SF System: Repeater and Equalizer Design

The undersea portion of the SF System includes repeaters spaced at 18.52 kilometer ( ~ 1 0 nautical mile) intervals along the undersea cable to compensate signal loss in the cable across the two nominal transmission bands of 564 to 2788 kHz and 3660 to 5884 kHz. This allocation permits 720* three-kilohertz two-way message channels. Since it is not possible to make the repeater gain characteristic exactly match the cable loss, an equalizer corrects the error accumulated after every 20 repeaters. Physically both repeater and equalizer units are sealed, encapsulated, and shock mounted within a sealed beryllium copper pressure hull. The repeaters of the SF System use transistor and semiconductor diode circuitry. The advantages gained include small size, improved power efficiency, and low aging. It should be noted that total repeater size and weight are little altered by the use of transistors, since most other repeater components are not scaled down in size. However, the smaller physical size of the transistors shortens the feedback loop and thus contributes to the wider bandwidth. Careful attention to design detail, lengthy testing programs, and proven electrical and mechanical concepts refined to meet system re* The basic SF design objective was 720 channels. On the 1300-nautical mile system between Florida and the Virgin Islands, out of band misalignments are small enough to permit possible use of as many as 820 channels. The recently completed TAT-5 System is able to meet objective for over 820 channels.