JP2004186546A

JP2004186546A - Multilayer electric wave absorber

Info

Publication number: JP2004186546A
Application number: JP2002353579A
Authority: JP
Inventors: Yoshiyuki Moriyama; 義幸森山; Naonobu Taniguchi; 直延谷口; Yuji Matsumoto; 勇二松本
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2002-12-05
Filing date: 2002-12-05
Publication date: 2004-07-02
Anticipated expiration: 2022-12-05
Also published as: JP4240363B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer electric wave absorber which is ready for high frequency waves in an electromagnetic environment where TE waves, TEM waves and TM waves in 2.45 GHz band and 5.2 GHz band, circular polarized waves in 5.8 GHz band are used together and whose return loss is ≥18 dB. <P>SOLUTION: The multilayer electric wave absorber is formed of several wave absorbing layers exhibiting different electric wave absorbing characteristics. It is composed of a first electric wave absorbing layer 20 which has the peak of its return loss at 2.8-3.8 GHz as the sole electric wave absorbing characteristics and a second electric wave absorbing layer 10 which has the peak of its return loss at 5-6 GHz as the sole electric wave absorbing characteristics. It exhibits double-humped characteristics, namely has peaks of its return loss at 2.2-2.8 GHz and at 4.8-5.5 GHz. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、２．２〜６ＧＨｚの広い周波数帯域において２．４５ＧＨｚ付近と５．２ＧＨｚ付近で２つの電波吸収特性のピークを有する積層型電波吸収体に関する。また、ＴＭ波、ＴＥ波、ＴＥＭ波と円偏波の両方に好適な高周波対応の積層型電波吸収体に関する。
【０００２】
【従来の技術】
複数の電子機器の間で、無線ＬＡＮ（ＬｏｃａｌＡｒｅａＮｅｔｗｏｒｋ）システムを構築してデータの送受信を行うことが行われ、ＩＥＥＥ８０２．１１規格では、用いることができる無線周波数帯として２．４５ＧＨｚ帯が規定されている。また、近年５．２ＧＨｚ帯も開放された。電波としてはＴＥＭ（ＴｒａｎｓｖｅｒｓｅＥｌｅｃｔｒｏｍａｇｎｅｔｉｃ）波、ＴＥ（ＴｒａｎｓｖｅｒｓｅＥｌｅｃｔｒｉｃ）波、ＴＭ（ＴｒａｎｓｖｅｒｓｅＭａｇｎｅｔｉｃ）波である。従って、電波吸収体に対しても２．４５ＧＨｚ帯と５．２ＧＨｚ帯の２つの周波数帯に対応したものに対するニーズが大きくなった。無線ＬＡＮは、建物内で無線を使用して電子機器間のネットワークを形成する。
また、これらの電子機器は自動車などに搭載され、ＥＴＣ（ＥｌｅｃｔｒｏｎｉｃＴｏｌｌＣｏｌｌｅｃｔｉｏｎＳｙｓｔｅｍ：自動料金収受システム）などで電子化された道路システムの中で使用されることが多くなった。将来的には携帯電話で道路通行料金の収受をすることになりそうである。このシステムでは５．８ＧＨｚ帯の円偏波が用いられている。
【０００３】
このような電波環境の中で、電波吸収体にも広い周波数帯域で優れた電波吸収特性を発揮することが求められる。
このニーズに応える一手段として、帯域幅を広げ且つ薄型の電波吸収体を構成するために、電波吸収特性の異なる複数の電波吸収層を積層した積層型電波吸収体によって反射減衰量のピークを２つにした双峰性のものが知られている（例えば、特許文献１）。
この発明は、電波入射側から、コンクリートでなる第１層と、空気でなる第２層と、コンクリートでなる第３層と、フェライト磁性体でなる第４層と、反射メッシュの第５層とを有し、反射減衰量のピークを有する双峰性特性を呈する積層型電波吸収体である。
【０００４】
【特許文献１】
特開平１１−２６１２８２号公報（図２他）
【０００５】
【発明が解決しようとする課題】
従来の積層型電波吸収体（例えば、特許文献１記載）においては、ＶＨＦ帯とＵＨＦ帯のせいぜい７７０ＭＨｚ程度の周波数に対応するものであり、はるかに高周波の２．４５ＧＨｚ帯と５．２ＧＨｚ帯のＴＭ波、ＴＥ波、ＴＥＭ波と、５．８ＧＨｚ帯の円偏波が共に使用される電磁環境下における高周波対応の電波吸収体として反射減衰量が１８ｄＢ以上の満足のゆく特性を発揮できるものでは無かった。
【０００６】
そこで本発明は、２．４５ＧＨｚ帯と５．２ＧＨｚ帯のＴＭ波、ＴＥ波、ＴＥＭ波と、５．８ＧＨｚ帯の円偏波が共に使用される電磁環境下における高周波対応の電波吸収体として反射減衰量が１８ｄＢ以上の積層型電波吸収体の提供を目的とする。
