DEVELOPMENT OF ELECTRICALLY SMALL PLANAR ANTENNAS WITH MATCHING CIRCUIT

Transcription

1 DEVEOPMENT OF EECTRICA SMA PANAR ANTENNAS WITH MATCHING CIRCUIT Haruichi Kanaya, Ryusuke Nabeshima, Ramesh K. Pokharel, and Keiji oshida Graduate School of Information Science and Electrical Engeerg, Kyushu University 744, Motooka, Fukuoka, , JAPAN Abstract: We designed and fabricated an electrically small antenna (ESA) with coplanar waveguide (CPW) matchg circuit. Matchg circuit is realized by usg terdigital gap and transmission le. We designed ESA with the aid of the coercial three-dimensional electro magnetic field simulator. We also made experiments on the ESA with CPW matchg circuits usg patterned circuit board.. INTRODUCTION In micro- and millimeter- wave deices, tegratg entire transceivers on a sgle chip is the vision for future wireless systems such as PDC, wireless AN, RF-ID and MIMO systems []-[3]. This has the benefit of cost and size reduction. However, antennas are considered to be the largest components of wireless systems, so that, it is necessary to miaturize antennas [4]. Studies are also made of an electrically small antenna (ESA), i.e., the antenna whose dimension is much smaller than one-wavelength, towards further reduction of the antenna size [5]. It is widely known, however, that order to realize the miaturized antenna, we must simultaneously realize a broadband impedance matchg circuit, which compensates the narrow bandwidth peculiar to the small antenna with low radiation resistance, and we must atta large impedance-matchg ratios to connect with semiconductor amplifiers with high ternal impedances. Also, the small antenna is sensitive to the conductor resistance because of its low radiation resistance, and the decrease of the radiation efficiency often makes serious problem. The terest for the coplanar waveguide (CPW) transmission les has creased significantly recent years. CPW transmission les have lower radiation leakage and less dispersion than microstrip les. Also, they are preferable for monolithic microwave tegrated circuit (MMIC s) and RFIC (radio frequency tegrated circuit) sce no via holes are required for tegration with devices [6, 7]. Slot antennas are suitable for CPW-fed configuration, and the conventional CPW-fed slot antenna is a one-wavelength center- fed type [8]. In our previous works, we designed the slot dipole antenna whose length is one-wavelength with a bandpass filter usg CPW les, which acts as an impedance matchg circuit as well [9-]. In order to reduce the total size of antenna, the slot loop antenna was also designed whose length is half wavelength perimeter, and it was tegrated to a low noise amplifier with the matchg circuit for terconnectg them [2]. Also, we have designed a slot dipole antenna whose dimension is much smaller than one wavelength with aid of EM-simulator and carried out the experiments on the slot dipole antenna with 2-poles bandpass filter, whose total size is 4. x 3.7, usg high temperature superconductors BCO th film on the MgO substrate with relative permittivity of 9.6, the 5. GHz band [3]. In this paper, we design the impedance matchg circuit, which connects an ESA fabricated on the normal metal to a semiconductor amplifier. The proposed matchg circuits have performances similar to those of the n-pole bandpass filter (BPF) [4]. By usg the quarter wavelength transmission le and admittance (J) -verter, we can reduce the total antenna size more than one-wavelength. In the begng, theoretical performances of the ESA with the CPW impedance matchg circuit are calculated by usg the transmission le model as well as the coercial electro-magnetic (EM) field simulator (Ansoft; HFSS ver.). In order to demonstrate the theory, we also carried out experiments on the ESA at 2.4 GHz, which has the size of 22.3 x 8.24, with the pole number of n matchg circuits usg FR4 prted circuit board. 2. DESIGN THEOR OF THE EECTRICA SMA ANTENNA (ESA) The conventional slot antenna is a one-wavelength center feed slot antenna as shown Fig.. The designed center frequency is 2.4GHz-band. The substrate has dielectric constant ε r 4.25 and tanδ.5. The thickness of the substrate and copper top metal is.8 and 8 μm, respectively. Three-dimensional EM simulator simulates the RF properties. Fig. 2 shows the return loss of the standard slot antenna. The put impedance is almost 5 Ω around 2.4 GHz, and db bandwidth is 65 MHz. Fig. 3 shows the simulated radiation pattern of the standard slot antenna shown Fig.. The peak ga and radiation efficiency are 3.92 dbi and

