Antenna Efficiency Measurements Research Articles

A Fast Time-Domain Method of Antenna Efficiency Measurement in a Reverberation Chamber

In this letter, a fast energy-based method of antenna efficiency measurement in a reverberation chamber (RC) in time domain (TD) is proposed. The antenna efficiency is determined by comparing the received energy from the antenna under test with that from the reference antenna. The measurement results are compared with that from the conventional reference antenna method in the frequency domain (FD). The proposed method needs no mechanical stirring or only a few mechanical stirrer positions, which could reduce the measurement time significantly.

Biased Estimation of Antenna Radiation Efficiency Within Reverberation Chambers Due to Unstirred Field: Role of Antenna Stirring

Reverberation chambers (RCs) are now a reliable alternative to anechoic chambers to perform antenna radiation efficiency measurements. Most methods, including the two-antenna one used in this paper, are based on the evaluation of the stirred components of the scattering parameters. They are obtained by subtracting the average scattering parameters from the measured ones. This averaging is performed thanks to mechanical stirring, that is sometimes supplemented by additional stirring such as antenna stirring (which includes platform stirring and source stirring). In this paper, we present an extensive study on the role of antenna stirring in the retrieved radiation efficiency. Four measurement campaigns were successively performed: without antenna stirring, with platform stirring only, with source stirring only, and with a combined stirring of both antennas. Performing only source stirring or platform stirring leads to a biased estimation of the radiation efficiency. Therefore, we showed that the stirring of both antennas is mandatory in order to obtain an unbiased estimation of the radiation efficiency.

Average Rician K-Factor-Based Uncertainty Model of Measured Antenna Efficiency Using the Reference Antenna Method in Reverberation Chambers

Along several decades of development, many methods based on reverberation chambers (RCs) have been proposed for accurate and efficient measurement of antenna efficiency. The standard reference antenna method (RAM) is applicable for almost all frequency bands of interest and all types of antennas, and thus, it may well be the most widely applied method in the industry. In this article, an average Rician <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -factor-based uncertainty model for the standard RAM is proposed. It takes both the number of independent samples and the nonideality of the RC into consideration. Extensive measurements are performed for different RC configurations. It is shown that the newly proposed uncertainty model provides good and stable estimations of the measurement uncertainties for both unloaded and loaded RCs. Moreover, it is experimentally verified that the differences in the radiation characteristics between the reference antenna and the antenna under test (AUT) have little effect on measurement uncertainty regardless of the loading conditions of the RC. The findings of this work allow more flexible and efficient antenna efficiency measurements, including estimation of the associated uncertainty.

The Effect of Noise on Reverberation-Chamber Measurements of Antenna Efficiency

In this work, we show that high antenna frequency selectivity can make it more challenging to accurately measure antenna efficiency in a reverberation chamber (RC) when using the two-antenna method since electronic noise can affect the measured stirred-energy component of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters. Recognizing whether the measured antenna-efficiency estimate is significantly affected by electronic noise is not trivial. The measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters used for calculation may have high signal-to-noise ratios (SNRs), but the antenna-efficiency estimate can still show large systematic errors due to noise. We model noise as a component of the measured stirred energy, showing that the measured reflection is much more susceptible to noise than the measured transmission. We do not focus on the origin of the noise, but rather on the effect it has on the measurement. We show the effect of noise on the overestimation of the stirred energy components of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters, which affects the two-antenna formulation more significantly than the three-antenna method one. We show that this overestimation can cause efficiency estimates calculated with the two- and three-antenna methods to be significantly over- or under-estimated and we show that uncertainties increase as well. Finally, we provide clear steps to recognize and reduce noise effects in antenna efficiency measurements.

Uncertainty in Reverberation-Chamber Antenna-Efficiency Measurements in the Presence of a Phantom.

