Arrangement to measure the deflection of a blade of a wind turbine, - wherein a transmitter is arranged at the tip end of the blade and wherein the transmitter is arranged within the blade, - wherein a receiver is arranged at the root end of the blade and wherein the receiver is arranged within the blade, - wherein a monitoring system is arranged at the root end of the blade and within the blade and close to the receiver, - wherein the monitoring system is arranged to generate a monitoring signal, - wherein the monitoring system is connected with the transmitter by an optical fiber, which is part of an optical communication system, so that the monitoring signal is transferred from the monitoring system to the transmitter, - wherein the optical communication system is arranged to transfer the monitoring signal as an optical modulated signal from the monitoring system to the transmitter, - wherein the transmitter, the receiver and the monitoring system are part of an active radar-system, - wherein the transmitter is prepared for a radio-frequency-transmission of the monitoring signal to the receiver, - wherein the receiver is prepared to receive the monitoring signal, which is transmitted as radio-frequency-signal by the transmitter, - wherein the monitoring system is connected by a cable-bound communication line with the receiver, so that the monitoring signal is transferred from the receiver to the monitoring system, and - wherein the monitoring system is arranged to determine the deflection of the blade based on angle-of-arrival data of the monitoring signal transferred from the receiver to the monitoring system. - wherein a transmitter is arranged at the tip end of the blade and wherein the transmitter is arranged within the blade, - wherein a receiver is arranged at the root end of the blade and wherein the receiver is arranged within the blade, - wherein a monitoring system is arranged at the root end of the blade and within the blade and close to the receiver, - wherein the monitoring system is arranged to generate a monitoring signal, - wherein the monitoring system is connected with the transmitter by an optical fiber, which is part of an optical communication system, so that the monitoring signal is transferred from the monitoring system to the transmitter, - wherein the optical communication system is arranged to transfer the monitoring signal as an optical modulated signal from the monitoring system to the transmitter, - wherein the transmitter, the receiver and the monitoring system are part of an active radar-system, - wherein the transmitter is prepared for a radio-frequency-transmission of the monitoring signal to the receiver, - wherein the receiver is prepared to receive the monitoring signal, which is transmitted as radio-frequency-signal by the transmitter, - wherein the monitoring system is connected by a cable-bound communication line with the receiver, so that the monitoring signal is transferred from the receiver to the monitoring system, and - wherein the monitoring system is arranged to determine the deflection of the blade based on angle-of-arrival data of the monitoring signal transferred from the receiver to the monitoring system.