【０００７】
【課題を解決するための手段】
本発明の第１手段は、電波吸収特性の異なる複数の電波吸収層を積層した積層型電波吸収体において、単独の電波吸収特性として２．８〜３．８ＧＨｚに反射減衰量のピークを有する第１の電波吸収層２０と、単独の電波吸収特性として５〜６ＧＨｚに反射減衰量のピークを有する第２の電波吸収層１０とでなり、２．２〜２．８ＧＨｚと４．８〜５．５ＧＨｚに反射減衰量のピークを有する双峰性特性を呈することを特徴とする積層型電波吸収体である。
【０００８】
本発明の第２手段は、前記第１の電波吸収層２０を反射層３０側に配置し、前記第２の電波吸収層１０を電波入射側に配置したことを特徴とする第１手段記載の積層型電波吸収体である。
【０００９】
本発明の第３手段は、電波入射側から、誘電率が５〜２０で厚さが２ｍｍ以下の第１吸収層４０と、誘電率が５以下で厚さが２〜１０ｍｍの第２吸収層５０と、磁性体層からなる第３吸収層７０と、反射層８０とを有し、反射減衰量のピークに双峰性特性を呈する積層型電波吸収体である。
【００１０】
本発明の第４手段は、電波入射側から、誘電率が５〜２０で厚さが２ｍｍ以下の第１層６１と、誘電率が５以下で厚さが２〜１０ｍｍの第２層６２と、誘電率が１０以下で厚さが２ｍｍ以下の第３層６３と、磁性体層からなる第４層６４と、反射層８０とを有し、反射減衰量のピークに双峰性特性を呈する積層型電波吸収体である。
【００１１】
本発明の第５手段は、２．４〜５．２ＧＨｚの入射電波に対して、前記第１層６１における反射率が３０〜９５％である第３または第４手段記載の積層型電波吸収体である。
【００１２】
本発明の第６手段は、２．２〜２．８ＧＨｚ（２．４５ＧＨｚ付近）においてＴＭ波、ＴＥ波、ＴＥＭ波に対する反射減衰量が１８ｄＢ以上であり、４．８〜５．５ＧＨｚ（５．２ＧＨｚ付近）において、ＴＭ波、ＴＥ波、ＴＥＭ波に対する反射減衰量が１８ｄＢ以上であり、かつ５．４〜６．２ＧＨｚ（５．８ＧＨｚ付近）において、円偏波に対する反射減衰量１８ｄＢ以上である第１手段ないし第５手段のいずれかに記載の積層型電波吸収体である。
【００１３】
【発明の実施の形態】
（作用）
本発明者は、積層型電波吸収体において各層の単独の電波吸収特性が組合せによりピークが低周波側にシフトする現象を積極的に利用して、双峰性を持たせて広帯域化すると同時にＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方に有効な反射減衰量を満足できる構成を見出した。
例えば、２．４５ＧＨｚに電波吸収能のピークを有する電波吸収層と、５．２ＧＨｚに電波吸収能のピークを有する電波吸収層とを単純に積層して積層型電波吸収体を構成した場合、両方のピークが重なり双峰性の電波吸収特性を呈するものの、そのピークは共に２ＧＨｚと４．５ＧＨｚとずれて、且つＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方を満足する特性が得られなくなることを見出した。
【００１４】
本発明者らは、各層の吸収周波数特性の設計を高周波伝送式を設計基本式として複素誘電率や複素透磁率の周波数特性をデータベース化して設計できるシステムを確立している（日刊工業新聞社の雑誌「工業材料」２００２年１１月号ｐ．４２−４５参照）。
本発明に係る積層型電波吸収体の設計にも応用して、本発明に係る技術的思想の具体的実現に活用した。
【００１５】
本発明に係る積層型広帯域電波吸収体を、図１を用いて説明する。図１は本発明の一形態に係る広帯域の積層型電波吸収体を示す断面図である。電波の到来方向から、ゴムや樹脂に磁性粉を分散した第２の電波吸収層１０、ゴムや樹脂に磁性粉を分散した第１の電波吸収層２０を積層した。更に、導電性材料や樹脂に導電性材料を分散した電波反射層３０を配設している。
ここで、電波反射層３０を一体化して積層型電波吸収体を構成することもできるが、第１の電波吸収層２０と第２の電波吸収層１０とで積層型電波吸収体を部品として構成して、使用に際しては前記の積層型電波吸収体を導電性外壁などの電波反射層３０として機能する部材に取付けて使用することもできる。
【００１６】
第２の電波吸収層１０を反射層３０側に配置し、第１の電波吸収層２０を電波入射側に配置することもできるが、単独の電波吸収特性として２．８〜３．８ＧＨｚに反射減衰量のピークを有する第１の電波吸収層２０を反射層３０側に、単独の電波吸収特性として５〜６ＧＨｚと第１の電波吸収層２０よりも高周波側に反射減衰量のピークを有する第２の電波吸収層１０を、電波入射側に配置した方が、より良好な電波吸収特性を得られる。反射しやすい高周波成分を、反射させることなく電波吸収体中に取り込んで熱エネルギーなどとして消散させる効果が高まるためと考えられる。
【００１７】
電波反射層３０は金属箔などの導電性材料や、樹脂に導電性材料を分散したもので構成できる。電波反射層３０に分散する導電性を有する材料は、例えばカーボン繊維や金属繊維であって、これを可撓性樹脂中に分散させシート状に成形する。電波反射層３０は面抵抗値を１００ｋΩ□以下とするのが望ましい。１００ｋΩ□を超えると電波が透過して効率よい吸収ができなくなるからである。
【００１８】
電波吸収層１０、２０に分散する磁性粉は、比重が６．０以上の金属や合金で、例えばＦｅ−Ｃｒ−Ａｌ系合金、カルボニル鉄合金、アモルファス合金、Ｆｅ−Ｓｉ系合金、モリブデンパーマロイ、スーパーマロイなどが使用できる。
あるいはフェライトなどの金属酸化物でもよい。また、Ｆｅ−Ｃｕ−Ｎｂ−Ｓｉ−Ｂ系からなるナノ結晶化合金から水アトマイズ法により粒形状粉をアトライタにて摩砕することにより製造した扁平形状粉であって、これを可撓性樹脂中に分散させシート状に成形してもよい。
これらの金属磁性粉の表面は、酸化防止剤が施されていることが好ましい。
【００１９】
