HF characteristic impedance
(mechanical dimensions of coaxial impedance standards)

For the determination of the characteristic impedance of coaxial HF reference impedance standards ("air lines"), the diameter of both inner and outer conductor must be exactly known. Additionally to use air lines as HF phase standards, their length must be precisely determined.

At PTB, these diameters are determined by use of a commercial diameter measuring setup ("air gauge") with analog voltage output, by comparing the diameter of the device under test with the diameter of three diameter standards (precise cylindrical bolts or rings) which are traced back to PTB length standards. To increase the measurement resolution 0.1 µm, the voltage value is A/D-converted with high resolution and acquired by a PC for data procedding. Simultaneously, the position of the device under test is measured by a commercial length measuring system with digital output to the PC.

A commercial length measuring system with digital display serves to determine the length dimensions of the air lines. The length of the air line inner and outer conductors is compared with the length of gauge blocks which are traced back to PTB length standards.

With these measuring setups, coaxial HF impedance standards in the coaxial line systems 7 mm (connectors PC-7 und N), 3.5 mm (PC-3.5), 2.92 mm (PC-2.92) and 2.4 mm (PC-2.4) can be mechanically characterised.

The PTB "air gauge" setup for measuring cross dimensions of coaxial air line impedance standards