Antenna calibration, critical field strength measurements and the validation of antenna and field strength test sites require special measure to establish test reference standards.
The Rohde & Schwarz HZ-12/13 sets include both built-in attenuators on both the balanced and unbalanced sides of a balun. The attenuators allow broadband stabilisation matching and define the total power attenuation between the dipole elements and the 50W connector.
As a result, a pair of dipoles coupled head to head will always show exactly calibrated attenuation. The HZ-12 is for 30 to 300 MHz and the HZ-13 is for 300 MHz to 1 GHz.