MIL-STD-1377 standard testing evaluates a weapon developer or designer with shielding effectiveness test methods. The methods of the shielding effectiveness test are for weapon enclosures, cables, and cable connectors.
Weapon enclosures can involve an enclosure that surrounds the weapon circuit, the cable is wiring that starts outside of the enclosure and enters inside and connected to circuits inside the weapon enclosure, and cable connectors are simply what connects everything together for the overall system.
Lastly, the test is performed over a frequency range of 100 kilohertz (kHz) up to 30 megahertz (MHz), then from 1 gigahertz (GHz) to 10 gigahertz (GHz).
As depicted in the figure below, the test setup is the same as other shielding effectiveness, with the determination of the dynamic range and the actual measurement of the signal for the sample in question. If the system involves a weapon enclosure, all internal components are removed so only the enclosure is tested.
Scope of the Military Shielding Effectiveness Compliance Standard
To gain a better understanding of the EMC test requirements of this standard, it is important to know the definitions of some of the key words and phrases. A weapon enclosure is the metal shell surrounding a weapon circuit, such as the weapon skin.
The weapon cable is any wiring outside of the weapon enclosure designed to be connected to circuits inside the enclosure. Surface transfer impedance is the ratio of the magnitudes of the longitudinal voltage drop on the outer surface of the shield to the current on the inside of the shield.
Testing to MIL-1377 shielding valuation requires a number of various pieces of equipment. The list below is not all inclusive, but it does provide an overview of the most common equipment needed.
- The signal source shall be any RF signal generator or power oscillator with a current output capability compatible with the ammeter and voltmeter. Any shielded RF current measuring device which can measure the center conductor current of a coaxial cable over the desired frequency range shall be used. Such a device can be constructed from 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 screen-shielded window in front of the box is used for viewing the meter. The sensitivity of the ammeter shall be compatible with the signal generator output current capability.
- It is generally necessary to construct an adapter to link the signal source and ammeter to the test cable. This can be constructed by placing the appropriate connectors on opposite sides of a small metal box and joining the proper center conductors inside. If a multiconductor cable is to be tested, the selection of the conductor(s) to be connected to the signal source is arbitrary.
- One end of the test cable shall be terminated with a short that provides shield integrity. An RF voltage measuring device or balanced or near balanced input design shall be wed. The voltmeter probe should be fitted with balanced probe leads, each approximately 3 inches in length. The voltmeter shall be coordinated with the signal generator such that it is capable of measuring RF voltages as small as 1 millivolt per ampere of signal generator output current capability.
- To avoid direct coupling to the voltmeter probe, the circuit from the signal source to the cable under test should be shielded. The effectiveness of this shield should be greater than that of the cable under test. This can be determined by shorting the cable adapter and measuring the voltage and the center conducting current with the signal generator on. The surface transfer impedance (STI) calculated from this arrangement shall be at least 20 decibels (db) below the STI of the test cable when calculated.
Expert MIL-STD 1377 Laboratory Testing
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