Caroline TV - Stratovision

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Stratovision

Stratovision was an airborne television transmission relay system from aircraft flying at high altitudes. It has been used for domestic broadcasting in the United States, and by the U.S. military in Vietnam and other countries.

In 1945 the Glenn L. Martin Co. and Westinghouse Electric Corporation advocated television coverage of small towns and rural areas, as well as the large metropolitan centres, by fourteen aircraft which would provide coverage for approximately 78% of the people in the United States

Because the broadcasting antenna for Stratovision is usually hung beneath the aircraft in flight, it naturally has a great command of Line-of-sight propagation. Although transmission distances are dependent upon atmospheric conditions, a transmitting antenna 30,000 feet (9 km) above the Earth's surface has a line of sight distance of approximately 211 miles (340 km).

A Stratovision 25 kW transmitter operating from 30,000 feet (9 km) at 600 MH will achieve a field intensity of 2 millivolts per meter for a 30-foot (9 m) high receiving antenna up to 238 miles (383 km) away from the aircraft.

Stratovision tests were undertaken between June 1948 and February 1949. The first phase was undertaken by the Glenn L. Martin Co. and Westinghouse Electric Corporation using a twin-engine PV-2 aircraft flying at 25,000 feet (7.6 km) transmitting with 250 watts on 107.5 MHz and 5 kW on 514 MHz at Baltimore, Maryland so that recordings could be made at various locations ranging from Norfolk, Virginia to Pittsburgh, Pennsylvania and Boston, Massachusetts.

The second phase of testing was undertaken by these companies using a stripped-down B-29 Superfortress flying at 30,000 feet (9.1 km). The plane was equipped to receive a relay transmission from WMAR-TV, the Westinghouse television studios in Baltimore, which was then relayed over a 5 kW video transmitter and a 1 kW audio transmitter for reception on 82-88 MHz with a television set tuned to Channel 6.

The aircraft received its originating signals from circular dipoles attached to a streamlined 8 foot (2.5 m) mast on top of the aircraft's vertical tail fin. The retractable 28 feet (8.5 m) long broadcasting antenna hung vertically beneath the aircraft. It was composed of a two-element turnstile array for video and a single-element circular dipole for sound transmissions.

The receivers, transmitters and necessary air-conditioning were all powered by the plane's engines using three 15 kVA, 500 Hz alternators. Without air conditioning the transmitters in the interior of the aircraft would have generated a temperature of 134 degrees Fahrenheit (57 degrees Celsius) with an outside air temperature of 25 degrees Fahrenheit (minus 4 degrees Celsius).

On 23 June 1948 the system's airborne transmitter rebroadcast the Republican National Convention, being held in Philadelphia, Pennsylvania, to the surrounding nine-state area during the 9 to 10 pm EDT time period. As part of the activity, a receiver was set up in a hall in Zanesville, Ohio, a small city on the outskirts of the broadcast area (to demonstrate to the invited newspaper reporters that the system was capable of reaching "small town and farm homes").

The tests were watched by many television viewers who sent in reception reports. From these reports it was calculated that Stratovision would require only eight relay planes to provide a transcontinental network and six additional planes to provide coverage to 78% of the United States. Mr. C.E. Nobles who was the head of Stratovision for Westinghouse said in his report:-

"The major technical problems of the system have been solved, and the commercial development awaits only the crystallisation of public demand for the expanded services offered by airborne broadcasting, application of the system by the radio industry to meet this demand, and the clarification of channel facilities available to make possible this application."

In 1961 a nonprofit organisation, Midwest Program on Airborne Television Instruction (MPATI), started a Stratovision service from the airfield of Purdue University as a three-year experiment funded by the Ford Foundation. The programme organised, produced and transmitted educational television programmes four days a week from a DC-6AB aircraft flying at 23,000 feet (7,010 m) over the community of Montpelier in north central Indiana.

MPATI delivered its programmes to television channels 72 (KS2XGA) and 76 (KS2XGD) in the UHF band, by transmitting videotaped lectures from the aircraft to an estimated potential 5,000,000 students in 13,000 schools and colleges. The aircraft were equipped with two 2-inch (51 mm) videotape machines and two UHF transmitters.

The service ended in 1968 when MPATI became embroiled in legal action over their application of Stratovision in a controversy with the Westinghouse company.

During the war in Vietnam, the United States Navy used Stratovision television technology when it flew Operation Blue Eagle from 1966 to 1972 over the Saigon area of South Vietnam. The television programmes were aimed at two audiences on two channels. One for the general public and the other was for the information and entertainment of US troops who were stationed in South Vietnam.