電波吸収層１０、２０に金属磁性体粉を用いる場合、その分散量は６０〜９０ｍａｓｓ％が好ましい。６０ｍａｓｓ％未満であると吸収性能が低下し、９０ｍａｓｓ％を超えると材料代が高価になるばかりでなく、重量が重く、柔軟性、耐久性等が低下し実用上好ましくない。
【００２０】
電波吸収層１０、２０にフェライトを用いる場合、その分散量は６５〜９２ｍａｓｓ％が好ましい。６５ｍａｓｓ％未満であると吸収性能が低下し、９２ｍａｓｓ％を超えると生産性が悪くなり、重量が重く、柔軟性、耐久性等が低下し実用上好ましくない。
【００２１】
電波吸収層１０、２０や電波反射層３０のバインダとして用いる樹脂やゴムは、柔軟性があり、比重が１．５以下であり、耐候性を有する例えばアクリル樹脂、クロロプレンゴム、ブチルゴム、ウレタンゴム、シリコン樹脂、塩化ビニル樹脂、フェノール樹脂等である。
【００２２】
電波反射層３０、電波吸収層１０、２０の層厚さは、それぞれ０．５〜５ｍｍが好ましい。０．５ｍｍ未満であると、吸収性能が低下し、５ｍｍを超えると積層した場合の材料代が高価になるばかりでなく、重量が重く、柔軟性が低下し実用上好ましくない。
また積層した全体の厚さは３〜８ｍｍとすることが好ましい。３ｍｍ未満であると、吸収性能が低下し、８ｍｍを超えると積層した場合の材料代が高価になるばかりでなく、重量が重く、柔軟性が低下し実用上好ましくない。
【００２３】
図２は、本発明に係る別の実施形態を示す断面図である。電波入射側から、誘電率が５〜２０で厚さが２ｍｍ以下の第１吸収層４０と、誘電率が５以下で厚さが２〜１０ｍｍの第２吸収層５０と、磁性体層からなる第３吸収層７０と、反射層８０とを有し、反射減衰量のピークに双峰性特性を呈する積層型電波吸収体である。
【００２４】
図２に示す実施形態では、第１吸収層４０は、反射率が３０〜９５％の範囲内にあることが好ましく、例えば樹脂にカーボンや金属繊維を混合したものなどが使用できる。第２吸収層５０は、例えば発泡ウレタンやスチレンなどを用いることができ、空気層としてもよい。第３吸収層７０は、前述の電波吸収層１０，２０と同じ材質を用いることが出来る。
【００２５】
第１吸収層４０において所定範囲内の反射率を有する材料を用いる理由は、入射電波に対して、第１吸収層４０で反射した電波と透過して下の層で反射した電波が逆位相となって相殺し吸収される効果を利用するためである。
第１吸収層４０における反射率を３０〜９５％に限定する理由を説明する。反射率が３０％未満だと反射した電波との相殺効果が減少し、反射率が９５％を超えると電波が積層型電波吸収体に入射せず電波吸収体として利用できないからである。
【００２６】
第１吸収層４０の誘電率を５〜２０、厚さを２ｍｍ以下とした理由、第２吸収層５０の誘電率を５以下、厚さを２〜１０ｍｍとした理由は共に、この範囲外の組合せだとＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方を満足する所望の双峰性特性が得られなくなるからである。
【００２７】
図３は、本発明に係る更に別の実施形態を示す断面図である。電波入射側から、誘電率が５〜２０で厚さが２ｍｍ以下の第１層６１と、誘電率が５以下で厚さが２〜１０ｍｍの第２層６２と、誘電率が１０以下で厚さが２ｍｍ以下の第３層６３と、磁性体層からなる第４層６４と、反射層８０とを有し、反射減衰量のピークに双峰性特性を呈する積層型電波吸収体である。
【００２８】
図３に示す実施形態では、第１層６１は、反射率が３０〜９５％の範囲内にあることが好ましく、例えば樹脂にカーボンや金属繊維を混合したものなどが使用できる。第２層６２は、例えば発泡ウレタンやスチレンなどを用いることができ、空気層としてもよい。第３層６３は、例えば樹脂にカーボンや金属繊維を混合したものなどを用いることができる。第４層６４は、前述の電波吸収層１０，２０において述べたものを用いることが出来る。
【００２９】
第１層６１の誘電率を５〜２０、厚さを２ｍｍ以下とした理由、第２層６２の誘電率を５以下、厚さを２〜１０ｍｍとした理由、第３層６３の誘電率を１０以下で厚さを２ｍｍ以下とした理由は共に、この組合せの範囲外だとＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方を満足する所望の双峰性特性が得られなくなるからである。
【００３０】
本発明の第１，第２，第６手段に係る積層型電波吸収体は導電材料で裏打ちした２層型、本発明の第３，第５，第６手段に係る積層型電波吸収体は導電材料で裏打ちした３層型、本発明の第４，第５，第６手段に係る積層型電波吸収体は導電材料で裏打ちした４層型であり、共に電波吸収の周波数特性に双峰性を有する。
【００３１】
以下、実施例によって本発明を具体的に説明する。
【００３２】
（実施例１）
図１に示す電波反射層３０として厚み０．１ｍｍのアルミ箔を用いた。第１の電波吸収層２０を、フェライト粉をクロロプレンゴム中に８５ｍａｓｓ％分散させ１．３ｍｍの厚さにシート化し形成した。反射減衰量のピークが３．５ＧＨｚとなるように調製している。
第２の電波吸収層１０を、カルボニル鉄粉をクロロプレンゴム中に７８ｍａｓｓ％分散させ２．５ｍｍの厚さにシート化し形成した。反射減衰量のピークが５．４ＧＨｚとなるように調製している。
これら３種類のシートを順次積層し一体化することにより、図１に断面を模式的に示すように、全体の厚さが３．９ｍｍの積層型電波吸収体を形成した。
この積層型電波吸収体の電波吸収性能を、入射角３０度のタイムドメイン法で評価した結果、ＴＭ波と円偏波の両方に対して、おのおの表１、表２（単位は［ｄＢ］）に示すように、２．２〜２．８ＧＨｚ（２．４５ＧＨｚ付近）においてＴＭ波に対する反射減衰量が１８ｄＢ以上であり、４．８〜５．５ＧＨｚ（５．２ＧＨｚ付近）において、ＴＭ波に対する反射減衰量が１８ｄＢ以上であり、かつ５．４〜６．２ＧＨｚ（５．８ＧＨｚ付近）において、円偏波に対する反射減衰量１８ｄＢ以上という所望の電波吸収特性が得られた。なお、ＴＭ波のみならずＴＥ波、ＴＥＭ波についても同様に良好であった。
【００３３】
【表１】

