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Cezar Palconet
The title say’s “ Radiated Efficiency : a true measure of antenna performance ...
Bill.St.John
Radiated efficiency: A true measure of antenna performance
Jouni Lifländer, RF Designer, Pulse Finland Oy
12/7/2010 8:15 AM EST
Evaluation Metrics
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.
Next: Gain
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.
Next: Gain
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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
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Fabrico
12/14/2010 1:43 PM EST
Can I have the permission to translate that into french with pictures?
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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?
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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.
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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)
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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.
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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.
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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??
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Bill.St.John
12/15/2010 5:49 PM EST
Actually David, a dipole has 2.1 dB gain over an isotrope.
Bill
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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.
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