Neda Khiabani

Measurement and Performance of Textile Antenna Efficiency on a Human Body in a Reverberation Chamber

With the advent of on-body communication research in recent years, there is a growing need for antenna developments that satisfy a wide criteria (one being minimal efficiency degradation) in order to be integrated successfully onto human subjects; one promising development is the textile antenna. In this paper we investigate the efficiency performance of some newly designed small sized textile antennas on live human subjects using a reverberation chamber. First, we show that the material selection of these textile antennas can have a crucial effect on the on-body frequency detuning and efficiency levels, as via a comparison we determine that a lossier textile antenna in free space can actually outperform a higher free space efficient textile antenna when placed on-body. This has a profound impact on the material design choices for these small sized antennas. Second, we investigate the performance effects under bent conditions and finally we show that the overall performance of the textile antenna can be mitigated somewhat by variations in on-body distances from the human subject. It is revealed that in some cases a small 20 mm distance from the body is sufficient for the radiation efficiency to approach the free space levels. Theoretical, simulated and experimental evidence is presented to verify the conclusions.

Theoretical Modeling of a Photoconductive Antenna in a Terahertz Pulsed System

The most common device for terahertz (THz) wave generation and detection is a THz photoconductive antenna. This paper examines the factors which affect the radiated power and optical-to-THz power conversion efficiency of the antenna. A novel equivalent circuit model using lumped elements is developed for analyzing the performance of these antennas. In this model, whilst keeping the simplicity of the lumped element approach, the underlying physical behavior of the device is taken into account when calculating the circuit elements. Based on the model, the influence of various parameters on the optical-to-THz power conversion efficiency and radiated power is then investigated. The simulated results agree well with published measured results. The model predicts that an increase in the laser power and/or bias voltage, and a reduction in reflections from the air-substrate can improve the optical-to-THz power conversion efficiency of the device. This novel model is very useful for both designing a THz antenna and tuning a THz system to achieve maximized optical-to-THz power conversion efficiency and THz radiated power.

Terahertz photoconductive antenna efficiency

Terahertz (THz) photoconductive antennas are the most common device for the generation and detection of THz waves and are very different from conventional RF/microwave antennas. One of the major problems of the current photoconductive antennas is that the antenna efficiency is very low, thus it is difficult to obtain high power THz waves. In this paper a comparison of the conventional and photoconductive antennas is made and various parameters and aspects of photoconductive antennas are examined with the aim to maximise its efficiency factor. A new antenna efficiency expression is derived which clearly shows how the efficiency is linked to various parameters. An example is used to demonstrate why the total efficiency is very low and what the major limiting factors are. Recommendations for how to increase the efficiency are presented.

Wideband antenna efficiency measurements

Over the past 10 years or so, many broadband and ultra-wideband antennas have been developed for broadband high data rate mobile communications. It is realized how to make efficient and accurate measurements for such antennas could be a challenging issue. In this paper, we examine the antenna efficiency measurement techniques which have been used up to now, and identify their advantages and disadvantages for broadband measurements. It is shown that the classic Wheeler cap method, which is the best for electrically small antennas, has a lot of attractive features even for broadband antenna measurements in terms of the cost and efficiency although the newly proposed reverberation chamber method may become a good alternative method. A “source stirred” method is proposed as a new approach for the broadband antenna efficiency measurement and its major advantages are cheap, convenient, and efficient. Both the experimental and numerical results are provided. It is shown that the loss due to the imperfection of the cavity can cause a serious problem for obtaining an accurate result. How to eliminate the effects of the cavity loss and improve the measurement accuracy for this method is an issue to be resolved.

Time variant source resistance in the THz photoconductive antenna

Terahertz (THz) photoconductive antennas are the most common devices for the generation and detection of THz waves. One of the main problems of the current photoconductive antennas as the emitter is that the optical-THz conversion efficiency is very low, thus it is difficult to obtain high power THz radiation. Impedance mismatching between the antenna and the photoconductive material (which acts as the source impedance) is one of the primary factors on this low conversion efficiency. In order to assess this issue, proper evaluation of the resistance of the photoconductive material is required. In this paper, the time dependant conductivity of the photoconductive material based on pulsed system is first calculated. Then, through this conductivity, the source conductance (resistance) in THz photoconductive antenna is determined which illustrates influences of different parameters of laser pulses, photoconductive material properties and THz antenna. This new formula can aid in a better theoretical assessment as part of total optical-THz conversion efficiency calculation as the source conductance (resistance) can be more accurately derived.