【００３４】
【表２】

【００３５】
（比較例１）
電波反射層３０は実施例１と同じ厚み０．１ｍｍのアルミ箔を用いた。第１の電波吸収層２０を、フェライト粉をクロロプレンゴム中に８８ｍａｓｓ％分散させ１．３ｍｍの厚さにシート化し形成した。反射減衰量のピークが２．４５ＧＨｚとなるように調製している。第２の電波吸収層１０を、カルボニル鉄粉をクロロプレンゴム中に８２ｍａｓｓ％分散させ２．５ｍｍの厚さにシート化し形成した。反射減衰量のピークが５．２ＧＨｚとなるように調製している。
実施例１と同様に入射角３０度のタイムドメイン法で評価した結果、表１、表２に示すように所望の電波吸収特性が得られなかった。
【００３６】
（実施例２）
本発明に係る別実施例の断面図を図２に示す。電波反射層８０は（実施例１）と同じく厚み０．１ｍｍのアルミ箔を用いた。第１吸収層４０は、誘電率が９で厚さ１．５ｍｍであり、カーボン繊維を０．８ｍａｓｓ％、クロロプレンゴムに混合したものを用いた。第２吸収層５０は、誘電率が１．２で厚さが７ｍｍの発泡スチレンを用いた。第３吸収層７０は、厚さが１．５ｍｍでカルボニル鉄粉をクロロプレンゴムに混合したものを用いた。
この積層型広帯域電波吸収体の電波吸収性能を評価した結果、ＴＭ波と円偏波の両方に対して、おのおの表３、表４（単位は［ｄＢ］）に示すように、２．２〜２．８ＧＨｚ（２．４５ＧＨｚ付近）においてＴＭ波に対する反射減衰量が１８ｄＢ以上であり、４．８〜５．５ＧＨｚ（５．２ＧＨｚ付近）において、ＴＭ波に対する反射減衰量が１８ｄＢ以上であり、かつ５．４〜６．２ＧＨｚ（５．８ＧＨｚ付近）において、円偏波に対する反射減衰量１８ｄＢ以上が得られた。なお、ＴＭ波のみならずＴＥ波、ＴＥＭ波についても同様に良好であった。
【００３７】
【表３】

【００３８】
【表４】

【００３９】
（比較例２）
第２層５０の厚さを１２ｍｍとした以外は（実施例２）と同一条件を用いた。この場合、表３、表４に示すように所望の電波吸収特性が得られなかった。
【００４０】
（実施例３）
本発明に係る更に別実施例の断面図を図３に示す。図２を用いて説明した（実施例２）において更に第２層５０と第４層７０との間に第３層６０を介装した。
電波反射層８０は（実施例１）と同じく厚み０．１ｍｍのアルミ箔を用いた。第１層６１は、誘電率が９で厚さ１ｍｍであり、カーボン繊維を０．８ｍａｓｓ％、クロロプレンゴムに混合したものを用いた。第２層６２は、誘電率が１．２で厚さが８．５ｍｍの発泡スチレンを用いた。第３層６３は、誘電率が９で厚さが１ｍｍであり、カーボン繊維を０．８ｍａｓｓ％、クロロプレンゴムに混合したものを用いた。第４層６４は、厚さが２ｍｍでカルボニル鉄粉をクロロプレンゴムに混合したものを用いた。
この積層型広帯域電波吸収体の電波吸収性能を評価した結果、ＴＭ波と円偏波の両方に対して、おのおの表５、表６（単位は［ｄＢ］）に示すように、２．２〜２．８ＧＨｚ（２．４５ＧＨｚ付近）においてＴＭ波に対する反射減衰量が１８ｄＢ以上であり、４．８〜５．５ＧＨｚ（５．２ＧＨｚ付近）において、ＴＭ波に対する反射減衰量が１８ｄＢ以上であり、かつ５．４〜６．２ＧＨｚ（５．８ＧＨｚ付近）において、円偏波に対する反射減衰量１８ｄＢ以上が得られた。なお、ＴＭ波のみならずＴＥ波、ＴＥＭ波についても同様に良好であった。
【００４１】
【表５】

