A 3.5mm twin line coaxial calorimeter is the first microwave power calorimeter in this line size to be constructed at the National Physical Laboratory. The instrument consists of two terminated coaxial lines with a length of high thermal resistance line between an APC-3.5mm input connector and a tractorial load. The principle of operation is to measure RF power dissipated in one of the loads in terms of DC power dissipated in the other load. A temperature change due to the dissipation of power is detected by platinum resistance thermometers (PRTs) fixed on the coaxial line in the region of the load. Two PRT sensors connected in series on both sides of each coaxial line are incorporated in an AC bridge and detection circuit. Initially the output from the detection circuit is zero but once power is applied to the RF line the bridge goes out of balance. This signal is then used to control a feedback voltage to the DC line until the bridge output is returned to zero. The final DC feedback voltage that balances the bridge is then used to determine the RF power to the calorimeter. The instrument operates over the frequency range DC to 26.5GHz at power levels 0.1--10mW. The calculated effective efficiency for the calorimeter ranges from 102.4/100 at 0.1 GHz to 99.3/100 at 26.5GHz. The VSWR of the RF line ranges from 1.006 at 1 GHz to 1.06 at 26.5GHz with a maximum of 1.12 at 25.1 GHz.
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