LF/MF Communications Equipment Aboard USS Ling (SS-297) - Part 2

By Howard F. Holden WB2AWQ

Neither the RAK nor the RAL receivers are lightweight units. The receiver with-out power supply weighs around 75 pounds. A look at the interior of one shows why. These babies are built like the proverbial battleship.

The front panels each boast no less than 12 controls, and there is also a filament voltage meter and an audio output meter. The receivers drive headphones directly, or feed a control box that allows the operator to listen to either the RAK or the RAL, or both simultaneously. There is a separate power supply for each receiver. Each power supply weighs 43 pounds. The power supplies use three tubes, a 5Z3 full wave rectifier and an 874 regulator (Ed.-similar to the VR90. but with a 4-pin base), and one #876 ballast tube².

The RAK has not yet been restored to operation; the RAL gives a fine account of itself on HF, but suffers from severe BC overload on the lower frequency ranges. It even does a creditable job on SSB signals, once one gets used to juggling the regeneration control along with the main tuning. Typically one wants the regeneration control just over the point of oscillation, which happens to offer the best beat reception selectivity (highest tuning Q). The frequency vernier control can be used to fine-tune SSB signals, as any change in the regen control will also change the detector/oscillator frequency.

The RAK is designed to operate with an antenna tuning system that is intended to compensate for the relative shortness of the submarine' s antennas on VLF and LF, while the RAL operates directly off the antennas. The long-wires are each about 80 ft long, and run from the sides of the conning tower to the bow, where they are supported by a T -mast. Any point along the antenna wires is at best 10 feet off the sub's main deck!

The antennas exit the radio room through a dome in the overhead and an eight-foot-tall watertight shaft. They exit the through two insulators protruding through the bulkhead.

We can hear a number of signals below 535 kHz on the RAL (which tunes down to 300 kHz). Navtex (an automated mode B SITOR)³transmission of weather and hydrological information for ships) on 518 kHz is very strong.

These signals are broadcast from the US Coast Guard in Boston, MA and Portsmouth, VA among others. We can also hear several airport marker beacons in the NYC metro area, plus some other unidentified digital signals, one around 487 kHz, and one near 400 kHz.

² A. P. Jacobi, The Ballast Tube Handbook, Feb 1991, published by Antique Electronic Supply, Tempe, AZ, page 54, and The ARRL Radio Amateur's Handbook, third edition, first printing, October 1927, page 78.

³ SITOR is the commercial equivalent of AMTOR, or "Amateur Teleprinting Over Radio," and mode B is the FEC or "forward error correction" method, used in one-way SITOR or AMTOR transmissions.