Wideband EM architecture of buildings : a six - to - one dual - passband filter for indoor wireless environments

The application of closely packed elements to obtain wideband isolation at mobile phone frequency bands while allowing transparency at the emergency TETRA and the Bluetooth bands is proposed. The intention is to modify the EM architecture of buildings over a very wide frequency range, permitting access at these two very separated bands. In particular, we have in mind buildings with restricted mobile phone usage such as public libraries, theatres and secure offices. Experimentally validating the design required scaling the operating frequencies by a factor of 10. Introduction: With the recent and ever increasing number of communication applications using the frequency spectrum between about 300 MHz and 5 GHz, owing to the favourable balance there between range and bandwidth, there is a need for the capability to establish “electromagnetic zones” in the built environment, between which communications, or the interference level, can be managed [1]. As a result, there is an interest in Frequency Selective Surfaces (FSS) and their use in buildings to establish the necessary electromagnetic architecture. A major challenge for FSS design for such applications at these wavelengths are the high proportional frequency ranges that may have to be covered, the factor of 6 for example between the 400MHz emergency service band and the 2.4GHz Bluetooth band. The design of suitable FSS for a specific application is similar to that of antennas: while a number of parameters can be varied (eg. the spacing between elements, type and amount of dielectric in hybrid or multi-layer FSS), the most significant difficulty is generally associated with the generation of elements with geometries that would fit the specific application at hand. There is no obvious general methodology to choose one particular element configuration suited to dual-band filter applications at selected frequencies. FSS made of the simplest patch elements are generally reflective in a single frequency band (and, to a lesser extent, at harmonic frequencies) and suffer from a relatively slow roll-offs, with a typical edge ratio of 1:3 between the frequencies corresponding to the -0.5dB edges of the low frequency transmission band and the reflection band. An extreme example of such behaviour is that of superdense FSS [2] made of tightly packed elements disposed on a skewed lattice, which give reflection bands which can greatly exceed an octave in width, but with very slow roll-off rates. Using the Fabry-Perot technique described in [3] and [4], faster roll-offs can be produced, by correctly spacing two layers of the densely packed structure. As a guide to a way forwards to meet the challenge posed by the need for wideband operation, in this letter we describe a singly polarised FSS that is transparent in both the TETRA (around 400MHz) band and in the unlicensed wireless land band (2.4-2.5GHz), and mostly opaque at frequencies in between. Design and results: Floquet modal software and the frequency domain solver of CST Microwave studio were used in the computational modelling. The simulated plane wave response for such an FSS is shown in Fig. 2. The dimensions of the structure as given in Fig.1 were: L = 168mm, W = 8mm, Px = 172mm and Py = 20mm. The space S between the two layers was 48mm which could be implemented in a thin partition wall in a building, or further tuned to thicker wall structures. In the simulations there are two clear passbands, one at around 400MHz and the other at 2.4GHz. At the lower band the resulting widths between the -1dB points are 40MHz and 130MHz at the higher band, while the -5dB bandwidths are about 130MHz and 180 MHz respectively. W