Microstrip-coupled Transition Edge Sensors (TESs) are important because they can be combined with waveguide-horn
technology to produce sensitive bolometric detectors with well-defined, single-mode beam patterns and
polarisation characteristics. They also allow superconducting RF filters to be included on the detector chips.
Our own design of TES uses a finline taper to transform between waveguide and superconducting Nb microstrip.
The microstrip transports the signal to a matched Au-Cu resistor, which is deposited on a thermally isolated SiN
membrane. The dissipated RF power causes the resistance of a Mo-Cu TES bilayer to increase, and the resulting
reduction in bias current is read out by a SQUID. We have fabricated TES bilayers with critical temperatures
of 400 to 600mK, and deduced dark NEPs as low as 3x10<sup>-17</sup>W/√Hz at 150GHz. In this paper we describe a
number of experiments that were carried out in order to investigate the electrothermal behaviour of microstrip-coupled
TESs. We show that the electrothermal behaviour of microstrip-coupled TESs can be as good as that
of free-space TESs, and therefore that they are suitable for high-performance astronomical applications.
In this paper we present a novel design of an antenna coupled TES
direct detector for high performance applications. In particular, the
design of the detector has been optimised to be suitable for the
measurement of the weak B-mode signal in the CMB polarization. An
important feature of this design is that it employs corrugated horn
antennas for coupling the astronomical signal to the detector. This
allows us to feed the telescope with a well collimated beam with low
sidelobes and cross polarization. The paper contains simulations
demonstrating the suitability of individual electromagnetic components
to be used in the instrument.