The effect of a user's proximity on wireless device performance is critical to test the device under realistic conditions. In this work, we propose and demonstrate an improved uncertainty estimation method for antenna efficiency measurements in a reverberation chamber. The improved method separately computes uncertainties due to the effects of chamber loading by a phantom and the effects of antenna mismatch introduced by this phantom, illustrating the sensitivity of uncertainty to close-proximity user effects. We demonstrate that, while the impact of the phantom may be significant on antenna efficiency, and, it has some influence on the uncertainty in the measurement, its impact on overall uncertainty may be insignificant. This is demonstrated using the two-antenna method in the presence of a phantom close to the antenna under test. We illustrate the method by summarizing the antenna efficiencies with their uncertainties and the impact of the phantom for important communication bands. Due to the large effect of the user on antenna performance, this type of measurement and its uncertainty evaluation is a valuable way to characterize antenna efficiency including user effects.

Complete Framework for Frequency and Time-Domain Performance Assessment of Vibrating Intrinsic Reverberation Chambers

This article presents a complete framework able to characterize the performances in both frequency and time domains of “vibrating intrinsic reverberation chambers” made of a metallized tent of shape continuously changing under the action of the stirring process. In the frequency domain, the analysis is based on a method proposed recently in order to assess from which frequency the chamber can be “well-stirred.” The method uses S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> measurements of the emitting antenna in only one position of the VIRC for different stirring conditions, here at different instants. The performances of the VIRC are shown to be similar to the ones obtained in a “classical” parallelepipedic RC of similar volume. In the time domain, a method is proposed in order to measure the decorrelation time defined as the minimum interval of time to wait in order to collect two successive independent samples and to detect any periodical repetition of the stirring process. This time-domain analysis emphasizes that the number of collectable samples is proportional in a VIRC to the measurement time. The use of a VIRC could therefore contribute to severely decrease the uncertainty budget of some RC measurements such as antenna efficiency measurements or over-the-air testing of wireless devices.

Statistical Analysis of Antenna Efficiency Measurements With Non-Reference Antenna Methods in a Reverberation Chamber

Reverberation chambers (RCs) have become a popular testing facility in antenna efficiency measurements. Unlike the standard reference antenna method, the non-reference antenna methods have been proposed to measure the antenna efficiency in the absence of a reference antenna. However, the statistical distributions of the measured (estimated) total antenna efficiencies using the non-reference antenna methods have not been derived before, making it difficult to perform a rigorous statistical analysis of the antenna efficiency measurement. In this paper, the distributions of the measured total antenna efficiencies using two non-reference antenna methods in an RC are derived, based on which the statistics (e.g., expectation and variance) of the measured total antenna efficiencies are also derived. It is shown that the original non-reference antenna methods (estimators) are only asymptotically unbiased and are biased with limited samples, thus corresponding unbiased estimators are proposed and analyzed. The derived analytical expressions of the statistics of the total antenna efficiency estimators are verified by simulations and measurements in an RC. The performances of different non-reference antenna methods are discussed.

Benchmarking a High-End Smartphone’s Antenna Efficiencies

In an effort to create a benchmark for antenna designs for mobile applications, the total and radiation efficiencies of a modern high-end smartphone’s antennas are measured. To this end, an adapter board is designed that facilitates the connection to a calibrated measurement system. Its shape is chosen to closely resemble the original phone’s dimensions while allowing connection of the antennas via spring fingers to a coaxial measurement system. The characterization of this mock-up is performed in a reverberation chamber, which offers the advantage of an on-average isotropic environment, making it ideal for antenna reflection coefficient and efficiency measurements. The reflection coefficients, total efficiencies, and radiation efficiencies of the mock-up are then measured from 0.75 to 6 GHz. The total antenna efficiencies, not accounting for a possible improvement due to adaptive matching networks, are found to be below 15% up to 4 GHz and up to 25% below 6 GHz. Meanwhile, the radiation efficiencies are up to 15% below 2.5 GHz and up to 40% below 6 GHz. Such antenna efficiency measurements are the first and serve well as a benchmark for future designs and concepts.

