Mohsen Koohestani

A Novel, Low-Profile, Vertically-Polarized UWB Antenna for WBAN

This paper proposes a novel, low-profile UWB antenna for wireless body area network (WBAN) applications. The antenna has a polarization perpendicular to the body-free-space interface, which is interesting in order to minimize the coupling into the body. Its structure comprises a modified mono-cone with a top-cross-plate and is coaxially fed through the ground plane. The higher frequency band |S11| performance is due to the mono-cone while the top-cross-plate is responsible for the lower frequency band. This plate also leads to a height reduction when compared to conventional mono-cone antennas. A comprehensive parametric study is done to provide design guidelines. Both frequency- and time-domain results have been measured and presented to validate the design. Results show that the antenna operates from 3.06 to beyond 12 GHz based on |S11| ≤ -10 dB, radiates omni-directionally in the H-plane, and has a radiation efficiency over 95%. The system-fidelity factor for UWB signals is adequate for pulse transmission. Finally, the influence of the human proximity on the antenna matching was tested. Results show that its impedance is nearly unchanged as compared to free-space.

A Novel Compact CPW-Fed Polarization Diversity Ultrawideband Antenna

A new compact coplanar-fed antenna suitable for polarization diversity in ultrawideband (UWB) applications is presented. The antenna consists of two identical monopoles that are printed on a low-loss substrate with 3 mm spacing and positioned perpendicular to each other. Both frequency- and time-domain results have been measured and presented to validate the design. Results show that the proposed antenna has not only ultra-wide bandwidth ( ~115% for port 1 and ~107% for port 2), but also good port isolation above 22 dB over the entire band of interest. Moreover, radiation patterns demonstrate good orthogonal polarization operation. Furthermore, the system fidelity factor is adequate for pulse transmission with averages of 85% and 75% for port 1 and 2, respectively. Finally, the envelope correlation coefficient (ρe) has been calculated to evaluate the diversity performance. Results indicate that ρe ≤ -20 dB across the ultra-wide bandwidth. These results show the suitability of the proposed antenna for future UWB diversity applications.

Radiation efficiency of a coplanar-fed ultra-wideband antenna

This paper presents a study on the radiation efficiency of Ultra-Wideband (UWB) antennas. Measurement were made using a cavity-based method with source stirring. A novel post-processing filtering technique of the measured data is introduced. The approach was tested on a coplanar-line-fed UWB monopole. The efficiency of this antenna was also computed with two commercial electromagnetic packages. Simulations and measurement agree to predict a radiation efficiency of over 95% through the 3.1-10.6 GHz operating bandwidth.

Diversity Gain Influenced by Polarization and Spatial Diversity Techniques in Ultrawideband

This paper presents an experimental investigation of diversity gain influenced by polarization and spatial diversity techniques in ultrawideband (UWB) radio technology. To this aim, two different coplanar-fed UWB diversity antennas having the same size and employing identical monopoles were taken and both effective and apparent diversity gain were measured in a reverberation chamber. To extract the diversity gain, three commonly used approaches to combine diversity (i.e., selection, equal gain, and maximal ratio) have been considered. Results showed that >1 dB (~ 26 %) improvement in diversity gain is obtained over most of the considered band for polarization diversity case as compared with spatial case, showing the usefulness of polarization diversity for future UWB diversity applications.

Novel Approach to the Measurement of Ultrawideband Antenna Efficiency

This letter presents a new cavity-based approach for measuring the radiation efficiency of ultrawideband (UWB) antennas. The procedure is based on a generalized Wheeler cap method that is only practical for narrowband measurements, and so this letter introduces an extension to UWB using source-stirring. The number of required measurements needed is relatively low and independent of the frequency span, therefore making the technique convenient for UWB. A complete post-processing procedure to automatically identify and remove erroneous radiation efficiency predictions is presented. The new method was tested with a coplanar-fed UWB monopole and compared to existing UWB efficiency cavity-based measurement methods. The results show good agreement with simulations and compare favorably to other UWB-centric methods, confirming the proposed approach as a good alternative to measure UWB antenna radiation efficiency.

Influence of dielectric loading on the fidelity factor of an Ultra Wideband monopole antenna

This paper reports a study of the influence on the fidelity factor due to loading an Ultra Wideband monopole with a pair of dielectric substrate pads. The fidelity factor was calculated, with and without loading, using the simulated transmitted and received pulses in three points in the azimuth plane. Results show that this dielectric loading technique does not cause significant pulse distortion. The fidelity factor is affected only by an average of 2.8% relative to the unloaded antennas in all situations considered. Two versions of the same antenna design were studied: coplanar- and microstrip-fed. It was found that the influence of feeding on the fidelity factor is also minimal.

Fidelity concepts used in UWB systems

Since its introduction the fidelity concept has been used to evaluate the time behavior of UWB antennas. However, fidelity has been employed with different meanings. This paper clarifies the differences between fidelity factor, system fidelity factor, and fidelity factor of the system. A recently developed UWB antenna has been taken as a representative one to illustrate the differences among these concepts.

Novel approach to the measurement of ultra wideband antenna efficiency

This letter presents a new cavity-based approach for measuring the radiation efficiency of ultrawideband (UWB) antennas. The procedure is based on a generalized Wheeler cap method that is only practical for narrowband measurements, and so this letter introduces an extension to UWB using source-stirring. The number of required measurements needed is relatively low and independent of the frequency span, therefore making the technique convenient for UWB. A complete post-processing procedure to automatically identify and remove erroneous radiation efficiency predictions is presented. The new method was tested with a coplanar-fed UWB monopole and compared to existing UWB efficiency cavity-based measurement methods. The results show good agreement with simulations and compare favorably to other UWB-centric methods, confirming the proposed approach as a good alternative to measure UWB antenna radiation efficiency.