Sains Malaysiana 36(1): 65-67 (2007) 

Electromagnetic and Absorption Properties of Some TPNR/Fe₃O₄/YIG Microwave Absorbers as Revealed by a Microwave Vector Network

Analyser and Specular Absorber Method

(Sifat Electromagnet dan Penyerapan Beberapa Penyerap Gelombang Mikro

TPNR/Fe₃0₄/YIG yang diperlihatkan oleh Penganalisis Rangkaian

Vector Gelombang Mikro dan Kaedah Penyerap Spekular)

Ahmad Nazlim Yusoff

Diagnostic Imaging & Radiotherapy Programme

Faculty of Allied Health Sciences, Universiti Kebangsaan Malaysia

50300 Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia

Junaidah Mat Sani

Physics Department, Faculty of Applied Sciences

MARA University of Technology

26400 Jengka, Pahang Darul Makmur, Malaysia.

Mustaffa  Hj. Abdullah

Physics Programme, Faculty of Science & Technology

Universiti Kebangsaan Malaysia

43600 Bangi, Selangor Darul Ehsan, Malaysia

Sahrim Hj. Ahmad & Nazree Ahmad

Materials Science Programme, Faculty of Science & Technology

Universiti Kebangsaan Malaysia

43600 Bangi, Selangor Darul Ehsan, Malaysia

ABSTRACT

The complex scattering parameters (S₁₁* and S₂₁*), relative dielectric permittivity (e_r* = e_r' - je_r"), relative magnetic permeability (m_r* = m_r' - jm_r") and  absorption characteristics of some thermoplastic natural rubber (TPNR) – Fe₃O₄/YIG composites were investigated by means of a microwave vector network analyser and specular absorber method. The measurements were performed in the frequency range of 1 – 13 GHz with and without the presence of a transverse external magnetic field with magnitude of 1 T. The specular absorber method is used in determining the reflection loss (R_L) of the composites and its dependence on material properties, thickness and the external magnetic field. In toroidal form, the composites under study seem to transmit more (P_t > 60%) but absorb and reflect less (P_a + P_r < 30%) microwave power in the frequency range used in this study and in both unmagnetised and magnetised states. The external magnetic field is seen to have the effects of reducing S₁₁*, S₂₁*, e_r', e_r", m_r' and m_r". The suppression of the relaxation and resonance behaviours on the m_r' and m_r" plots for all samples are observed for measurements performed in the presence of the external field. The composites, as revealed by the specular absorber method, show conditions of minimal reflection on R_L versus frequency plot for all sample thickness where more than 95% of the microwave power is not reflected back. The location of the dips of minimal reflection on the frequency domain depend on the thickness, the dielectric and magnetic properties of the materials.  The conditions of minimal reflection was found to occur when the thickness (t) of the sample equals the odd number multiple of a quarter wavelength in the material (l_m), t = nl_m/4 (n = 1, 3, 5, 7 …), where a geometrical cancellation took place at the surface of the absorber between the reflected waves, resembling a thin filem inteference phenomenon. The dips are suppressed and shifted to a higher frequency in the presence of the external magnetic field. With the knowledge of the dielectric permittivity and magnetic permeability of a material, the specular absorber method may provides a simple theoretical graphic aids in determining the absorption characteristics and location of the matching condition in the frequency domain.

Keywords : specular absorber method; thermoplastic natural rubber; yttrium iron garnet; reflection loss; quarter wavelength thickness 

ABSTRAK

Parameter penyerakan (S₁₁* dan S₂₁*), ketelusan dielektrik relatif (e_r* = e_r' - je_r") dan ketelapan magnet relatif (m_r* = m_r' - jm_r") kompleks serta ciri-ciri penyerapan bagi beberapa komposit getah asli termoplastik (GATP) – Fe₃O₄/YIG diselidiki menggunakan penganalisis rangkaian vektor gelombang mikro dan kaedah penyerap spekular. Pengukuran dilakukan dalam julat frekuensi 1 – 13 GHz dengan dan tanpa kehadiran medan magnet luar melintang dengan magnitud 1 T. Kaedah penyerap spekular digunakan dalam menentukan kehilangan pantulan (R_L) bagi komposit dan kebergantungannya terhadap sifat dan ketebalan bahan serta medan magnet luar. Dalam bentuk toroid, komposit yang dikaji didapati lebih banyak menghantar (P_t > 60%) tetapi kurang menyerap dan memantulkan (P_a + P_r < 30%) kuasa gelombang mikro dalam julat frekuensi yang digunakan dalam kajian ini untuk kedua-dua keadaan termagnet dan tidak termagnet. Medan magnet luar dilihat sebagai mempunyai kesan mengurangkan S₁₁*, S₂₁*, e_r', e_r", m_r' dan m_r". Penekanan kelakuan relaksasi dan resonans ke atas plot m_r' and m_r" untuk kesemua sampel dapat diperhatikan untuk pengukuran yang dilakukan dengan kehadiran medan magnet luar. Kesemua komposit, seperti yang diperolehi daripada kaedah penyerap spekular, menunjukkan keadaan pantulan minimum pada plot R_L melawan frekuensi untuk semua ketebalan sampel dengan lebih daripada 95% daripada kuasa gelombang mikro tidak dipantulkan semula. Lokasi keadaan dengan pantulan minimum (puncak minimum) pada domain frekuensi bergantung kepada ketebalan, sifat dielektrik dan magnet bahan. Keadaan dengan pantulan minimum didapati berlaku apabila ketebalan (t) bahan bersamaan dengan gandaan ganjil bagi seperempat panjang gelombang di dalam bahan  (l_m), t = nl_m/4 (n = 1, 3, 5, 7 …), pada mana suatu pembatalan secara geometri berlaku pada permukaan penyerap di antara gelombang-gelombang terpantul, menyerupai fenomena inteferens filem nipis. Puncak minimum didapati tertekan dan teranjak ke frekuensi yang lebih tinggi dengan kehadiran medan magnet luar. Dengan adanya maklumat mengenai ketelusan dielektrik dan ketelapan magnet bahan, kaedah penyerap spekular dapat  menyediakan suatu bantuan grafik secara teori dalam menentukan ciri penyerapan dan lokasi keadaan kesepadanan dalam domain frekuensi.

Kata kunci: kaedah penyerap spekular; getah asli termoplastik; garnet itrium besi; kehilangan pantulan; ketebalan seperempat panjang gelombang

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