Evaluation of chamber effects on antenna efficiency measurements using non‐reference antenna methods in two reverberation chambers

In this study, the evaluation of chamber effects on antenna efficiency measurements using non-reference antenna (NRA) methods is presented. Two reverberation chambers have been employed and they differ in dimensions and paddle stirrer configurations, therefore the corresponding electrical characteristics such as quality factors, decay constants, and enhanced backscatter coefficients are envisaged to be different. However, it is found that these differences have little influence on the efficiency measurement of antennas-under-test (AUTs). The AUTs used for this study were a directional antenna and an omnidirectional antenna. The discrepancy in the efficiency of the two antennas measured between the two chambers is <10% within their operational frequency bands. These results demonstrate that the NRA measurement techniques are robust. Further investigation shows that the directional antenna is slightly more prone to polarisation mismatch than the omnidirectional antenna is. Therefore, polarisation stirring should be implemented when using the NRA methods for antenna efficiency measurement especially for directional antennas.

Improved Antenna Efficiency Measurement Uncertainty in a Reverberation Chamber at Millimeter-Wave Frequencies

We provide results of antenna radiation and total radiation efficiency at millimeter-wave frequencies gathered with a new open-ended waveguide-plate method that is compared to a well-known two-antenna method. The new method yields improved uncertainty in antenna efficiency measurements. Both methods are based on use of a reverberation chamber. Measurement results are compared to numerical simulations and good agreement (~3% maximum difference) is achieved. Before performing the efficiency measurements, the chamber configuration was assessed with respect to the Rician $K$ -factor, number of uncorrelated paddle orientations, and coherence bandwidth. We calculated the uncertainty using the NIST microwave uncertainty framework capable of performing parallel sensitivity and Monte Carlo analyses. The framework enables us to capture and propagate the uncertainties in the S-parameter measurements to the final efficiency result. The expanded uncertainty that we achieved for these antenna efficiency measurements is 2.60%.

An Improved Method for Efficiency Measurement of All-Excited Antenna Array in Reverberation Chamber Using Power Divider

Reverberation chambers (RCs) are ideally suitable for antenna efficiency measurements. For the measurement of the efficiency of a passive all-excited antenna array in an RC, a power divider is normally used to excite all the ports of the array simultaneously. The presence of the power divider introduces additional insertion loss to the antenna array. How to calibrate the insertion loss accurately is a problem for such a measurement. In the existing method, the effect of the mismatch between the array elements and the power divider was not taken into consideration, which could result in inaccurate evaluation of the efficiency of the antenna array. In this paper, an improved method of measuring the antenna array efficiency is presented. It allows the de-embedding of the power divider which removes the insertion loss of the power divider, thus it obtains the efficiency of an antenna array more accurately. This is realized by introducing proper attenuators between antenna array elements and power divider ports. Simulations and measurements have been conducted to verify the proposed method. It is shown that this method is accurate even when the array elements are not well matched with the power divider, especially for the wideband antenna arrays, where good impedance matching of array elements is difficult to maintain. The advantages and limitations of this approach are also discussed. The proposed method should be very useful for accurate and efficient measurement of antenna array efficiency.

Development and Application of a Microwave Reactor Radiating through a Leaky Coaxial Antenna

AbstractA novel microwave (MW) reactor radiating through a leaky coaxial antenna with an impedance matching unit was developed. The MW energy could be effectively transmitted from the power source to the reactor with a uniform distribution of the electromagnetic field. Simulations of the MW radiating indicated the excellent radiation capacity of electromagnetic waves and a uniform axially extended cylinder MW field around the antenna. The simulation result was in accordance with the antenna efficiency measurements and the temperature distribution. Model reactions of benzoyl hydrazine preparations performed in an open vessel under atmospheric pressure and the preparation of 3‐acetylcoumarin in a sealed reactor under pressure proceeded smoothly on a scale of hundreds of grams.

Generalized Statistics of Antenna Efficiency Measurement in a Reverberation Chamber

Statistics of the measured antenna efficiency using the reverberation chamber (RC) technique (proposed in IEC 61000-4-21) has been derived by assuming that the measurements of the antenna under test (AUT) and the reference antenna have the same independent sample number. This work generalizes the statistics of the measured antenna efficiency by allowing the AUT measurement and the reference measurement to have different numbers of independent samples. The derived statistics are verified by simulations as well as RC measurements. Moreover, practical issues about how to allocate the total measurement time to the AUT and reference measurements for different cases are discussed.

