Standard antennas designed with electro-optic modulators and optical fiber linkage

We describe the design of standard reference antennas that use an electrooptic transducer together with optical-fiber linkage to preserve the amplitude and phase information of the received signal. They will be used over a range from 10 MHz to 2 GHz at our open area test site in order to reduce measurement uncertainties attributable to the ambient electromagnetic spectrum. The transducer consists of an optical-fiber directional coupler with unbalanced legs and LiNbO 3 phase modulators. The output signal is processed to servocontrol the wavelength of the laser and maintain the optimal optical bias point in changing environments. The complementary rf signals are subtracted to provide common mode noise rejection. The output signal is modeled to determine the design and operating parameters required for good repeatability and accuracy. The results show that spurious reflections in the modulator legs need to be < -50 dB in order to obtain the desired stability.