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9th of September 2021
Low frequency tests at Parkes
by Ron Beresford
Trials were undertaken recently at Parkes with a small, potentially portable, VHF antenna for pulsar detection to explore the potential for global timing event synchronization. Experiments were conducted east of the Parkes 64m dish using a single Murchison Widefield Array (V3) antenna as the receptor, as used in 4x4 array tiles at the MRO. The antenna is a broadband VHF 70-300MHz dual polarization bowtie dipole structure with a span of 74cm. When positioned 10cm above a wire mesh ground plane the antenna field pattern has a broad lobe maximized skyward with a drop in gain below 30 degrees elevation, improving rejection to terrestrial radio interference. The dipole elements connect to a balanced HEMT FET Low Noise Amplifier design located in the plastic hub of the antenna. The LNA noise contribution is less than the galactic sky noise contribution with a mid-band gain of 19dB. Using a single polarization output, we attenuate (>20dB) unwanted FM radio signals with a 88-108MHz band-stop filter then directly modulate a DFB laser diode RFoF single-mode fiber span over 500m to the DFB4 1GS/s digitizer located in the tower. The antenna and fiber optics in the paddock are powered by a small 12V7AHr battery. No pulsar detections were made but a second attempt with an improved signal chain dynamic range and different DFB4 settings is planned. The weather during the experiment could only be described as atrocious, with heavy snow at Orange on the return to Marsfield!

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