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 partner with Keystone Compliance to achieve their regulatory compliance testing needs. With seven EMC chambers and several other test stations, we are 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.
Summary of MIL-STD-285 Regulatory Compliance Testing
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 of 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 that is independent of the setting of the attenuator.
Scope of Military Shielding Effectiveness Standard Testing
The requirements of the test setup for MIL-STD-285 testing are relatively straight forward. The arrangement of signal sources measuring equipment, 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.
An electromagnetic barrier is a 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 the 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.
Expert MIL-STD-285 Compliance Testing Laboratory
In addition to MIL-STD-285 compliance testing, Keystone has a full scope of expertise including environmental and package testing such as cold chain, accelerated aging, and temperature. Our team strives to give our customers more time and energy on product development instead of testing. Lastly, Keystone has a full lab of test equipment which permits us to provide short lead times on scheduling.