【００４２】
【表６】

【００４３】
（比較例３）
第２層６２の厚さを１２ｍｍに換えた以外は（実施例３）と同じ条件を用いた。この場合、表５、表６に示すように所望の電波吸収特性が得られなかった。
【００４４】
【発明の効果】
本発明によれば、積層型電波吸収体において単独の電波吸収特性が異なる各層の誘電率、厚さなどの条件を適切に組み合わせることによって双峰性で且つＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方に有効な反射減衰量を満足できる電波吸収体を提供できる。従って、インテリジェント交通システムに限らず、今後進展が期待されるコンビニエンスストアなどにおける料金収受システムなどＩＴＳ，ＥＴＣ，ＤＳＲＣと無線ＬＡＮの両方が共存するシステムへの発展に寄与は大きい。
【図面の簡単な説明】
【図１】本発明の１実施例を示す断面図である。
【図２】本発明の別実施例を示す断面図である。
【図３】本発明の更に別実施例を示す断面図である。
【符号の説明】
１０：第２の電波吸収層
２０：第１の電波吸収層
３０：反射層
４０：第１吸収層
５０：第２吸収層
６１：第１層
６２：第２層
６３：第３層
６４：第４層
７０：第３吸収層
８０：反射層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a laminated radio wave absorber having two radio wave absorption characteristic peaks at around 2.45 GHz and around 5.2 GHz in a wide frequency band of 2.2 to 6 GHz. The present invention also relates to a high-frequency-capable laminated electromagnetic wave absorber suitable for both TM waves, TE waves, TEM waves, and circularly polarized waves.
[0002]
[Prior art]
2. Description of the Related Art A wireless LAN (Local Area Network) system is constructed between a plurality of electronic devices to transmit and receive data. The IEEE 802.11 standard specifies a 2.45 GHz band as a usable radio frequency band. Have been. In addition, the 5.2 GHz band has recently been opened. The radio waves include a TEM (Transverse Electromagnetic) wave, a TE (Transverse Electric) wave, and a TM (Transverse Magnetic) wave. Therefore, the need for a radio wave absorber corresponding to two frequency bands of the 2.45 GHz band and the 5.2 GHz band has increased. A wireless LAN forms a network between electronic devices in a building using wireless.
In addition, these electronic devices are mounted on automobiles and the like, and are often used in road systems that are digitized by ETC (Electronic Toll Collection System). It is likely that mobile tolls will be used to collect road tolls in the future. In this system, circularly polarized waves in the 5.8 GHz band are used.
[0003]
In such a radio wave environment, a radio wave absorber is required to exhibit excellent radio wave absorption characteristics in a wide frequency band.
One way to meet this need is to reduce the peak of the return loss by a stacked type radio wave absorber in which a plurality of radio wave absorption layers having different radio wave absorption characteristics are laminated in order to increase the bandwidth and form a thin radio wave absorber. One bimodal type is known (for example, Patent Document 1).
According to the present invention, from a radio wave incident side, a first layer made of concrete, a second layer made of air, a third layer made of concrete, a fourth layer made of a ferrite magnetic material, and a fifth layer made of a reflective mesh are provided. And has a bimodal characteristic having a peak of return loss.
[0004]
[Patent Document 1]
JP-A-11-261282 (FIG. 2 and others)
[0005]
[Problems to be solved by the invention]
The conventional laminated electromagnetic wave absorber (for example, described in Patent Document 1) corresponds to a frequency of about 770 MHz at most in the VHF band and the UHF band, and has a much higher frequency of 2.45 GHz band and 5.2 GHz band. As a radio wave absorber for high frequency in an electromagnetic environment where both TM wave, TE wave, TEM wave and circularly polarized wave in the 5.8 GHz band are used, it can exhibit satisfactory characteristics with a return loss of 18 dB or more. There was no.
[0006]
Therefore, the present invention reflects as a radio wave absorber corresponding to a high frequency in an electromagnetic environment in which a TM wave, a TE wave, a TEM wave, and a 5.8 GHz circular polarized wave are both used in the 2.45 GHz band and the 5.2 GHz band. It is an object of the present invention to provide a laminated electromagnetic wave absorber having an attenuation of 18 dB or more.
[0007]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a laminated type radio wave absorber in which a plurality of radio wave absorption layers having different radio wave absorption characteristics are stacked, and the second radio wave absorber has a return loss peak at 2.8 to 3.8 GHz as a single radio wave absorption characteristic. The first radio wave absorption layer 20 and the second radio wave absorption layer 10 having a peak of return loss at 5 to 6 GHz as a single radio wave absorption characteristic, and are 2.2 to 2.8 GHz and 4.8 to 5. A laminated electromagnetic wave absorber characterized by exhibiting a bimodal characteristic having a return loss peak at 5 GHz.
[0008]
The second means of the present invention is characterized in that the first radio wave absorbing layer 20 is arranged on the reflection layer 30 side and the second radio wave absorbing layer 10 is arranged on the radio wave incident side. It is a laminated type radio wave absorber.
[0009]
The third means of the present invention comprises, from the radio wave incident side, a first absorbing layer 40 having a dielectric constant of 5 to 20 and a thickness of 2 mm or less, and a second absorbing layer 40 having a dielectric constant of 5 or less and a thickness of 2 to 10 mm. 50, a third absorption layer 70 made of a magnetic material layer, and a reflection layer 80, and is a laminated radio wave absorber exhibiting a bimodal characteristic at the peak of the return loss.
[0010]
The fourth means of the present invention comprises, from the radio wave incident side, a first layer 61 having a dielectric constant of 5 to 20 and a thickness of 2 mm or less, and a second layer 62 having a dielectric constant of 5 or less and a thickness of 2 to 10 mm. , A third layer 63 having a dielectric constant of 10 or less and a thickness of 2 mm or less, a fourth layer 64 made of a magnetic material layer, and a reflective layer 80, exhibiting a bimodal characteristic in the peak of the return loss. It is a laminated type radio wave absorber.
[0011]
A fifth aspect of the present invention is the laminated type radio wave absorber according to the third or fourth aspect, wherein a reflectance of the first layer 61 is 30 to 95% with respect to an incident radio wave of 2.4 to 5.2 GHz. It is.
[0012]
According to a sixth aspect of the present invention, in 2.2 to 2.8 GHz (around 2.45 GHz), the return loss with respect to a TM wave, a TE wave, and a TEM wave is 18 dB or more, and 4.8 to 5.5 GHz (5.5. In the vicinity of 2 GHz), the return loss for the TM wave, the TE wave, and the TEM wave is 18 dB or more, and in the range of 5.4 to 6.2 GHz (around 5.8 GHz), the return loss for the circularly polarized wave is 18 dB or more. The laminated electromagnetic wave absorber according to any one of the first means to the fifth means.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
(Action)
The inventor of the present invention has developed a bimodal broadband by using the phenomenon that the peak of the individual radio wave absorption characteristics of each layer is shifted to the lower frequency side by combination in the laminated type radio wave absorber, and at the same time, the ITS , ETC, DSRC and a configuration that can satisfy the return loss effective for both wireless LAN.
For example, when a radio wave absorption layer having a radio wave absorption peak at 2.45 GHz and a radio wave absorption layer having a radio wave absorption peak at 5.2 GHz are simply laminated to form a laminated radio wave absorber, Have overlapping peaks and exhibit bimodal radio wave absorption characteristics, but both peaks are shifted from 2 GHz to 4.5 GHz, and characteristics satisfying both ITS, ETC, DSRC and wireless LAN cannot be obtained. Was.
[0014]
The present inventors have established a system that can design the absorption frequency characteristics of each layer using a high-frequency transmission formula as a basic design formula and design the frequency characteristics of the complex permittivity and the complex magnetic permeability into a database. Magazine "Industrial Materials", November 2002, p.42-45).
The present invention was also applied to the design of the laminated electromagnetic wave absorber according to the present invention, and utilized for the specific realization of the technical idea according to the present invention.
[0015]
The laminated broadband radio wave absorber according to the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view showing a broadband laminated wave absorber according to one embodiment of the present invention. A second radio wave absorbing layer 10 in which magnetic powder is dispersed in rubber or resin and a first radio wave absorbing layer 20 in which magnetic powder is dispersed in rubber or resin are laminated in the direction of arrival of radio waves. Further, a radio wave reflection layer 30 in which a conductive material is dispersed in a conductive material or a resin is provided.
Here, the radio wave reflecting layer 30 may be integrated to form a laminated radio wave absorber, but the first radio wave absorbing layer 20 and the second radio wave absorbing layer 10 constitute the laminated radio wave absorber as a component. In use, the laminated electromagnetic wave absorber can be used by attaching it to a member functioning as the electric wave reflection layer 30 such as a conductive outer wall.
[0016]
The second radio wave absorbing layer 10 can be arranged on the reflection layer 30 side and the first radio wave absorbing layer 20 can be arranged on the radio wave incident side. However, as a single radio wave absorbing characteristic, the light is reflected at 2.8 to 3.8 GHz. The first radio wave absorption layer 20 having the peak of the attenuation amount is provided on the reflection layer 30 side, and the single radio wave absorption characteristic has a peak of 5-6 GHz and the reflection attenuation amount on the higher frequency side than the first radio wave absorption layer 20. By arranging the second radio wave absorbing layer 10 on the radio wave incident side, better radio wave absorption characteristics can be obtained. It is considered that the effect of taking the easily reflected high frequency component into the radio wave absorber without reflecting it and dissipating it as heat energy is enhanced.
[0017]
The radio wave reflection layer 30 can be formed of a conductive material such as a metal foil or a resin in which a conductive material is dispersed. The conductive material dispersed in the radio wave reflection layer 30 is, for example, carbon fiber or metal fiber, which is dispersed in a flexible resin and formed into a sheet. It is desirable that the radio wave reflection layer 30 has a sheet resistance value of 100 kΩ □ or less. This is because if it exceeds 100 kΩ □, radio waves pass through and efficient absorption cannot be performed.
[0018]
The magnetic powder dispersed in the radio

