![schelkunoff electromagnetic waves schelkunoff electromagnetic waves](https://images-na.ssl-images-amazon.com/images/I/5174t--9b2L.jpg)
For a fixed distance between loops, the SE was found to be roughly independent of the distance of the sheet from the transmit loop (as predicted by the plane wave spectrum theory). The receive loop open circuit voltage was measured both with the sheet present and absent. The power source was a 200 W McIntosh amplifier with a low impedance, providing a roughly constant voltage on the source loop. Both transmit and receive loops were 7 cm in diameter. He measured three materials - copper, aluminum and steel - each roughly 1/16- and 1/8-inch thick. 6 This formulation matches the experimental setup, 5 which was loop-to-loop, and is rigorous as long as the loops are sufficiently small to maintain constant current.Ĭarefully measured data on the SE of metal sheets are sparse Moser's 5 data are used here. This work was recently extended to include an electrically small receiving loop, again in the near-field regime.
![schelkunoff electromagnetic waves schelkunoff electromagnetic waves](https://d3i71xaburhd42.cloudfront.net/43fab5f8c97cf624acd05ad68acd21b7fc8011ad/14-Figure6-1.png)
The loop magnetic field was expanded into an integral of plane waves each plane wave component was propagated through the wall. In a classic paper, Moser 5 obtained the SE of a metal wall when the source antenna was an electrically small loop, and when the wall was in its near field. The wall must be infinitely large or, if part of a shielded room, sufficiently large to encompass most of the incident field. As will be shown, use of this modified near-field wave impedance produces very good results. Whitehouse modified Equation 6 to include the effect of loop radius, replacing R with√R 2 + a 2 where a is the loop radius. (Transverse fields apply when the plane of the loop is parallel to the wall other loop orientations will have a different wave impedance.) This relationship results in a wave impedance of Whitehouse 4 demonstrated that the correct wave impedance uses the ratio of transverse fields, where H trans = H r sin + H cos. The small loop near-field wave impedance is usually computed as the ratio of the well-known near-field components E f /H. Īt low frequencies most antennas (and inadvertent radiators) are loops. The critical contributions of Schelkunoff were to formulate transmission through a wall - long before the Radiation Laboratory work - and to recognize that near-field effects could be subsumed into the external wave impedance Z wave. The sheet thickness is t and its impedance and propagation constant are Z sheet and. The terms, of course, result from the exact formula for transmission of a plane wave through a sheet of arbitrary material.
![schelkunoff electromagnetic waves schelkunoff electromagnetic waves](https://image3.slideserve.com/6318675/slide1-n.jpg)
These formulas have been applied to plane sheets and used by many workers. He showed that SE can be written as three terms (in decibels) representing the primary reflection at the air-metal interface, the attenuation (diffusion loss) for a single pass through the metal wall and a correction term to account for the internal reflections and additional attenuation in the material. This article reviews formulas for the near-field case.Ĭylindrical and spherical shields were investigated by Schelkunoff 1 many decades ago. For these cases, the shielding effectiveness (SE) (the ratio of the incident field to the exit field) may be significantly different than for the plane wave case. Often the frequencies of interest are sufficiently low that the metal wall is in the near field of a source or test antenna.
![schelkunoff electromagnetic waves schelkunoff electromagnetic waves](https://alchetron.com/cdn/sergei-alexander-schelkunoff-50ce2598-e2f5-466f-9992-b3c8c4414e0-resize-750.jpeg)
For plane wave sources there are well-known simple formulas for transmission through a metal sheet (or through a multiple-layer wall) using A-B-C-D or equivalent chain matrices. Many devices such as electronic typewriters and computers emit ELF radiation. Electromagnetic shielding of rooms or equipment through the use of metal walls or sheets has been important for many decades, and is now even more necessary with the burgeoning number of RF transmitters of all types, both inside and outside of buildings.