Proceedings of the 40th European Microwave Conference
Ferrite Supported Steerable Antenna on Metamaterial CRLH Transmission Line Gheorghe Sajin, Stefan Simion, Florea Craciunoiu, Alina-Cristina Bunea, Adrian Dinescu, Andrei A. Muller National Institute for Research and Development in Microtechnologies, IMT Bucharest, Str. Erou Iancu Nicolae 126A, 077190 Bucharest, Romania, Phone: +40-21-269 0775
[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] Abstract — The aim of this paper is to describe the results obtained on the radiation pattern scanning of a ferrite supported resonating CPW antenna based on metamaterial transmission lines approach. The scanning effect is obtained as an effect of remarkable properties of the ferrite substrate in the microwave frequencies domain when applying a biasing magnetic field. Data obtained by simulation with a suitable electromagnetic analysis software tool indicate an azimuthal characteristic deviation of ± 9.780 at a working frequency f = 13 GHz for a variable magnetic biasing field between 0 T and 0.16 T. This result is in very good agreement with the experimental data showing a characteristic displacement of +120 … -100.
(a)
I. INTRODUCTION The CRLH cell opened a new class of circuit topology for many devices such as coupled-line directional couplers [1] and various types of antennas [2], [3]. In the domain of antennas made on the basis of metamaterials approach, a lot of contributions were produced, more recent being [4] - [8]. Concerning the angle scanning of a CRLH antenna characteristic, this ability was electronically achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to some varactor structures [9]. There are very few contributions found in literature concerning CRLH microwave and millimeter wave devices supported on magnetically biased ferrite. Recently, authors [10] compared four related CRLH leaky-wave antennas with dispersion controlled by an applied magnetic field for fixed frequency external tuning. In this contribution we present the radiation characteristic scanning of a CRLH CPW zeroth order resonator antenna having a magnetically polarized ferrite as supporting substrate. II. COMPUTING THE SCANNING OF THE RADIATION PATTERN The antenna is an array of three CRLH cells, each having a T circuit topology formed of two series connected CPW interdigital capacitors and two parallel short-ended CPW transmission lines as inductors cf. [11]. The antenna layout is shown in Fig. 1 (a), while Fig 1 (b) shows a photo detail of the processed area around the junction between the CPW interdigital capacitors and the CPW inductive lines.
978-2-87487-016-3 © 2010 EuMA
(b) Fig. 1. The antenna layout (a) and a photo detail of the area around the junction between the CPW interdigital capacitors and CPW inductive lines.
The substrate used in this work was a polycrystalline garnet with the saturation magnetization 4πMs = 550 G, permittivity ε = 13.5 and resonance linewidth ΔH = 16.8 kA/m. The thickness of the ferrite substrate was 0.5 mm and the metallized surface was previously mirror polished. The antenna layout was analyzed and modeled using the IE3D – Zeland software for an externally applied biasing field Happl = 0 T, namely, for the ferrite substrate in unmagnetized state. For the layout of one of the CRLH cells the following results were obtained: the CPW inductive line length – 1.5 mm; the width of the CPW central conductor – 100 μm; the width of the CPW slot – 100 μm; the length of the interdigital capacitor at the end of antenna – 1 mm and 0.5 mm for the internal interdigital capacitor; the width of the metallic finger of the interdigital capacitor – 10 μm; the space between two fingers of the interdigital capacitor – 10 μm; the space between the interdigital capacitor and the ground planes of
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the CPW structure – 100 μm. The number of the metallic fingers of the interdigital capacitor is equal to 10. For this layout, the elements of the CRLH equivalent circuit for the interdigital capacitor and for the short-ended inductive CPW lines are: shunt inductance LL = 0.55 nH, series interdigital capacitance CL = 0.18 pF, series inductance LR = 0.3 nH and shunt capacitance CR = 0.23 pF (see [11]). The whole layout of the zeroth-order resonating antenna consists of three such CRLH cells, each one having the above dimensions. An input CPW line of 4.5 mm length was used for the connection to the measurement system. When the ferritic substrate is biased by a dc magnetic field (Happl) applied normally on the ferrite substrate, the permeability changes its values from unpolarized state. The effective permeability μeff of the magnetically biased ferrite substrate was computed by inserting the relations (1)…(7) in a suitable software, cf. [12].
