datasheets.com EBN.com EDN.com EETimes.com Embedded.com PlanetAnalog.com TechOnline.com
Events
UBM Tech
UBM Tech

Design Article

#### Comment

Cezar Palconet

1/12/2011 5:20 AM EST

The title say’s “ Radiated Efficiency : a true measure of antenna performance ...

More...

Bill.St.John

12/15/2010 5:49 PM EST

Actually David, a dipole has 2.1 dB gain over an isotrope.

Bill

More...

# Radiated efficiency: A true measure of antenna performance

## 12/7/2010 8:15 AM EST

Gain
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.

Summary
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.

janine.love

12/7/2010 8:29 AM EST

I asked Pulse for the text of this white paper because it is a good, general description of important performance parameters.The original can be downloaded here. http://www.pulseelectronics.com/file.php?id=3720

Fabrico

12/14/2010 1:43 PM EST

Can I have the permission to translate that into french with pictures?

Txema

12/8/2010 5:37 AM EST

"Even if the signal is equally strong in all directions, gain doesn’t provide a measure to determine that"...confusing... if the signal is equally strong in all directions, then it is an isotropic pattern, and gain should be 0dB...
Or am I misunderstanding something?

Jouni Lifländer

12/14/2010 3:16 AM EST

The gain provides a single fiqure which determines antennas capability to radiate towards some specific direction and thus it doesn't tell whether the radiation is equally distributed towards all directions or not.

If the signal is equally strong in all directions, the radiation pattern is omnidirectional. In practice such antenna has some gain value less than 0dB. An isotropic antenna is an hypothetical lossless antenna having equal radiation in all directions. Such lossless antennas exists only in theory.

David Ashton

12/14/2010 4:39 AM EST

Txema: From the article: "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)"

Makes sense to me. Also it's important to distinguish between gain relative to a dipole or relative to a (hypothetical) isotropic antenna Hence the dBi used in the diagram on Page 4 (of the PDF).

Some antennas (eg the quoted "monopoles with parasitic elements") may radiate in a pattern that is equally strong in all HORIZONTAL directions. So you might get a radiation pattern that is like a much flatter doughnut than that shown on Page 3 (again of the PDF).

Jouni - could you say that antenna efficiency is the ratio of the actual radiated power in the direction of interest (usually that of max. radiation) divided by the theoretical predicted radiated power in that direction? and what sort of figures would you find in practice?

Great article - thanks!
(and RF ed - thanks for the PDF link)

kinnar

12/9/2010 3:19 AM EST

This is quite true that Radiated Efficiency is the true measure of the Antenna but in case of the Handhelds and Notebooks it is better if consumer can come to know about the property of the antenna, but generally they never come to know about this.
Even the laptops does not come with the reception pattern or radiation pattern of the antenna.

t.alex

12/9/2010 3:26 AM EST

Very well written article. I really enjoy the explanation of gain - which is misleading for most users.

David Ashton

12/14/2010 4:46 AM EST

Jouni - just looking at the diagram on page 3 (of the PDF)- it appears that even in the direction of maximum radiation a dipole has very little gain over an isotropic antenna - one dB or so at the most??

Bill.St.John

12/15/2010 5:49 PM EST

Actually David, a dipole has 2.1 dB gain over an isotrope.

Bill

Cezar Palconet

1/12/2011 5:20 AM EST

The title say’s “ Radiated Efficiency : a true measure of antenna performance ”
This is an interesting paper and deserves further thought, in any measure, it is always a comparison between two values, where one may be an accepted standard. In this article I have not seen any agreement to the later as being accepted as a value, in theory, antennas of the same configuration in terms of electrical and physical, will exhibit very close if not identical performance values, setting this aside, but not completely losing sight of it, a focus on determining what the title says, I agree that the amount of efficiency for an antenna is the ratio of how it efficiently radiates the power delivered to its feed point, thus a reference antenna would have to be conceived for this purpose, one that has a material superior above all others, and a physical configuration that will represent a true Omni radiation pattern, specifically under laboratory conditions.
Theoretically an isotropic reference antenna would fill this spot, however in that theory it did not mention the material that will exhibits such characteristics, thus in the real world, no two isotropic antenna of two material, of the same physical shape produce the same radiation characteristics efficiency. Gain is an element of physical configuration, thus for any specific applications where miniaturization and aesthetics are an issue, the unit gain will be compromised.