The New Arecibo Ionospheric Modification HF Facility Dual Array Cassegrain Antenna – History and Design

A new HF facility has been built at Arecibo Observatory, Puerto Rico, that has replaced the prior Islote heater that was destroyed by Hurricane Georges in 1998. It was decided to use the 1000 foot dish for this new heater antenna instead of rebuilding the previous installation. This will make it possible to have all research activities with ionospheric modification including the 430 MHz incoherent scatter radar (ISR) to be located at the Observatory. This will be perfect to provide the ability to study the upper atmosphere, study plasma effects, and other future experiments. Historically, ionospheric modification has been carried out before at Arecibo using originally a Yagi and later a crossed-log periodic antenna hanging from the platform. These both had logistic and electrical arcing problems, and that was what led to the construction of the Islote facility on the north coast of Puerto Rico. The Islote facility also had logistic and arcing problems from both the wires in the antenna and the wire cage pseudo-coaxial transmission lines. The transmission lines were upgraded, and this improved performance and reliability greatly just before the hurricane destroyed the facility. The first feasible concept to be considered for the current design was a dual-band crossed-Yagi that would hang with cables from pulleys and winches on the three support towers. The total power for each polarization would then travel up a 4-wire open transmission line from below. A combining and phasing system design was formulated for the six 100 kilowatt transmitters in this concept. It was later decided to use another design based on a Cassegrain concept of a phased-array at the bottom of the dish feeding energy to a sub-reflector mesh hanging from cables and winches from the three support towers. This paper will describe some of the history and the design of the present antenna that is currently being used for ionospheric modification at Arecibo.