μ eff = where: μ' = 1 +
2 2 μ' − K '
(1)
μ'
[
(
)]
2 2 2 ωM ωL ωL − ω 1 − α 2 2 2 2 2 2 2 ωL − ω 1 + α + 4 ω ωL α
[
( )] 2 2 2 ωM ω[ωL − ω (1 + α )] K' = [ω2L − ω2 (1 + α 2 )]2 + 4ω2ω2Lα 2
The technological process for the antenna manufacturing consists of a standard wet etching photolithography process of 500 Å Cr / 0.6 μm Au metallization obtained by evaporation on the mirror polished side of the ferrite wafer. A microscope photo showing the configuration of the interdigitated capacitors and part of the inductor lines is shown in Fig. 1 (b). The antenna active area is 3,9 mm×3,4 mm, smaller by ~30%, than a λ/2 patch antenna. Two antenna structures mounted on suitable high frequency test fixtures are shown in Fig. 2.
(3) Fig. 2. Test fixtures supporting the fabricated antennas on a ferrite substrate.
(4)
where: ΔH = resonance linewidth; Ms = saturation magnetization of the ferrite substrate; Hi = internal magnetic field in the ferrite substrate. For the geometric shape of the ferrite substrate – a thin wafer having the thickness much smaller than the other two dimensions – with the biasing magnetic field applied normally on its surface, the internal magnetic field (Hi) is computed using the following equation: Hi = Happl – Ms
III. EXPERIMENTAL RESULTS
(2)
where:
γ M s = ωM ; γ H i = ωL ; α ≅ ΔH/2Hi
amplitude of the radiation characteristic. Cf. [13], the correspondence between the frequency shift rate and the angular position of the radiation pattern maximum is about 30 / 100 MHz. Considering these data a scanning angle of about 150 results for our present modeling.
(5)
A preliminary computing using previous relations shows that if the magnetic field applied to this ferrite substrate changes from Happl = 0 T to Happl = 0,16 T, the effective permeability (μeff) of the ferrite substrate decreases from μeff ≅ 1 to μeff ≅ 0.95. This value was used in the IE3D software in order to compute the antenna frequency shift following the magnetic biasing of the ferritic substrate. The conclusion of this preliminary computing is that if a magnetic biasing is applied normally on the substrate, the resonating frequency of the ferrite supported antenna changes with about 500 MHz. As it was shown in a previous work [13] a frequency change of the CRLH antenna (on ferritic or non-ferritic substrate) implies a change of the maximum azimuthal
Fig. 3. The measuring arrangement (a) and the antenna structure in the measuring setup for measurements in the transverse plane θ.
A photo of an experimental setup able to measure the power radiated by the antenna at various angles while applying a biasing magnetic field normally on the ferrite substrate is shown in Fig. 3. The emitting antenna on ferrite is fixed close to a mobile electromagnet armature. The receiving is made by a horn antenna having the possibility to rotate around the emitting antenna. The angle of this rotation is read on a circular dial marked in degrees. The Hall probe was put close to the fixed armature and does not appear in the photo in Fig. 3.
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A. Measurement of the radiation characteristic in transverse plane (θ)
Scanning of the radiation characteristics for Happl=0 T; 0.16 T and -0.16 T, respectively 90
The measurements of the radiation pattern were made both in transverse and longitudinal planes, defined as in Fig. 4.
1 60
120 0.8 0.6
30
150 0.4
P/Pmax
0.2
180
0
210
330
Fig. 4. The transverse (θ) and the longitudinal (ϕ) measurement planes
The experiments were made with biasing magnetic fields having the values: Happl = 0 T, Happl = 0.16 T and Happl = −0.16 T, the last value being obtained by the reversal of the current through the electromagnet. The power levels measured at different angles normalized to the maximum value at Happl = 0 T / θ = 0°, is given in Table I and the correspondent radiation antenna pattern is shown in Fig.5. TABLE I THE RATE P/PMAX AT DIFFERENT TRANSVERSE ANGLES (θ) FOR THREE VALUES OF THE BIASING MAGNETIC FIELD
0.15 0.19 0.14 0.24 0.33 0.33 0.44 0.72 0.81 0.91 0.91 0.7 0.4 0.27 0.11
Happl = 0 T
0.1 0.12 0.15 0.13 0.29 0.68 0.87 1 0.91 0.81 0.47 0.22 0.2 0.21 0.21
θ (deg)
Happl = 0 T Happl = 0.16 T Happl = -0.16 T
*Normalized for the maximum value at Happl = 0 T
Fig. 5. Scanning the radiation pattern of the ferrite supported antenna in polar representation.