wave absorbing layers

10 and 20 is a metal or alloy having a specific gravity of 6.0 or more, such as an Fe-Cr-Al alloy, a carbonyl iron alloy, an amorphous alloy, an Fe-Si alloy, a molybdenum permalloy, Supermalloy can be used.
Alternatively, a metal oxide such as ferrite may be used. A flat powder produced by milling a granular powder by a water atomization method with an attritor from a nano-crystallized alloy composed of an Fe-Cu-Nb-Si-B system, which is a flexible resin. It may be dispersed therein and formed into a sheet.
It is preferable that an antioxidant is applied to the surface of these metal magnetic powders.
[0019]
When a metal magnetic powder is used for the radio wave absorption layers 10 and 20, the dispersion amount is preferably 60 to 90 mass%. If it is less than 60 mass%, the absorption performance decreases, and if it exceeds 90 mass%, not only is the material cost high, but also the weight is heavy, and the flexibility and durability are reduced, which is not practically preferable.
[0020]
When ferrite is used for the radio wave absorption layers 10 and 20, the amount of dispersion is preferably 65 to 92 mass%. If it is less than 65 mass%, the absorption performance will be reduced, and if it exceeds 92 mass%, the productivity will be poor, the weight will be heavy, the flexibility, the durability and the like will be reduced, which is not practically preferable.
[0021]
The resin or rubber used as a binder for the radio

wave absorbing layers

10 and 20 and the radio wave reflecting layer 30 is flexible, has a specific gravity of 1.5 or less, and has weather resistance such as acrylic resin, chloroprene rubber, butyl rubber, urethane rubber, Silicon resin, vinyl chloride resin, phenol resin and the like.
[0022]
The thickness of each of the radio wave reflection layer 30 and the radio wave absorption layers 10 and 20 is preferably 0.5 to 5 mm. If it is less than 0.5 mm, the absorption performance is reduced, and if it exceeds 5 mm, not only is the material cost when laminating becomes expensive, but also the weight is heavy and the flexibility is reduced, which is not practically preferable.
Further, the total thickness of the laminated layers is preferably 3 to 8 mm. If it is less than 3 mm, the absorption performance will be reduced, and if it is more than 8 mm, not only will the material cost when laminating become expensive, but also the weight will be heavy and the flexibility will be reduced, which is not practically preferable.
[0023]
FIG. 2 is a cross-sectional view showing another embodiment according to the present invention. From the radio wave incident side, a first absorption layer 40 having a dielectric constant of 5 to 20 and a thickness of 2 mm or less, a second absorption layer 50 of a dielectric constant of 5 or less and a thickness of 2 to 10 mm, and a magnetic layer This is a laminated radio wave absorber having a third absorption layer 70 and a reflection layer 80 and exhibiting a bimodal characteristic at the peak of the return loss.
[0024]
In the embodiment shown in FIG. 2, the first absorption layer 40 preferably has a reflectivity in the range of 30 to 95%, and for example, a material obtained by mixing carbon or metal fiber with a resin can be used. The second absorbing layer 50 can be made of, for example, urethane foam or styrene, and may be an air layer. The third absorbing layer 70 can be made of the same material as the above-mentioned radio

