Undersea cables use an armorless coaxial cable design. The repeaters have a rigid housing and directional filters which permit transmitting in both directions through one cable and a single amplifier in each repeater. The lower portion of the transmission band goes in one direction, the higher portion in the other direction, with an unused guard-band between the two. Each repeater contains anoscillator with a unique frequency, which is used for fault location and to determine gains and losses along the length of the system. Those repeaters are powered in series, through the center conductor, from the two ends. Ideally, the gain of each repeater would exactly compensate, at each frequency, for the loss of the associated cable.There will be small differences between average repeater gain and average cable loss, as well as variations from repeater to repeater and cable section to cable section. Both of these types of variations lead to deviations from the actual zero net gain of a cable and its associated repeater. Any such differences between these gains and losses have a signal-to-noise penalty associated with them, which is roughly a function of the total net gains and losses between any two repeaters. To hold these penalties to an acceptable level (a few decibels, ocean block equalizers are inserted at regular intervals along the system (every 20 or 30 repeaters). These equalizers are passive; they use gain from an adjacent repeater which has much less than a full cable section associated with it (in the case of SG, 1 nmi rather than 5 nmi). These equalizers contain fixed networks to compensate for deviations known at the time the equalizers are manufactured. They also contain a family of networks that can be switched in or out just prior to the laying of each equalizer to compensate for deviations that first become apparent, as the system is layed. It is the determination and setting to the optimum choice among the available networks for each equalizer that is the most important transmission function during laying. To make it simple, underwater repeaters are powered by high voltage dc power supplies established at the shore terminals. Each repeater and equalizer contains a power separator filter for extracting the dc current from the center conductor of the coaxial cable while allowing signal transmission.