Efficiency measurement of UWB and UHF antennas in small cavities of arbitrary shape

AbstractA new method that extends the application of spherical Wheeler cap to relatively small cavities of any shape is presented for radiation efficiency estimation. We introduce a modified Huynh formulation to take into account cavity losses when they are non‐negligible for high frequencies. These assumptions are consistent with experimental results, which are based on antenna measurements in cubic cavities and for different frequency bands. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2193–2196, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24552

Time-Domain Measurement of Antenna Efficiency in Reverberation Chamber

In this communication, we present a new and quick measurement method of antenna efficiency in reverberation chamber. The technique involves the processing of the measured time-domain response of transmit/receive antennas to a short pulse. Thereafter, a large number of multipath taps for an intended frequency and bandwidth are extracted from the channel delay response. Owning to the large delay profile of the channel, overall estimated power of the multipath taps is less variant with the mechanical stirring of cavity modes, i.e., an approximately uniform field throughout much volume of the chamber is obtained. The radiation efficiency of different antennas are measured by the time-domain approach and compared to the traditional method using frequency domain and mechanical stirring. The proposed method has good accuracy even without mechanical stirring of the cavity modes. Consequently, the radiation efficiency of a test antenna can be measured for several frequencies with one single experiment.

Antenna Efficiency Measurements in a Reverberation Chamber Without the Need for a Reference Antenna

This letter proposes a reverberation chamber approach for antenna efficiency measurement that does not require a reference antenna, that allows the antenna efficiency to be determined over a potentially wide frequency, range and which is applicable to electrically large antennas, thus demonstrating clear advantages over currently employed techniques for antenna efficiency measurement. This letter outlines the proposed method and presents results for a monopole antenna that demonstrate the efficiency can be obtained with reasonable accuracy over a range of frequencies.

Efficiency measurement of UWB antennas using time reversal in reverberation chambers

Using the reverberation chamber to obtain antenna radiation efficiency is considered. The reverberation chamber, which has for many years been used for electromagnetic compatibility measurements, can also be used with great advantage for antenna measurements since it simulates effectively a uniform multipath propagation environment. How a reverberation chamber can be used to measure the ultra-wideband (UWB) antenna efficiency in a very short time is described. The procedure based on time domain is described and some experimental results are given and compared to measurements obtained by the Wheeler cap method extended by Schantz to UWB antennas.

A Near Field Focused Microstrip Array for a Radiometric Temperature Sensor

A low-cost radiometric temperature sensor system is described, with emphasis on the use of a 12.5 GHz near-field focused microstrip array to provide a low-profile compact product suitable for commercial and industrial applications. An overview of the temperature sensor system is given, followed by design details of the focused microstrip array, including near-field patterns and antenna efficiency measurements. Temperature measurements made with the radiometric system in a food-processing plant field test show that the antenna can resolve individual hamburger patties at a practical measurement distance

Efficiency measurement of antennas for on‐body communications

AbstractThe Wheeler cap method is used to measure the efficiency of antennas for use in on‐body communication systems. A monopole and a planar inverted F antenna on small ground planes were mounted in contact with body phantom material and their efficiency measured using the Wheeler cap and by a conventional anechoic chamber method. Results from the two methods show close agreement and also are close to results of simulations using commercial software. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2256–2259, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21930

Electromagnetic coupling effects on the cavity measurement of antenna efficiency

Electromagnetic coupling effects on the antenna in a conducting cavity are studied theoretically and experimentally. It is observed in experiments that at the resonant frequencies of the cavity, the input resistance of the antenna attains values two or three orders of magnitude higher than that at frequencies away from resonance. It is shown via theoretical analysis that the input resistance of the antenna measured at the resonant frequencies of the cavity is not merely the loss resistance desired in computing the antenna efficiency, but is actually the sum of the loss resistance of the antenna and the coupling resistance between the antenna and cavity. This coupling effect is demonstrated quantitatively by numerical computations for dipole and monopole antennas. The computational results for the input resistance are in agreement with the measured data. A method is proposed to avoid the cavity-antenna antiresonance in the measurement.