wave absorbing layers

10 and 20.
[0025]
The reason for using a material having a reflectance within a predetermined range in the first absorption layer 40 is that, with respect to the incident radio wave, the radio wave reflected by the first absorption layer 40 and the radio wave transmitted and reflected by the lower layer have opposite phases. This is to utilize the effect of being canceled out and absorbed.
The reason for limiting the reflectance of the first absorption layer 40 to 30 to 95% will be described. If the reflectivity is less than 30%, the effect of canceling out the reflected radio wave is reduced, and if the reflectivity exceeds 95%, the radio wave does not enter the laminated radio wave absorber and cannot be used as the radio wave absorber.
[0026]
The reason why the dielectric constant of the first absorption layer 40 is 5 to 20 and the thickness is 2 mm or less, and the reason why the dielectric constant of the second absorption layer 50 is 5 or less and the thickness is 2 to 10 mm are both out of this range. This is because a desired bimodal characteristic satisfying both ITS, ETC, DSRC and wireless LAN cannot be obtained if the combination is used.
[0027]
FIG. 3 is a sectional view showing still another embodiment according to the present invention. From the radio wave incident side, a first layer 61 having a dielectric constant of 5 to 20 and a thickness of 2 mm or less, a second layer 62 having a dielectric constant of 5 or less and a thickness of 2 to 10 mm, and a dielectric layer having a dielectric constant of 10 or less and a thickness of 10 or less. This is a laminated electromagnetic wave absorber having a third layer 63 having a thickness of 2 mm or less, a fourth layer 64 made of a magnetic layer, and a reflective layer 80, and exhibiting a bimodal characteristic at the peak of the return loss.
[0028]
In the embodiment shown in FIG. 3, the first layer 61 preferably has a reflectivity in the range of 30 to 95%, and for example, a material obtained by mixing carbon or metal fiber with a resin can be used. The second layer 62 can be made of, for example, urethane foam or styrene, and may be an air layer. The third layer 63 can be made of, for example, a mixture of resin and carbon or metal fiber. As the fourth layer 64, those described in the aforementioned radio wave absorption layers 10 and 20 can be used.
[0029]
The dielectric constant of the first layer 61 is 5 to 20, the thickness is 2 mm or less, the dielectric constant of the second layer 62 is 5 or less, the thickness is 2 to 10 mm, and the dielectric constant of the third layer 63 is The reason why the thickness is set to 10 or less and the thickness is set to 2 mm or less is that if the combination is out of the range, desired bimodal characteristics satisfying both ITS, ETC, DSRC and wireless LAN cannot be obtained.
[0030]
The laminated electromagnetic wave absorber according to the first, second and sixth means of the present invention is a two-layer type electromagnetic wave absorber lined with a conductive material, and the laminated electromagnetic wave absorber according to the third, fifth and sixth means of the present invention is electrically conductive. The three-layer type material-backed laminated electromagnetic wave absorber according to the fourth, fifth, and sixth means of the present invention is a four-layer type lined with a conductive material, and both have bimodal characteristics in frequency characteristics of electromagnetic wave absorption. Have.
[0031]
Hereinafter, the present invention will be specifically described with reference to examples.
[0032]
(Example 1)
An aluminum foil having a thickness of 0.1 mm was used as the radio wave reflection layer 30 shown in FIG. The first electromagnetic wave absorbing layer 20 was formed by dispersing 85 mass% of ferrite powder in chloroprene rubber and forming a sheet having a thickness of 1.3 mm. It is adjusted so that the peak of the return loss is 3.5 GHz.
The second radio wave absorption layer 10 was formed by dispersing 78 mass% of carbonyl iron powder in chloroprene rubber and sheeting it to a thickness of 2.5 mm. It is adjusted so that the peak of the return loss is 5.4 GHz.
By sequentially laminating and integrating these three types of sheets, a laminated radio wave absorber having a total thickness of 3.9 mm was formed as schematically shown in cross section in FIG.
As a result of evaluating the radio wave absorption performance of this laminated type radio wave absorber by a time domain method at an incident angle of 30 degrees, Table 1 and Table 2 (unit: [dB]) were obtained for both TM waves and circularly polarized waves. As shown in the figure, the return loss for the TM wave is not less than 18 dB at 2.2 to 2.8 GHz (around 2.45 GHz), and the reflection loss for the TM wave is at 4.8 to 5.5 GHz (around 5.2 GHz). A desired radio wave absorption characteristic with an attenuation of 18 dB or more and a reflection attenuation of 18 dB or more for circularly polarized waves was obtained at 5.4 to 6.2 GHz (around 5.8 GHz). In addition, not only the TM wave but also the TE wave and the TEM wave were similarly good.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
(Comparative Example 1)
As the radio wave reflection layer 30, the same aluminum foil as in Example 1 having a thickness of 0.1 mm was used. The first radio wave absorption layer 20 was formed by dispersing 88 mass% of ferrite powder in chloroprene rubber and forming a sheet having a thickness of 1.3 mm. The peak value of the return loss is adjusted to be 2.45 GHz. The second radio wave absorbing layer 10 was formed by dispersing 82 mass% of carbonyl iron powder in chloroprene rubber and sheeting it to a thickness of 2.5 mm. It is adjusted so that the peak of the return loss is 5.2 GHz.
As a result of evaluation by the time domain method at an incident angle of 30 degrees in the same manner as in Example 1, as shown in Tables 1 and 2, desired radio wave absorption characteristics could not be obtained.
[0036]
(Example 2)
FIG. 2 shows a sectional view of another embodiment according to the present invention. As the radio wave reflection layer 80, an aluminum foil having a thickness of 0.1 mm was used as in (Example 1). The first absorption layer 40 has a dielectric constant of 9, a thickness of 1.5 mm, and a mixture of 0.8 mass% of carbon fiber and chloroprene rubber. For the second absorption layer 50, foamed styrene having a dielectric constant of 1.2 and a thickness of 7 mm was used. As the third absorption layer 70, a material having a thickness of 1.5 mm and carbonyl iron powder mixed with chloroprene rubber was used.
As a result of evaluating the radio wave absorption performance of the laminated type broadband radio wave absorber, as shown in Tables 3 and 4 (unit: [dB]), 2.2 to 2.2 for both the TM wave and the circularly polarized wave. At 2.8 GHz (around 2.45 GHz), the return loss for the TM wave is 18 dB or more, and between 4.8 and 5.5 GHz (around 5.2 GHz), the return loss for the TM wave is 18 dB or more, and At 5.4 to 6.2 GHz (around 5.8 GHz), a return loss of 18 dB or more for circularly polarized waves was obtained. In addition, not only the TM wave but also the TE wave and the TEM wave were similarly good.
[0037]
[Table 3]

