Radiated efficiency: A true measure of antenna performance
Jouni Lifländer, RF Designer, Pulse Finland Oy
12/7/2010 8:15 AM EST
Gain is an antenna measure that combines the antenna’s efficiency and its directivity into one figure. Directivity means that the antenna radiates with greater efficiency to one particular direction in its surrounding space than to the others. Antenna absolute gain is defined as the ratio of the radiation intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated evenly to all directions of surrounding space (i.e. isotropically). If no specific direction is stated, the direction of maximum radiation is used
to determine gain. Directivity alone is pretty close to this, but actually it describes only the directional properties of the antenna and is therefore controlled only by the pattern. The term “antenna gain” is actually a bit misleading, because an antenna is a passive component that does not have any amplifying characteristics.
The antenna’s radiation pattern sometimes is purposely steered to some specific direction. These so-called directional antennas are used commonly in base stations, but also in some other applications such as GPS where some directional characteristics are desirable. Since the GPS satellites are in the sky, the antenna should have the greatest radiating efficiency towards the upper hemisphere. Gain is measured in an anechoic chamber by feeding some power to the antenna and then measuring the strength of the radiated electromagnetic field in different angles of the surrounding space. From this data the direction of maximum radiation is determined.
Gain is the measure usually used to measure antenna performance. It is a useful measure in cases where the antenna is required to have directional characteristics, for example in base stations situated on rooftops or at the end of office corridors, etc. and for GSM antennas used in some navigation applications. When directional radiation is needed, measuring gain determines if the power fed to the antenna is being optimally used. For example, in a wall-mounted base station antenna there is no point in directing the signal to the wall as it reduces the gain of the antenna
to the preferred direction (assuming that the total efficiency level remains the same in both cases). Purely measuring antenna efficiency would provide the same performance information whether the antenna’s greatest radiation is directed optimally as intended or not.
Using gain as a measure of performance, even for directional antennas, has some disadvantages. For most of the mobile terminals, their orientation with respect to the closest base station is coincidental. The signal scatters and reflects several times from basically any object on its way from the base station to the mobile terminal. From the antenna point of view this means that the signal arriving to the receiver can pretty much arrive from any direction of the surrounding space and no optimal direction for receiving the signal can be determined in advance. Thus in mobile antennas, the antenna radiation pattern should be designed as omni-directional as possible, making it equally effective in all directions. The fact is that an antenna with a big gain value
in some specific direction may not receive the signals arriving from other directions very well.
Gain is the most prevalent way of measuring the efficiency of an antenna and, therefore, the performance of an antenna. However, gain doesn’t measure the overall efficiency of an antenna. It only determines the efficiency of radiated output in one direction at a time. It tells nothing about the efficiency level that the antenna is achieving in all the other directions surrounding the antenna. That is fine if it is an application where all or most of the output from the antenna needs to be directed to a specific location and the location of the transmitting antenna is fixed with respect to the receiving antennas. If, on the other hand, as in many other wireless applications including the use of mobile phones, the signal emitted from the antenna needs to be equally strong in more than one direction, then gain is a poor measure of performance. Although performance might be good when the signal is facing the direction of maximum performance,
it could be weak, non-existent, and/or degraded in other areas. Even if the signal is equally strong in all directions, gain doesn’t provide a measure to determine that.
Because increasingly there are more applications that require a non-directed signal, companies are using radiated efficiency as the preferred test method. With radiated efficiency, one can determine what the efficiency and performance are for all areas surrounding the antenna. This knowledge enables the engineer to assess if the antenna will meet performance parameters
or if it is necessary to design an antenna that gives maximum antenna performance uniformly
or to multiple directions, rather than only in one direction. The antenna should have as omni-directional a radiation pattern as possible because the likelihood for angle of arrival of the signal is uniformly distributed over the whole space.