Communications: directive radio wave systems and devices (e.g. – Radio wave absorber
Reexamination Certificate
1999-10-14
2002-04-16
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radio wave absorber
C342S004000
Reexamination Certificate
active
06373425
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an electromagnetic wave absorber and more particularly, to a composite electromagnetic wave absorber for use in a compact anechoic chamber where there is to be no reflection of electromagnetic waves. More specifically, this invention relates to a composite electromagnetic wave absorber comprising ferrite powder dispersed non-polar resin, of pyramidal shape and a ferrite tile.
The present invention relates further to an anechoic chamber using said composite absorber and a method of fitting the same in the compact anechoic chamber.
Troubles of electromagnetic wave jamming that an electronic product imparts to other electronic products or troubles of electromagnetic wave jamming of the electronic product imparted by other electronic products, on the contrary, have been increasing in recent years. Under such circumstances, manufacturers of the electronic products have been urged to insure that their products are free from troubles of jamming even when electromagnetic wave jamming is inputted to their products and that their products do not in turn generate and emit electromagnetic wave jamming that might exert adverse influences on the products of other manufacturers. In other words, electromagnetic compatibility (EMC) for satisfying these two requirements has become necessary for electronic products in general.
A measurement chamber for evaluating these two requirements is therefore necessary. To remove the influences from any other than the electronic product that is to be measured, the outer wall of the measurement chamber is covered with a metallic plate for shielding electromagnetic waves, and its inner wall is covered with an electromagnetic wave absorber (hereinafter called merely the “absorber”) lest the electromagnetic waves emitted from the electronic product are reflected by the wall. Such a measurement chamber is generally referred to an “anechoic chamber”.
The anechoic chamber includes two types: the first is a large anechoic chamber for conducting EMC examination of large products (such as automobiles, large electronic appliances, and so forth) and the other is a compact anechoic chamber for examining relatively small electronic products.
The frequency range measured inside the anechoic chamber has been from 30 MHz to 1 GHz in the past. For, the frequency at which the problem of radiation of electromagnetic waves occurring is from around 30 MHz and electronic products using a frequency exceeding 1 GHz have not existed in practice.
The EMC evaluation measurement conducted inside the anechoic chamber includes a measurement of electromagnetic emission (30 MHz to 1 GHz) from the electronic product, and a radiation electromagnetic field immunity measurement (26 MHz to 1 GHz) of the electronic product to external jamming waves. In these evaluation measurements, the upper limit frequency is standardized to 1 GHz, and the conventional anechoic chambers have been designed to correspond to this standard.
Recently, however, electronic appliances that Tim use a frequency higher than 1 GHz have appeared on the market with diversification of electronic products. They are, for example, cellular telephones (1.45 GHz), microwave ranges (2.45 GHz), satellite broadcasting (4 GHz, 6 GHz), and so forth. Therefore, the standard of the measurement method in the anechoic chamber has to be changed unavoidably from the conventional standard to the one including higher frequency band than 1 GHz. The absorbers used in the anechoic chamber, too, have to be changed from those for frequency band up to 1 GHz of the prior art to those for higher frequency band than 1 GHz.
The absorbers used for the large anechoic chamber have been generally composite absorbers formed by superposing the ferrite tile and the carbon pyramid. Though only the carbon pyramid was used at first for the absorber, its absorption performance in a low frequency range of 200 GHz or below was not sufficient. Therefore, a predetermined absorption performance (generally 20 dB) had to be secured by using the ferrite tile in addition to the carbon pyramid in the composite configuration. This ferrite tile/carbon pyramid composite absorber is free from deterioration of absorption characteristics of the carbon pyramid itself for higher frequency band than 1GHz and renders no particular problem for the increase of the measurement frequency in the large anechoic chamber.
More concretely, the typical electromagnetic wave absorber used in the anechoic chamber includes a ferrite tile absorber of a sintered body of ferrite and a carbon pyramid absorber formed by shaping a carbon-impregnated resin foam into a pyramidal shape. The ferrite tile absorber exhibits excellent absorption performance in a low frequency range of a long wavelength with its thickness of only about 5 to 7 mm, but its adaptive frequency band is narrow.
Though the carbon pyramid absorber has broader frequency band absorption characteristics owing to its pyramidal configuration, the height of the pyramid must be increased in accordance with the wavelength, and results in a large construction for the low frequency band. These absorbers of different sizes are used either as a single body or in combination in accordance with the specification of the anechoic chamber and its size. They are so designed as to reduce their size as much as possible, and exhibit generally a reflection damping performance of at least 20 dB.
On the other hand, the carbon pyramid used in the large anechoic chamber is not used in the compact anechoic chamber. For, the carbon pyramid has a height of 180 cm or more, and an effective measurement space can hardly be secured in the compact anechoic chamber from the size limitation of the chamber itself. For this reason, the conventional compact anechoic chamber has been produced by bonding only the ferrite tile as the absorber, or by putting a short carbon pyramid on the ferrite tile. The compact anechoic chamber to which the ferrite tiles are merely bonded without carbon pyramids exhibits absorption characteristics that corresponds exactly to absorption characteristics provided by the ferrite tile itself, and its absorption performance sometimes fails to satisfy the absorption requirement of 20 dB in the frequency band of 30 to 60 MH and higher than 500 MHz. Quite naturally, this kind of compact anechoic chamber cannot be used in practice in the high frequency band of 1 GHz or above.
The compact anechoic chamber that uses the composite structure of ferrite tile and the short carbon pyramid (about 15 to 40 cm high) exhibits absorption performance of at least 20 dB at the frequency of 1 GHz or more, but reflection of the electromagnetic waves occurs in a frequency band of a few hundreds MHz. In this case, absorption characteristics of the ferrite tile itself are deteriorated.
In other words, the excellent absorption characteristics of the ferrite tile absorber are impeded. When the balance with absorption performance in a low frequency band of a few hundreds MHz is taken into consideration, the size of the pyramid must be increased eventually, and the height of the pyramid increases as much as 45 cm to 1 m. If this carbon pyramid absorber is added to the existing anechoic chamber comprising the ferrite tile absorber alone, the effective space of the chamber decreases and this decrease naturally impedes the measurement in the anechoic chamber. When the size of the chamber is enlarged to cope with the carbon pyramid absorber, the cost increases as much. Therefore, these measures are not practically advantageous.
To solve these problems of the absorbers of the compact anechoic chamber, the inventors of the present invention had proposed a porous ferrite absorber, that exhibits absorption performance of at least 20 dB in a high frequency band of 1 GHz or more without deteriorating absorption characteristics of the ferrite tile, in Japanese Patent Laid-Open Publication Nos. 302991/1995 and 130388/1996. The present inventors also proposed a porous ferrite/ferrite tile composite absorber produced by dis
Hayashi Toshikatsu
Inoue Shigeo
Shimada Kazuo
Kabushiki Kaisha Riken
Kubovcik & Kubovcik
Sotomayor John B.
LandOfFree
Composite electromagnetic wave absorber and method of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Composite electromagnetic wave absorber and method of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Composite electromagnetic wave absorber and method of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2905583