Non dispersive, UWB, leaky lens radiated links

Abstract

For most radar and communication applications that require a wide frequency bandwidth the linearity of the phase of the antenna transfer function is of great importance. If the antenna is dispersive (i.e. the phase does not vary linearly with frequency) compensations at system level must be included, such as sub-divisions of the bands, digital signal processing, inclusions of filters, etc. [1]. The most linear phase responses are associated to antennas derived from TEM-like horns. For instance the Vivaldi antenna, which is a low cost implementation of TEM horns, is a very successful example, see [2]. However, it is well known that its phase center moves along the longitudinal axis. This implies that the phase linearity is only limited to the broadside direction. The stability of the phase center as a function of the frequency is only a necessary condition for the linearity of the phase in all observation directions. The most successful wideband antennas that present fixed phase centers are often derived from spirals or from the log periodic concept. As an example, the Eleven antenna [3] has recently been proposed for decade Bandwidth (BW) reflector's excitation. However, as shown in [2] the log periodic concept is intrinsically associated to a non linear phase variation of the transfer function. The same applies for spirals. Overall, it appears that no antennas are presently known capable of radiating non dispersively over the entire main beam and over very large bandwidths. When the attention is shifted from a single antenna to a radiated link, the linearity of the phase of the antenna transfer function is not sufficient for the reception of a non distorted replica of the input impulse. Also the amplitude of the spectrum of the transmitted impulse must be preserved. The planarly fed Leaky Lens antenna, described in paper [4], appears to be well performing with respect to both dispersivity and amplitude distortion. It is an enhanced version of the antennas presented in [5] and [6]. In [4], the concept and design of the antenna was presented. In this paper, corresponding hardware prototypes are described and experimentally characterized. To demonstrate the ultra-wide bandwidth performance while retaining manageable lens dimensions, 12 GHz has been chosen as lowest operating frequency. The planar feed has been realized in standard printed circuit board (PCB) technology. Both types of antennas discussed in [4], with and without matching layers, have been manufactured. The matching layers reduce the reflection at the dielectric/air interface and this is therefore the preferred implementation of the Leaky Lens antenna design. However, for applications at sub-mm wave frequencies the realization of the matching layers poses serious technological challenges. Thus, the performance degradation of the planarly fed Leaky Lens antenna without the matching layers has also been quantified. A link between two prototypes has been experimentally characterized. From the measured S-parameters, the pulse preserving property of the antenna link demonstrates the phase linearity and phase center stability of each antenna as well as the spectral amplitude preservation of the link. While the phase preserving properties arise from the Enhanced Leaky Lens radiation mechanism [4], the spectral amplitude preservation is strictly related to the specific geometry chosen for the dielectric lens.