2 %, respectively, which is the characteristics of dipole antenna. 38. W 8. feed 5. edge. 74. feed W Cross section.8 edge copper.8 FR4: ε r 4.25 tanδ.5(@.8ghz) Fig.. ayout of the standard slot dipole antenna. Rreturn oss [db] Fig. 2. Return loss of the standard slot dipole antenna. metal loss, and a is the reactance of the antenna. Because the quarter-wavelength parallel resonance appears at 2.74 GHz, the size of this antenna is electrically smaller than quarter wavelength when the antenna works at 2.4GHz. a is 42.5+j385 Ω at 2.4 GHz, which is far from 5Ω, so that, return loss is almost db. It is shown that most RF signals reflect for the impedance miss-match (.67λ ) Antenna Impedance ( a ) [Ω] (.8λ ) a R a +j a Fig. 4. ayout of the ESA. R a a -2 φ φ φ 9 Return oss [db] Fig. 3. Simulated radiation pattern of the standard slot dipole antenna. Fig. 4 shows the simulated layout of an electrically small antenna (ESA). The physical properties of the substrate are the same as those of standard antenna. The antenna size is.8λ and.67λ, respectively, where λ is the wavelength the vacuum at 2.45 GHz, which is especially smaller than that of the standard dipole slot antenna. Figs. 5 and 5 show the a ( a R a +j a ) and return loss of the antenna, where R a represents radiation resistance and Fig. 5. Input impedance and return loss of the ESA without matchg circuit. 3. DESIGN THEOR OF IMPEDANCE MATCHING CIRCUIT BETWEEN ESA AND SEMICONDUCTOR AMPIFIER In order to connect between ESA and RF front-end, impedance matchg must be attaed between the antenna and amplifiers.

3 The present matchg circuit is based on the bandpass filter (BPF) composed of the transmission le and J-verters. Fig. 6 shows the n BPF, where, is the admittance of the load and B is the susceptance the parallel resonator with a susceptance slope parameter b. The design parameters of the n BPF are J J 2 w w b g g ω ω B b ω ω b g g 2 () (2) (3) where, w is the relative bandwidth and g i is the filter parameter. Fig. 6 shows the equivalent circuit model of the Fig. 6 at the center frequency. In the figure, G S and G, and Q ei are the equivalent conductance and external quality factor, respectively. J, jb J,2 e b2 jb G Qe2 Fig. 6. Circuit model of the n bandpass filter and equivalent circuit model at center frequency. Antenna b G s Q a A A C m B B Amp. n. In this figure, a (/ a ) denotes the put impedance of ESA and (/ ) represents the put impedance of NA or the output impedance of PA. (/ ) and i are characteristic impedance and electrical length of the transition le. Fig. 7 shows the equivalent circuit model of the Fig. 7 at the center frequency. In Fig. 7, is the admittance for lookg the antenna from A-A, and is the admittance for lookg amplifier from A-A and given by, ' G ' G' + jb' ' + jb ' a + + j tan j tan + j + / jωcm + j + / jωc (4). (5) Fally, the comparison of the Fig. 6 and 7, the proposed design value,, 2, C m are led the numerical value as follows: B' ( B ω ω b' g g Q e G' w ' ( 2 ω ω (6) (7) (8) (9).. () Fig. 8 shows the simulated layout of the ESA with CPW transmission le which has and (See A-A Fig. 7 ). Fig. 9 shows the frequency dependence of the. We can see the parallel resonance around 2.4 GHz. a m π 2) b' g g Q e2 G ' w π 2) 2 ω B'(, ) b' 2 ω ω ω tan( ) 2 tan( ) 2 2 G jb Fig. 7. Circuit model of the n impedance matchg circuit and equivalent circuit model at center frequency. Fig. 7 shows the equivalent circuit model for the smallest matchg circuit, correspondg to BPF with A A Fig. 8. ayout of the electrically small antenna with CPW transmission le.

