Radiation in a high-frequency circuit or system can be difficult to control. For that reason, RF/microwave absorber materials are often added to circuits to suppress or eliminate unwanted radiationeither from the circuit to the outside, or to minimize the effects of radiated signals outside a circuit (such as broadcast signals) on the performance of the circuit. To better appreciate RF/microwave absorbers and how to properly apply them, Emerson & Cuming Microwave Products is offering a 19-page application note, "Theory and Application of RF/Microwave Absorbers," for free download on their website.
This application note steps through the fundamental theory of how absorbers work to suppress radiation, with a brief review on dielectric materials and how they are characterized in terms of relative dielectric constant and dielectric loss tangent. It then extends the discussion to how absorber materials are characterized by such parameters as magnetic loss tangent, and how the permittivity and permeability of an absorber material will vary dramatically with frequency. It then differentiates inherently narrowband absorber materials and those that are more broadband in nature, including those that employ an impedance gradient.
For evaluating the characteristics of absorber materials, the application note even includes a sidebar story on the NRL Arch, originally designed by the Naval Research Laboratory (NRL) and accepted as an industry standard method for nondestructive testing of microwave absorber materials over a wide frequency range. An NRL arch consists of a transmit and receive antenna, both of which are oriented toward a metal plate. To measure incident reflectivity of a material, the antennas are located as physically close as possible to each other. Usually, a microwave vector network analyzer (VNA) is used to collect test results on a material within the NRL Arch. A calibration is performed by measuring the resultant power reflecting off the metal plate over a broad frequency range, which is considered the 0-dB level. The material under test is then placed on the plate and the reflected signal measured in dB. Time-domain gating may be used on the VNA to eliminate antenna cross talk and reduce the error introduced by room reflections. The size of the absorber material under test and the antenna-to-plate distance are determined by the frequency range of test.
This informative application note reviews different types of absorber materials, additional forms of measurement techniques, and a wide variety of applications for RF/microwave absorbers. It is generously supported by equations explaining absorber theory, including a description of how electromagnetic (EM) waves are reflected and transmitted at a material boundary.
Emerson & Cuming Microwave Products
28 York Ave., Randolph, MA 02368