[0038]
[Table 4]

[0039]
(Comparative Example 2)
The same conditions as in (Example 2) were used except that the thickness of the second layer 50 was 12 mm. In this case, desired radio wave absorption characteristics could not be obtained as shown in Tables 3 and 4.
[0040]
(Example 3)
FIG. 3 is a sectional view of still another embodiment according to the present invention. In Embodiment 2 described with reference to FIG. 2, a third layer 60 is further interposed between the second layer 50 and the fourth layer 70.
As the radio wave reflection layer 80, an aluminum foil having a thickness of 0.1 mm was used as in (Example 1). The first layer 61 has a dielectric constant of 9, a thickness of 1 mm, and a mixture of carbon fibers mixed with 0.8 mass% of chloroprene rubber. For the second layer 62, foamed styrene having a dielectric constant of 1.2 and a thickness of 8.5 mm was used. The third layer 63 has a dielectric constant of 9 and a thickness of 1 mm, and is made of a mixture of 0.8 mass% of carbon fiber and chloroprene rubber. For the fourth layer 64, a mixture of carbonyl iron powder and chloroprene rubber having a thickness of 2 mm was used.
As a result of evaluating the radio wave absorption performance of this laminated broadband radio wave absorber, as shown in Tables 5 and 6 (unit: [dB]), 2.2 to 2.2 for both the TM wave and the circularly polarized wave. At 2.8 GHz (around 2.45 GHz), the return loss for the TM wave is 18 dB or more, and between 4.8 and 5.5 GHz (around 5.2 GHz), the return loss for the TM wave is 18 dB or more, and At 5.4 to 6.2 GHz (around 5.8 GHz), a return loss of 18 dB or more for circularly polarized waves was obtained. In addition, not only the TM wave but also the TE wave and the TEM wave were similarly good.
[0041]
[Table 5]

[0042]
[Table 6]

[0043]
(Comparative Example 3)
The same conditions as in (Example 3) were used except that the thickness of the second layer 62 was changed to 12 mm. In this case, desired radio wave absorption characteristics could not be obtained as shown in Tables 5 and 6.
[0044]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it is bimodal by combining conditions, such as a dielectric constant and thickness of each layer in which a single electromagnetic wave absorption characteristic differs in a laminated type electromagnetic wave absorber, and it is ITS, ETC, both DSRC and wireless LAN. The electromagnetic wave absorber which can satisfy the effective return loss can be provided. Therefore, the present invention is not limited to the intelligent transportation system, but greatly contributes to the development of a system in which both ITS, ETC, DSRC, and wireless LAN coexist, such as a toll collection system in a convenience store, which is expected to progress in the future.
[Brief description of the drawings]
FIG. 1 is a sectional view showing one embodiment of the present invention.
FIG. 2 is a sectional view showing another embodiment of the present invention.
FIG. 3 is a sectional view showing still another embodiment of the present invention.
[Explanation of symbols]
10: second radio wave absorption layer 20: first radio wave absorption layer 30: reflection layer 40: first absorption layer 50: second absorption layer 61: first layer 62: second layer 63: third layer 64: first Four layers 70: third absorbing layer 80: reflective layer

Claims

A first radio wave absorption layer having a plurality of radio wave absorption layers having different radio wave absorption characteristics, a first radio wave absorption layer having a peak of return loss at 2.8 to 3.8 GHz as a single radio wave absorption characteristic;
A second radio wave absorption layer having a return loss peak at 5 to 6 GHz as a single radio wave absorption characteristic,
A laminated electromagnetic wave absorber having a bimodal characteristic having peaks of return loss at 2.2 to 2.8 GHz and 4.8 to 5.5 GHz.

Disposing the first radio wave absorption layer on the reflection layer side;
2. The laminated electromagnetic wave absorber according to claim 1, wherein said second electromagnetic wave absorbing layer is arranged on a radio wave incident side.

From the radio wave incident side,
A first absorption layer having a dielectric constant of 5 to 20 and a thickness of 2 mm or less;
A second absorbing layer having a dielectric constant of 5 or less and a thickness of 2 to 10 mm;
A third absorbing layer comprising a magnetic layer,
Having a reflective layer,
A laminated wave absorber that exhibits bimodal characteristics at the peak of return loss.

From the radio wave incident side,
A first layer having a dielectric constant of 5 to 20 and a thickness of 2 mm or less;
A second layer having a dielectric constant of 5 or less and a thickness of 2 to 10 mm;
A third layer having a dielectric constant of 10 or less and a thickness of 2 mm or less;
A fourth layer comprising a magnetic layer,
Having a reflective layer,
A laminated wave absorber that exhibits bimodal characteristics at the peak of return loss.

The multilayer radio wave absorber according to claim 3 or 4, wherein a reflectance of the first layer with respect to an incident radio wave of 2.4 to 5.2 GHz is 30 to 95%.

At 2.2 to 2.8 GHz (around 2.45 GHz), the return loss of the TM wave, TE wave, and TEM wave is 18 dB or more, and at 4.8 to 5.5 GHz (around 5.2 GHz), the TM wave, The return loss for a TE wave and a TEM wave is 18 dB or more, and the return loss for a circularly polarized wave is 5.4 dB to 6.2 GHz (around 5.8 GHz). 2. The laminated electromagnetic wave absorber according to 1.