to the amplifier chain is connected directly to the balun on the antenna (with no intermediate transmission cable), as shown in Figure \[f\_edges\_photos\]. While absolute calibration is limited in this configuration by the effect of the unknown phases of the reflections on the measured spectrum, the compact size of the antenna and the small signal path delays result in a smooth spectral response.

The amplifier module is connected to the analog-to-digital conversion module by three low-loss coaxial transmission cables. The cables provide power, switching control, and signal transmission, respectively. Common-mode current on these cables (i.e. current that is on the outer surface of the shielding in the coaxial cable, or current that is unidirectional on both the central conductor and inner surface of the shielding) is also capable of producing reflections and additional sinusoidal ripples in the measured spectrum. The ferrite core balun used between the antenna and amplifiers allows common-mode current of approximately 10% of the differential mode. Although most of this current is transferred to the ground screen by direct contact between the amplifier module casing and the ground screen, some current persists and leaks through the casing of the amplifier module and onto the shielding of the 