In order to better approximate the scanning angle, the measured values were rated to the maximum value for each applied magnetic biasing field. The result is shown in Fig.6. Scanning of the radiation characteristics for Happl=0 T; 0.16 T and -0.16 T, respectively 90
Happl = −0.16 T
1
120
0.15 0.19 0.14 0.24 0.81 0.87 0.77 0.72 0.7 0.64 0.55 0.47 0.4 0.27 0.11
60 0.8 0.6
150
30 0.4 0.2
P/Pmax
−35 −30 −25 −20 −15 −10 −5 0 5 10 15 20 25 30 35
Happl = 0.16 T
300 270
P / Pmax
0
θ()
240
180
0
330
210
300
240 270
One may observe that, if the ferritic substrate supporting the antenna is biased by a magnetic field Happl = 0.16 T, the maximum of the radiation characteristic is moved to the value θ ≅ 120. If the orientation of the biasing magnetic field is reversed, keeping the initial magnitude, Happl = −0.16 T, the maximum of the radiation characteristic is moved in the opposite direction to the approx. value θ ≅ −100. This scanning of the radiation characteristic is shown in Fig. 5 in polar coordinates. In this representation the values were rated to the absolute maximum value at Happl = 0 T and θ = 00.
θ (deg)
*Normalized for the maximum value at each applied field
Happl = 0 T Happl = 0.16 T Happl = -0.16 T
Fig. 6. Scanning the radiation pattern of the ferrite supported antenna in polar representation. Values were rated to the maximum value for each applied magnetic biasing field.
B. Measurement of longitudinal plane (ϕ)
the
radiation
characteristic
in
In order to complete the characterization of the antenna’s radiation capability, measurements were also made in the longitudinal antenna plane (ϕ) – see Fig. 4. Measurements were done for 10 biasing magnetic field values but only the
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most representative values were retained. The measuring setup shown in Fig. 3 (b), was slightly modified and the measuring angle was in forward direction. The experimental results are shown in Fig. 7 where the radiated power at different angles in the (ϕ) plane in the forward direction for three values of the biasing magnetic field, were plotted. All the radiated power values were rated at the value P(0 T, 0 deg) meaning the measured value of the radiated power in absence of the biasing magnetic field (Happl = 0 T) in a point located on the normal line on the antenna center at ϕ = 00 (see Fig 4). All the measured values were rated to this value. From Fig. 7 one may find that the maximum radiated power occurs at an angle ϕ ≅ 50, regardless of the magnetic biasing of the ferritic substrate. The radiated power gradually decreases, as it can be seen in Fig. 7. The graph was limited to the angle value of ϕ ≅ 30° beyond this value the results being insignificant. One may see that while applying a magnetic polarization with field values between Happl = 0 T and Happl = 0.1 T, the radiated power increases.
Experimental data shows a radiation characteristic scanning ability between the angles θ ≅ 120 for Happl = 0.16 T and θ ≅ −100 for Happl = −0.16 T. The experimental results are consistent with the calculated values. In addition to the scanning effect one may see an increase of the attenuation of the emitted signal in the (θ) plane of the antenna. Concerning the antenna characteristic in the forward direction in the (ϕ) plane, we did not find a scanning effect, but an increase of the radiated power consecutive to magnetically biasing the ferrite substrate was observed. REFERENCES [1]
[2]
[3]
Scanning Radiation Antenna 1 - P/P(0 T, 0 deg)
[4]
1.6 1.4
P/P(0 T, 0 deg)
1.2 1
[5]
0T 0.03 T
0.8
0.09 T
0.6
[6]
0.4 0.2
[7]
0 0
5
10
15
20
25
30
Degrees in (ϕ) plane
[8]
Fig. 7 (a). Radiated power in (ϕ) plane for antenna structure.
The rate between the current power value and the reference value P(0 T, 0 deg) is almost 1.4 at ϕ = 50. This radiated power increase may be found for all the measurement points along the graph, the whole curve being “raised up”, but the maximum remains at the angle ϕ ≅ 50. Therefore, for the radiation characteristic along the (ϕ) plane of the antenna, no scanning effect was observed.
[9]
[10] [11] [12]
IV. CONCLUSIONS
[13]
We present a CRLH CPW zeroth-order antenna on ferrite substrate having the capability to scan the radiation pattern following the magnetic ferrite biasing. The antenna was analyzed using a full-wave electromagnetic software and the calculated data indicate a scanning possibility θ = ±150 for a magnetic biasing field Happl = ±0.16 T.
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