MIL-STD-1377 Effectiveness of Cable, Splice and Weapon Housing Shields and Filters in Preventing Electromagnetic Radiation Hazards

The standard “MIL-STD-1377 Effectiveness of Cable, Connector and Weapon Housing Shields and Filters in Preventing Electromagnetic Radiation Hazards” developed by the United States Department of Defense describes the shielding and filter effectiveness test methods for weapon design companies to determine whether the relevant requirements of the standard “MIL-P-24014 Military specifications: General requirements for preventing hazards from electromagnetic radiation in ammunition are implemented.

The purpose of these methods is to determine the effectiveness of cable, connector, and weapon enclosure shielding and filters in preventing electromagnetic radiation hazards.

This standard covers methods for measuring the shielding effectiveness of weapon housings, cables, and cable connections in frequency ranges between 100 kilohertz and 30 megahertz , and between 1000 megahertz and 10 gigahertz. Shielding effectiveness test methods are performed in two separate frequency ranges due to equipment limitations. Measuring shielding effectiveness between 30 megahertz and 1000 megahertz is unnecessary because the Navy has limited use of these frequencies, and experience indicates that shielding deficiencies in this range would likely manifest in one of the two measured frequency ranges.

The upper frequency for the test methods described in this standard may be increased to 40 gigahertz where applicable. This standard also covers methods for measuring the filtering effectiveness of radio frequency (RF) suppression devices (filters) for firearm firing circuits in the frequency range between 100 kilohertz and 10 gigahertz.

For the purposes of this standard, in addition to the terms and definitions given in the MIL-P-24014 standard, the following definitions apply:

• An octave is the interval between two frequencies that have a 2:1 ratio.
• A gun casing refers to the metal shell that surrounds a gun circuit, such as a gun shroud.
• A weapon cable is any wiring outside a weapon housing that is designed to connect to circuits inside the housing.
• A gun housing break is any break or connection in the metal gun housing.
• Surface transfer impedance is the ratio of the magnitude of the longitudinal voltage drop across the outer surface of the shield to the current inside the shield.
• Shielding effectiveness is a measure of shielding quality.
• Filter efficiency refers to the minimum attenuation of a filter.

The requirements for screening effect measurements on cables (100 kilohertz to 30 megahertz) are defined as follows:

• The test apparatus shall be arranged as shown in the figure given in the standard. The signal source shall be any RF signal generator or power oscillator having a current output capability compatible with the ammeter and voltmeter specified in this standard.
• Any shielded RF current meter capable of measuring the center conductor current of a coaxial cable in the desired frequency range should be used. Such a device consists of a small metal box and an RF panel ammeter. The ammeter, mounted inside the box, is connected to the center conductors of two bulkhead connectors mounted on opposite sides of the box. A small shielded window on the front of the box is used to view the meter. The sensitivity of the ammeter must be compatible with the output current capacity of the signal generator.
• An adapter is usually used to connect the signal source and the ammeter to the test cable. This can be done by placing the appropriate connectors on opposite sides of a small metal box and joining the appropriate center conductors inside. If a multi-conductor cable is to be tested, the choice of conductors to be connected to the signal source is optional.
• One end of the test lead should be terminated with a short circuit that ensures shield integrity.
• An RF voltage meter or balanced or near-balanced input design should be used. The voltmeter probe should be equipped with balanced probe tips, each approximately 8 mm long. The voltmeter should be coordinated with the signal generator so that it can measure RF voltages as small as 1 millivolt per ampere of signal generator output current capacity.
• To avoid direct connection to the voltmeter probe, the circuit from the signal source to the cable under test should be shielded. The effectiveness of this shielding should be greater than the effectiveness of the cable under test. This can be determined by short-circuiting the cable adapter and measuring the voltage and center conductor current with the signal generator on. The surface transfer impedance calculated from this arrangement should be at least 20 decibels lower than the surface transfer impedance of the test cable.

The requirements for shielding effect measurements at connectors (100 kilohertz to 30 megahertz) are defined as follows:

• The shielding effectiveness of cable connectors in the mated condition is normally measured as part of the cable measurement. If the application requires a connector to provide shielding during mating, shielding effectiveness measurements should be made as follows.
• The signal source and measuring equipment should be as specified above.
• Adapter cables: The two pieces of connector to be tested should be connected to short lengths of shielded cable by terminating one cable short as indicated above and the other being supplied with a cable adapter.
• Pin extension insert: A special pin extension insert should be used as shown in the figure. The purpose of the insert is to provide a path for the center conductor current and a method of locating the connectors relative to each other. The insert should be of a conductive material and long enough to keep the ends of the original mating pins at least one-tenth of an inch apart. If a multiple-pin connector is being tested, one of the outermost pins should be selected for extension. The conductor to this pin should be the conductor to which the signal is applied.
• Measurement technique: The arrangement of the device shall conform to the figure, except that a pair of test connectors with adapter cables shall be used instead of a test cable. When the output of the signal source is applied, the centre conductor current and the maximum voltage between the two connector shells shall be measured at frequencies of not more than 100 octave between 30 kilohertz and 1 megahertz. The connectors shall be manipulated relative to each other to obtain the maximum voltage. This shall be done with a non-conductive processing device.
• Surface transfer impedance shall be calculated by dividing the voltage measurement by the current measurement at each test frequency. Each calculation shall be reported separately.

This standard also defines requirements for the following shielding effect measurements:

• Measurements of shielding effectiveness in weapon enclosures (100 kilohertz to 30 megahertz)
• Measurements of shielding effectiveness on cables (1000 megahertz to 10 gigahertz)
• Measurements of shielding effectiveness in weapon enclosures (1000 megahertz to 10 gigahertz)
• Effectiveness measurements on filters below 30 megahertz for known load.
• Effectiveness measurements below 300 megahertz on filters for unknown load.
• Effectiveness measurements above 300 megahertz on filters for unknown load.

Our organization has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. Among the numerous test, measurement, analysis and evaluation studies provided for businesses in various sectors, there are also test services in accordance with the standard "MIL-STD-1377 Effectiveness of cable, connection and weapon enclosure shields and filters in preventing electromagnetic radiation hazards".