4 Input admittance (') [ms] G' -3 B' -4 Frequency (GHz) Fig. 9. Input admittance ( ) of the ESA with CPW transmission le. Fig. shows the layout of ESA with CPW matchg circuit. In order to realize 2 and C m, we adopted the terdigital gap and CPW transmission le, where is assumed to be 5 Ω by the experiment to be convenient. Fig. shows the put impedance ( ), which is the impedance for lookg the antenna from B-B Fig. 7 and Fig.. Fig. 2 shows the return loss and Smith chart of the ESA with CPW matchg circuit. is almost 5Ω around 2.4 GHz, so that, return loss is db at 2.4 GHz. Fig. 3 shows the simulated radiation pattern of the ESA with CPW matchg circuit. ayout 8.24 (.67λ ) Inverter 22.3 (.8λ ) Fig.. ayout of the ESA with CPW matchg circuit. Input Impedance ( ) [Ω] B B J-Space W. J Space.3 Tooth ength Toothength.2 Tooth Width ToothWidth.48 Fig.. Input impedance of the ESA with matchg circuit. W 2 Re[ ] Im [ ] -3 Freqency [GHz] Return oss [db] Fig. 2. Return loss and Smith chart of the ESA with CPW matchg circuit. φ φ φ 9 Fig. 3. Simulated radiation pattern of the ESA. 4. EPERIMENTA An ESA is fabricated on FR4 substrate by usg the prt board makg equipment (MITS; FP-2T model 4), which has μm-diameter millg cutter. Fig. 4 shows the photographs of the antenna fabrication system. In the figure, the close up of the high-frequency millg cutter is also shown. Fig. 5 shows the photographs of the ESA with CPW matchg circuit. In the figure, an terdigital gap is also shown. RF signal is put through MMC connecter, which has characteristic impedance 5 Ω. Fig. 6 shows the photographs of the RF measurg system. We measured the S-parameters by usg a GP-IB controlled vector network analyzer (HP; HP8722C). Prted board makg equipment PCB High-frequency Millg Cutter Fig. 4. Photographs of the antenna fabrication system.

5 Inverter result, we designed a small-size (22.3 x 8.24 ) slot antenna. Moreover, ESA with CPW matchg circuit was fabricated and measured the RF properties, thereby we demonstrated frequency characteristics as expected. Fig. 5. Photograph of the ESA. Return oss [db] - -2 Sim. Exp. -3 Vector Network Analyzer:HP8722C PC: Agilent ADS Fig. 7. Experimental result of the ESA. VNA MMC-SMA Connector Antenna Fig. 6. Photographs of the RF measurg system. 5. RESUTS AND DISCUSSIONS Fig. 7 shows the experimental results of the return loss of the ESA with CPW matchg circuit. In the figure, broken le shows the return loss of the EM simulated result. It seems that the bandwidth slightly decreases, and the center frequency shifts 37 MHz to lower side. Also, the return loss at the center frequency is 8 db. The observed experimental results are caused by an error edge part of the terdigital gap (see Fig. 5), an error of the dielectric constant of the FR4 substrate and residual loss of the connection between connector and antenna. Fig. 8 shows the comparison between experimental results and simulated results takg account of the permittivity error. In EM simulation, we made the ε r from 4.25 to 4.5, so that, the experiment results and the simulation values are correspondg well. Fig. 9 shows the comparison of the standard slot antenna with the ESA. The antenna size can be reduced to about 9 % and bandwidth design becomes possible. 6. CONCUSIONS In this paper, slot dipole antenna with a bandpass filter have been designed and tested. We succeeded realizg the circuit which matches the small radiation resistance of ESA to the amplifier. As a Return oss [db] Sim. Exp. Fig. 8. Experimental results and simulated results by takg account of permittivity error. Fig. 9. Comparison of the standard slot antenna with the ESA. ACKNOWEDGEMENT This work was partly supported by a Grant--Aid for Scientific Research (C) from the Japan Society

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