UWB antenna measurement methods

The methods for measuring antenna transfer functions with a network analyzer presented in the following use typically available measurement equipment for far-field antenna measurements with enhanced calibration procedures. The latter are necessary in order to obtain stable and consistent phase information for the transfer function in co- and cross-polarization. These methods complement the standardized antenna measurements known from [68]. A validation of the methods is presented with the prediction of a time domain impulse transmission, which is measured independently with a fast-pulse generator and an oscilloscope.

A prerequisite for the measurements is a stable measurement setup inside an anechoic chamber, which provides low reflections from the walls within the frequency range of operation. If multiple reflections cannot be avoided, the possibility of time gating for valid signals of the additional delay – due to the path lengths of wall or floor reflected signals – needs careful consideration. If the expected τr or even τFWHM overlap with the multiple reflections, the impulse response hAUT of the antenna under test (AUT) can no longer be measured accurately. Furthermore, far-field conditions are necessary in order to obtain a distance-independent impulse response estimation hAUT. The frequency domain far-field criterion r > (2D2)/λ (according to [24]) then applies for the highest frequency, i.e. the smallest wavelength.

For many scientists and engineers working in ultra-wideband technology, it seems that the idea of using signals with such a wide instantaneous bandwidth was spread by the US FCC with the accreditation of the frequency band from 3.1 to 10.6 GHz. But, if we look back in history, we find that even the first man-made electromagnetic waves were generated by sparks. Especially famous for electromagnetic research was Heinrich Hertz who, in the 1880s, verified the speed of propagation of electromagnetic waves, their polarization and interaction with objects, and the correct description of these waves by Maxwell's equations at our university in Karlsruhe, Germany. Before this time, electromagnetic waves could only be generated by the aforementioned sparks and were thus ultra-wideband.

Ultra-wideband was banned in the 1920s because it occupied too great a portion of the spectrum and from this point was primarily limited to military applications. This was until 1992 when Leopold Felsen, Lawrence Carin, and Henry Bertoni organized a conference on ultra-wideband, short-pulse electromagnetics in Brooklyn. Our institution, the Institut für Höchstfrequenztechnik und Elektronik (now the Institut für Hochfrequenztechnik und Elektronik) had the privilege of participating in this first conference on ultra-wideband. The topics at the conference were so fascinating that we decided to step into this area. The first research topics were in ground penetration radar, with the idea of detecting anti-personnel mines.

A well-known method of increasing the gain and lower the half-power beamwidth of the radiation pattern is to replace the single radiator by an antenna array. Its application in communications is interesting if, for example, a point-to-point connection is to be established. It is of special interest in MIMO systems, where a channel capacity might be increased if multiple radiators, either on the transmit or the receive side (or both) are used [75]. In radar systems an application of arrays is more common in order to achieve lower half-power beamwidths, which in general are used to increase the angular resolution.

In this chapter, specific design issues of UWB antenna arrays are described. According to the methods previously described, the frequency and time domain models are explained and used for the practical array design. In the second part of the chapter an extension of the monopulse technique to UWB systems is described, based on the array theory.

Array factor in UWB systems

The resulting array radiation pattern depends on the following parameters:

1. • number of array elements N

2. • distance between the elements d

3. • frequency f

4. • excitation coefficients – amplitude and phase

5. • radiation pattern of a single array element EF(f, ψ).