Absolute calibration of an LBA dipole

© Cristina-Maria Cordun

DISTURB is a soon-to-be-built interferometer that will monitor the Sun between 3 MHz and 3 GHz and give warnings when the solar activity exceeds a specific threshold. This warning system is required by the Dutch Ministry of Defense, to check whether the Sun is causing possible interferences in communications, or whether they have a different origin. The DISTURB prototype will consist of 9 LBA antennas (3 MHz - 100MHz) in a 3x3 compact square configuration, so it will have a field of view close to the full observable sky. Thus, using a calibrator source is not an option, and absolute calibration of the array is required.

To do an absolute calibration, we start by analyzing how the signal passes through all the components of an LBA dipole, such as the Antenna wires, Low-Noise Amplifier, Coax 9 cables, Receiver Unit, and Analog-to-Digital Converter. We note that the model that we made has a relative error of 20%, which is within the system's requirements. However, 90% of the analyzed dipoles do not have clean spectra and present a lower gain for the resonance frequency and a bump at around 60 MHz. We believe that this feature originates in the head of the LBA antenna, but we do not have a clear explanation for its origin, and thus, further measurements are required. Without understanding it, we cannot model it, and we cannot do an absolute calibration of the array.