Repeatability and uncertainty evaluations of on-body textile antenna efficiency measurements in a Reverberation Chamber

For the validation of body worn antennas, the human body should be taken into account as the performance of such antennas is likely to differ from their free space equivalents. How best to determine the human body effects on antenna performance is a pertinent point in which many questions still remain. In this paper we consider the efficiency of some newly developed textile antennas for on-body applications and directly establish that a Reverberation Chamber (RC) is an ideal solution for these types of measurements in conjunction with human beings. It is shown that a RC can provide a very repeatable series of measurements in conjunction with the human (in the order of 3%), proving that inevitable (slight) human movements do not affect the measurements accuracy, and further, that the on-body efficiency can be assessed with a low uncertainty (~ 0.22 dB) if the correct procedure is followed. The work proves that the RC facility is beneficial to any institution who seeks this knowledge for their designs.

A Novel Sub-THz Photomixer With Nano-Trapezoidal Electrodes

A new sub-terahertz (THz) photomixer with nano-trapezoidal fingers is analyzed, designed, fabricated, and measured. The results are compared with conventional THz photomixers without fingers (or bare gap) and with rectangular fingers. The new design is employed as an emitter as well as a detector and much improved THz power and signal-to-noise ratio (SNR) are obtained. As compared to the antenna with the bare gap and the antenna with rectangular finger tips under the same condition for the optical power and bias voltage, the new photomixer shows seven and two times power enhancement in the emission side, respectively, while in the detection side, it gives a better SNR of 15 and 10 dB, respectively.

Effects of substrate on the performance of photoconductive THz antennas

Photoconductive antennas have been widely used to generate and detect THz waves in recent years. Because the thickness of the substrate is usually larger than the wavelength of THz waves, surface/substrate modes may be generated and the effect of the substrate cannot be ignored - this has not been properly investigated. This paper is therefore to investigate this problem using a numerical simulation tool. The radiation pattern and power of photoconductive antennas with different sizes of substrates are obtained. The results show that the radiated power is sensitive to the dimensions of the substrate: most of the radiated power is radiated towards the substrate side rather than the free space, and a thin substrate is preferred to maximise the radiated power and control the radiation direction.

THz photoconductive antennas in pulsed systems and CW systems

In designing terahertz (THz) antennas with high output power and/or high optical-THz conversion efficiency, the photoconductor property is an important consideration. According to the type of the optical excitation, THz antennas can be divided into two categories: THz photoconductive antennas in a pulsed system and THz photomixing antennas in a Continuous-Wave (CW) system. How different excitation types can affect the performance of the antenna is an important issue in THz systems. Thus, the aim of this paper is to investigate and compare THz photoconductive antennas and THz photomixing antennas from an important design parameter point of view; namely the conductance of the photoconductive material substrate which acts as the source for a PC antenna. Through numerical analyses of example systems, it is shown that source resistance of a THz antenna in a CW system is greater than those in a pulsed system. Further, an important practical system setting, the dependency of alignment of two beams in CW systems on antenna gap size is also investigated. It is illustrated that mixing efficiency for large gap antennas is more sensitive to angle between two beams.

Photoconductive THz antennas

Terahertz (THz) antennas based upon photoconduction techniques in THz pulsed and continuous-wave (CW) systems are introduced and compared. It is addressed that the differences in the excitation of these antennas lead to different design requirements. In both antenna types, optical-to-electrical conversion efficiency is very low - this means that employing techniques which can enhance the generated THz photocurrent in the antenna gap is necessary. Comparison of impedance matching of these antennas demonstrates that matching efficiency is a much more severe problem in a CW system than in a pulsed system. Regarding the coupling of the THz wave to the air, both antenna types suffer from low radiation efficiency due to the losses in photoconductive substrate. Considering these analyses, a simplified block diagram on how different parameters can improve the radiated THz power and efficiency is presented and finally a new THz photomixer solution with improved performance is demonstrated.