Design Article
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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
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.
Next: Evaluation Metrics
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.
Next: Evaluation Metrics
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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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