TD-2

"TD2" redirects here. For other uses, see TD2 (disambiguation).

One of the former TD-2 relays in the Mojave National Preserve, California. The tower appears to be in use for other purposes; the vertical antennas at the top and the round dark grey dish are not part of the original system.

TD-2 was a microwave relay system developed by Bell Labs and used by AT&T to build a cross-country network of repeaters for telephone and television transmission. The same system was also used to build the Canadian Trans-Canada Skyway system by Bell Canada, and later, many other companies in many countries to build similar networks for both civilian and military communications.

The system began with the experimental TDX, completed in November 1947, carrying television and telephone between Boston and New York City. TD-2 was a minor improvement on TDX, moving to the 3.7 to 4.2 GHz band set aside in 1947 for common carrier use. The system had six channels, and using frequency-division multiplexing, each could carry up to 480 telephone calls or a television signal. The first TD-2 link between New York and Chicago opened on 1 September 1950, followed by a Los Angeles-San Francisco link on 1 September. The two coasts were linked in 1951.

Equipment improvements in 1953 increased capacity to 600 calls per channel. Looking to further improve throughput, Bell Labs introduced the TH system, which operated in a higher band, around 6 GHz. It also added polarization to the signals allowing two channels per band. This allowed it to carry 1,200 calls per channel, but required the use of horn antennas to retain polarization. After considerable research, Bell developed an antenna that worked for both TD-2 and TH, but these improvements also helped TD-2 and increased its capacity again to 900 calls, delaying a widespread rollout of TH which was added only to the busiest links.

Bell Canada began building a similar TD-2 system, the Skyway, which went into service 1958. The Canadian railway companies then built a second line using TH. By the late 1960s, almost all of the population of North America was linked using TD-2 and TH. Television signals moved to satellite distribution in the 1970s and 80s, and the network was mostly used for telephone from that time. During the late 1980s and especially 1990s, the installation of fibre optic lines replaced the microwave networks. Some of the towers are in use today for other purposes, but the majority of the sites are abandoned.