Achim Enders

A numerical study of photoconductive dipole antennas: the real emission frequency and an improved antenna design

In this paper we show that the real emission frequency of a photoconductive dipole antenna operating in conjunction with photomixers is not its natural resonant frequency, but a frequency where the antennas input resistance is the highest and the corresponding mismatch with the source is the smallest. We also introduce a new kind of antenna that offers a much higher input resistance to the photomixer and hence enhances the efficiency of the continuous-wave system.

Alignment and illumination issues in scaled THz RCS measurements

Radar cross section (RCS) measurements are widely employed for the target identification of objects such as aircraft. The RCS of large, complex geometries is usually determined in a well defined, scaled experimental setup. In such a setup, the measuring frequency equates the frequency of interest multiplied by the scaling factor of the model. For high scaling factors, frequencies well above 100 GHz result so that conventional microwave equipment reaches its technological limits. In this paper, we discuss the use of a fiber coupled terahertz time domain spectroscopy system for angle dependent RCS measurements. Several measurements on simple objects like metal plates with corresponding simulations are performed. We also investigate how angular misalignment and displacement of the scatterer effects the measurement results. Moreover, we discuss the influence of non planar incident wavefronts and compare experimentally obtained data to simulations using an enhanced physical optics model.

Scaled Measurements of Instrument-Landing-System Disturbances Due to Large Taxiing Aircraft

It is commonly known that the directional pattern of the instrument landing system (ILS)-localizer antenna system, which constitutes the landing-course information for approaching aircraft, can be disturbed by reflections from large aircraft on the ground. Therefore, airport operators are interested in knowing configurations of landed and taxiing aircraft that lead to such ILS disturbance scenarios. Measurements on a real airport for investigating such scenarios are very cost-intensive and not sufficiently possible due to availability reasons. Therefore, a new scaled measurement setup is presented. Scaling down an aircraft by the factor of 144 requires a scaled ILS-localizer operating at a frequency of nearly 16 GHz, the development and realization of which is presented in this paper. Using this scaled ILS-localizer, measurements with an Airbus A380 and a Boeing B747 are conducted showing the feasibility of the scaled approach. Several configurations are shown where those aircraft lead to interferences of the ILS, exceeding allowed tolerances. A comparison of the reflection behavior of both aircraft shows only slight differences. Additionally, this paper presents the derivation of the bistatic radar cross section of both aircraft to be applicable to scenarios with arbitrary ILS patterns.

Reliable transfer function measurements of PED excitations in aircrafts

The determination of reliable transfer function in a complex system represents major measuring problems. In this paper a new method is described which allows transfer function measurements under variable boundary conditions without using calibrated field sources. After a brief description of the new method, various results of measurements in different aircrafts are depicted. The influence of the aircraft size and the cabin interior furnishing will be presented.

Feasibility study on scaled bistatic RCS measurements of aircraft in W-band to investigate misguidance by the Instrument-Landing-System

In this paper a measurement technique is presented, which allows the derivation of scaled bistatic radar cross sections of objects at ILS frequencies with a very simple measurement setup and an easy post processing procedure. Its functionality is proven with simple reference objects, which can be calculated by common simulation tools. First sets of bistatic RCS are presented for the large aircraft A380 and B747. With a wider span of the observation angle and a more sophisticated measurement setup, enhancing RCS measurements within only one plane to a three dimensional observation area, it becomes feasible to measure the bistatic RCS profile, that is necessary to investigate ILS disturbances for a complete landing approach at decreasing height of aircraft. Additionally, the use of focusing lenses can improve the measurement dynamics and accuracy and is of interest for further investigations.

UWB aircraft transfer function measurements in the frequency range from 2 to 8 GHz

In this paper the determination of transfer functions from inside an Airbus A319 to outside the aircraft in a frequency range from 2 GHz to 8 GHz is presented. Two different aspects have been investigated. The first is the determination of the fuselage attenuation of the aircraft in order to see, if existing airport communications and radar systems could be disturbed by onboard installed equipment. The second concerns possible coupling from inside the aircraft to different relevant aircraft navigation and communication antennas which are mounted outside the aircraft along the fuselage. The different results of these two measurements are presented and possible interferences with existing systems are discussed.

Representation of absolute electric fieldstrength by superposition of radiated and guided waves

A commonly used method of calibrating sensors for high frequency electric fields is based on the use of closed TEM-waveguides. But this method does not allow the determination of errors produced by any coupling between the sensor and the waveguide. A new method of electric field sensor calibration is presented which allows the determination of coupling errors. This is done by a superposition of radiated and guided waves and this error can be quantified by measurements. Consequently a higher accuracy in the representation of electric fieldstrength and calibration of sensors and antennas can be obtained.

Contactless calibration of loop antennas in comparison to traditional methods

Magnetic loop antennas are commonly used for investigations on magnetic fields in a frequency range up to 30 MHz. As for all antennas the antenna factor is the result of a calibration procedure. Commonly used calibration routines like the standard field, standard antenna and three antenna method will be compared to a newer approach called impedance method, first introduced by Ishii [2]. Finally a new approach for contactless measurement of the antenna factor, the so-called in-situ impedance method, will be presented as well as a comparison of the resulting antenna factors of all calibration routines.

A Critique on Traceability in Site Validation Measurements

Site validation measurement procedures are specified in ANSI C63.4 and ANSI C63.5 in the US, and CISPR 16 internationally. The metrics for site performance for both standards are the site attenuation deviation (DeltaSA) or normalized site attenuation deviation (DeltaNSA). However, DeltaSA or DeltaNSA measured according to these standards for the same site-under-test are non-unique, leading to ambiguities in the reference for traceability. Because the antennas used for site-attenuation measurements are not specified, the variety of allowed antennas have a non-negligible effect on DeltaSAs. The main variable antenna characteristics which directly influence the apparent site response are differing antenna balun impedances, different near-field coupling effects for different antenna elements, and differing far-field radiation patterns. Results are presented for these influencing factors to show that several dB variations in DeltaSAs can be obtained for the same site. Consequently, it is suggested that standards development organizations (re-)consider specifying and adopting some type(s) of reference antenna(s).

EMC analysis including receiver characteristics - pantograph arcing and the instrument landing system

This contribution presents a sophisticated compliance study, whether emissions of transient sparks occurring between pantographs of rails and overhead lines can interfere with receivers of the instrument-landing-systems (ILS) onboard aircraft. Though the investigated interference scenario actually is quite specific and arose in the context of the enhancement of the airport Frankfurt in Germany it comprises several fields of electromagnetic compatibility where yet insufficient approaches exist. In particular, absolute field strength measurements of transient signals in a non-ideal measurement railway environment are conducted. Another new aspect in this study is, that the individual receiver characteristics are taken into account, e.g. non-linearity and band pass behavior, where usually only simple continuous wave signals in an interference path loss model are considered. Consequently, a setup is presented, where an individual measured interference signal from a real environment is synthesized and fed into a real EMC sink. This procedure however, could be generalized to any other receiver or interference source.