radio devices the isolation between antenna elements plays a very important role, as isolation between antennas should be kept as high as possible. In modern 4G applications that might include multiple antennas operating on the same frequency ranges (MIMO), the correlation between antenna elements should be minimized. Antenna correlation can be calculated using the data gathered from antenna efficiency measurements. With mobile devices that have a human operator a specific absorption rate (SAR) is determined to measure the rate?at which energy is absorbed by the user’s body when exposed to an electromagnetic field. SAR measurements are performed by authorized test laboratories equipped with special devices and knowledge for these highly controlled measurements.
Radiated efficiency is a measure of the power radiated through the antenna as an electromagnetic wave to the power fed to the antenna terminals. If an antenna could be made to be a totally?ideal electrical component, it would transform all of the power fed to its terminals to a radiating electromagnetic energy that propagates into the surrounding space. This is possible only in theory, and thus in real life some of the power fed to the antenna terminals is always lost. For example, the mismatch between the antenna element and the feeding network causes power losses. Also the actual antenna material loses energy just by its nature and creates unintended heat. All together these losses lead to situations where the antenna radiated efficiency in actual operation is always below 100% (equals 0 dB). The antenna efficiency is measured in an anechoic chamber by feeding some power to the antenna feed pads and measuring the strength?of the radiated electromagnetic field in the surrounding space. A good antenna, in general, radiates 50 – 60 % of the energy fed to it (-3 to -2.2dB).
Antenna efficiency is a useful and informative measure of antenna “economical efficiency.” With a quick glance, the antenna’s capability to use the power fed to the connection pads can?be evaluated and the amount of power required from the radio module can be determined in order to achieve a certain performance level. Antenna efficiency does not consider radiation direction and thus is a useful performance metric for measuring the efficiency of mobile?devices, which have an omni-directional radiation pattern. In mobile devices, no specific direction of radiation is emphasized. On the other hand, if the antenna is supposed to radiate?in a specific direction, (i.e. the antenna is designed to have some directive characteristics in its radiation pattern) then antenna gain is a better performance metric.
It is possible that an antenna has good efficiency, but in some specific direction or directions the radiation pattern has a null and thus no radiation is recorded in that specific direction.
Figure 2: A three dimensional representation of a typical dipole antenna radiation pattern on one frequency. Directions marked with blue indicate nulls in radiation pattern with low radiation efficiency. The 3D pattern gives a better idea of the entire antenna radiation pattern than the traditionally used two-dimensional patterns.
Figure 3: Typical dipole radiation pattern as seen from one direction only. The view is from the same antenna as the 3D picture shown above. This kind of view does not provide any information on antenna radiation towards other directions and dimensions. Typically 2D views are needed from multiple directions in order to gather even rough estimates of entire antenna radiation in surrounding space.