At the network access layer, optical fibre deployment continues at pace but copper cables containing twisted pairs will remain for some time and face an increasing bandwidth and data rate demand. Surface waves have been proposed to address these requirements. This paper reports and investigates the existence of stop bands reaching over 50 dB insertion loss on 1 metre long, typical final drop cables under surface wave excitation at a few GHz. Coupled mode analysis shows that lack of helical symmetry enables the formation of a stop band in systems containing twisted pairs. A representative core model containing a single twisted pair alongside a straight wire is thoroughly studied. Numerical simulations and measurements confirm the crucial dependence of the stop band frequency on the twist rate of the twisted pair. Further investigation into the role of the dielectric coating and the distance of the straight wire are performed as well. Finally, in systems with multiple twisted pairs, we find that the twist rate associated with any pair can create a stop band effectively limiting surface wave propagation. Thus, careful design and deployment strategies are required for use of surface waves on legacy copper networks.
This paper explores the behaviour of the ubiquitous twisted pairs at high frequencies and wideband excitation of twisted pairs up to 12 GHz. Higher carrier frequencies on twisted pairs can enable the data rates required by the future communication networks; hence, the existing copper infrastructure can be utilised on the last mile complementing the fibre networks. In this paper, we show a fundamental limit on the operating frequency of twisted pairs beyond which twisted pairs start to radiate and behave like an antenna. To validate our theoretical derivations through measurements, we designed a microstrip balun to excite the differential mode on the twisted pairs. At the end, we demonstrate that the standard twisted pairs used in the UK can be used up to 5 GHz carrier frequency without any radiation effect and this upper-bound can be moved to higher frequencies by decreasing the twist lengths.
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