  This paper explores the behaviour of the ubiquitous twisted pairs at high
frequencies and wideband excitation of twisted pairs up to 12GHz. Although
there is a large quantity of papers on twisted pairs, the papers in the
literature mostly focus on the sub-1GHz spectrum, where the current digital
subscriber line technologies operate. 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. Towards this objective, we derive analytical
expression for the electromagnetic fields and characteristic equation of
twisted pairs. With these derivations 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 theory through
measurements, we designed a microstrip balun in order to excite the
differential mode on the twisted pairs. Unlike off-the-shelf devices, this
balun has a nearly linear transmission curve across 1-12GHz. This linearity
allows us to detect the radiation which would not have been possible with an
off-the-shelf device. At the end, we demonstrate that the standard twisted
pairs used in the UK can be used up to 5GHz carrier frequency without any
radiation effect and this upper-bound can be moved to higher frequencies by
decreasing the twist lengths.
