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MIL-STD-285 Attenuation Measurements for Enclosures and Electromagnetic Shielding

MiL-STD-285 testing provides the framework for measuring the attenuation characteristics of electromagnetic shielding enclosures. Keystone Compliance has developed the reputation of being one of the best shielding effectiveness test labs in the country. Our capabilities are limitless and we provide constant communication throughout the test program.

Thousands of manufacturers partners with Keystone Compliance to achieve their regulatory compliance testing needs. With seven EMC chambers and several other test stations, we able to offer short lead times on scheduling. Our shielding effectiveness capabilities include the testing of small material samples up to entire buildings and structures.

For more information on our MIL-STD-285 shielding effectiveness testing expertise, please Contact Us or Request a Quote. For more information on MIL-STD-285, please read the remainder of this page.

As previously mentioned, MIL-STD 285 covers a method of measuring the attenuation characteristics of electromagnetic shielding enclosures. These enclosures are used for electronic test purposes over the frequency range 100 kilocycles to 10,000 megacycles.

To understand the requirements of shielding effectiveness testing, it is important to understand the terminology. One of the main components of shielding effectiveness is attenuation. Attenuation is a ratio, expressed in decibels (db). The ratio is of the received powers on opposite sides of a shield when the shield is illuminated by electromagnetic radiation. Per MIL-STD-285, this is the figure of merit to designate the shielding effectiveness of electromagnetic enclosures.

An attenuator may be a 50 or 72 ohm transmission line, low input impedance, step attenuator. A signal generator should be used to calibrate the attenuator. The attenuator shall be capable of measuring an insertion loss over 70 db. When an attenuator, the calibration of which is used as a basis for the desired measurements, is employed between either the receiver or the transmitter and its antenna, the antenna shall “look back” into an impedance which is independent of the setting of the attenuator.

An electromagnetic barrier is the topologically closed surface created to prevent or limit fields and conducted transients from entering the enclosed space. The electromagnetic barrier consists of the shield and POE treatments. This barrier encloses the protected volume and special protective volumes, if required.   

An electromagnetic closure is a treatment to prevent excessive electromagnetic field leakage at an aperture point of entry. Examples of closure techniques at a seam between two metal plates include welding, brazing, or soldering and metal-to-metal contact under pressure applied with a mechanical fastening.

Mission-critical equipment is defined as all equipment required to directly support a critical mission, success or failure of which could affect the outcome of battle. Mission-critical equipment often refers to all communications-electronics and support equipment that must be protected in order to perform critical trans- and post-HEMP attack missions.

A point-of-entry (POE) is a location on the electromagnetic barrier where the shield is penetrated and energy may enter the protected volume. This will occur unless an adequate POE protective device is provided. POEs are classified as aperture POEs or penetrating conductors according to the type of penetration. They are also classified as architectural, mechanical, structural, or electrical POEs according to the engineering discipline in which they are usually encountered.

The requirements of the test setup for MIL-STD-285 testing are relatively straight forward. The arrangement of signal sources measuring equipments, pick up devices and shielded enclosures are all detailed within the standard. All power lines, RF cables and other utilities entering the shielded enclosure must be in place when tests are conducted. Special care should be taken to make measurements in the vicinity of utility entrances, doors and access panels.

In need of shielding effectiveness or EMC testing? Learn firsthand why so many manufacturers partner with Keystone Compliance to meet their EMC compliance testing needs. Contact Us or Request a Quote to start the process. Our experts will guide you from the development of the test plan to the issuance of the test report.

In need of other EMC testing? We can take care of that too. From traditional EMC to indirect lightning to ESD, our EMC scope of accreditation is robust and all-encompassing. We specialize in a consultative approach with constant communication. We also take pride in taking on custom test plans that require custom setups.