From "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory" (Gebhard) - 25MB pdf
HIGH-FREQUENCY RECEIVER ANTENNA MULTICOUPLER 
(AT
RIGHT)
This multicoupler, developed by NRL (1961), provided a substantial advancement
in receiver multiplexing. Its use permits up to ten receivers to be accommodated
on a single antenna, with 40db isolation at 5 percent frequency separation from
adjacent transmitting channels ( types CU-1573, 2 to 6 MHz; CU-1574, 6 to 18
MHz; CU-1575, 10 to 30 MHZ). This multicoupler has been widely used in shore and shipboard
installations.
Receiver Multiplexing
Prior to 1923, radio reception aboard ship simultaneously with transmission was generally unsatisfactory due to the interference caused by spark and
arc transmitters. The elimination of these types of transmitters was an important step
in making simultaneous operation possible. Another important step was the Navy's decision, based on NRL reception work, to locate shipboard reception facilities forward and transmitting facilities aft, which provided considerable isolation of the two functions. Partial separation of these facilities was first accomplished on the battleship USS WYOMING, then the flagship of the scouting fleet (1923). A more comprehensive separation was next made on the battleship USS COLORADO. The arrangement was adopted and specified in the Navy's standard plans for ship installations
(1923).
Laboratory staff members prior to their transfer to NRL devised a "coupling tube-resistor network" system for receiver multiplexing with which up to eight receivers could be operated simultaneously on one aperiodic antenna without interaction (1922). This system was installed on the USS WYOMING and successfully demonstrated by NRL during fleet maneuvers in the Caribbean Sea and in Pacific waters in the early part of 1923. In view of this demonstration the Navy issued standard plans for the installation of the system specifying eight receivers on the larger ships. This receiver multiplexing system performed acceptably at the lower frequencies and also in the high frequency band as the latter came into general Navy use. The system found extensive application throughout the service and continued to be used long after World War II.
In recent years the Navy's requirements for ship communication channels had greatly increased, necessitating the closer spacing of receiver channels with respect to each other and also with respect to local transmitting channels. This, together with increased transmitter power level, made imperative much greater selectivity and linearity in multicouplers. While both transmitters and receivers can be satisfactorily operated simultaneously on the same antenna at low power, the high transmitter power used on shipboard makes the isolation of 20 dB and more, secured by proper spacing of transmitting and receiving antenna, of considerable value. However, in spite of this isolation, reliance must be placed heavily on the performance of multicouplers to avoid receiver intermodulation and injury to components which may result from the several hundred volts likely to be induced in the receiver system during transmission.
The first substantial step in advancing receiver multiplexing performance, over that of the early coupling tube-resistance network system, was the result of NRL's adaptation of its new multi-mesh resonant circuitry technique, previously developed for transmission. Its use in receiver multicouplers provided 40 dB isolation at 5 percent frequency spacing from an adjacent transmitter channel, and made possible accommodation of up to ten receivers on one antenna. NRL developed multicouplers including this technique which were designated the CU-1573 (2 to 6 MHz), CU-1574 (6 to 18 MHz), and CU-1575 (10 to 30 MHz) (1961). First installations were made on shore at U.S. Naval Communication Station, Keflavik, Iceland (1961) and on the missile ship USS NEWPORT NEWS (1963).
The next important step for multicouplers was NRL's provision of a terminated transmission line-combining network technique which permitted up to 20 receivers to be used on a single antenna (1966). Multicouplers utilizing this technique, such as the AN/SRA-38, 39, 40, 49, and 50, were obtained under formal procurement for wide Navy use (Pickard and Burns). A third step involved NRL's development of radio-frequency, energy-level-sensing and fast-acting overload devices and their embodiment in multicoupler input circuits (1961). These devices cause the receiver input circuits to be isolated when induced voltages from transmitters reach a level of about one volt, thus protecting the receiver system against injury. The devices also provide automatic restoration of operation when the voltages are removed. Through these steps, in combination, NRL provided the first receiver multicoupler system to accommodate up to 20 receiver channels on one antenna having 40 dB isolation at 5-percent frequency spacing with overload protection (1964). The techniques devised were proven to be of value in receiver multicouplers.