Analysis of a Proximity Coupled Patch Antenna on a Metalized Substrate

Jing Liang* and H.Y. David Yang

Department of Electrical and Computer Engineering, University of Illinois at Chicago

Abstract

This paper investigates the periodic metallic loading effects on a proximity coupled microstrip patch antenna fed by a microstrip line. Specifically a mushroom-type EBG structure is investigated. When the loading elements are small and capacitive, metallic loadings have the effect of reducing patch antenna size with the same center frequency with a slight decrease in bandwidth. When the EBG structure is sufficiently large and resonates at a similar frequency as the patch antenna, the high-impedance surface effect is not seen and the bandwidth actually decreases. This result is due to the similar dimensions of patch antenna and the loading elements.

Introduction Printed antennas are attractive for their low cost, low profile, and easy

integration with the rest of the system [1]. A common drawback is the narrow bandwidth, when a conductor backing is presented. Although an inverted printed F provides a much wider bandwidth by removing the ground backing underneath the wire antenna, it is not feasible for systems that require radiation isolation and ground backing. Bandwidth can be improved by using an electrically thick substrate so that the antenna and the ground are farther apart. But thick substrate will increase the system volume and weight, and more importantly will result in significant surface-wave energy trapped and lost within the substrate. The proximity-coupled feeding structures with a microstrip line electromagnetically coupled to the patch antenna a layer above are known to provide a better bandwidth than a probe-fed or edge-fed patch antenna [1].

In recently years, the applications of metalized electromagnetic band-gap (MEBG) substrate in low-profile antennas have attracted much attention. It has been shown that a plane-wave reflection from MEBG substrates has a zero-reflection phase, when the MEBG resonate with surface-wave suppression in the band-gap zone. MEBG substrates have been shown to reduce antenna profile and increase antenna array isolation. MEBG substrates are also shown to be able to shrink the antenna size and increase the antenna bandwidth in some extent [3, 4]. High-impedance (or artificial magnetic conductors) made of planar periodic elements are defined based on the far-field measurements of reflection coefficients from the planar periodic surfaces (or frequency selective surfaces) and are of questionable applications within the subjects of RF integrated circuits and antennas.

In this paper, rectangular proximity-coupled patch antenna with MEBG substrate is designed and analyzed with computer simulation (measurement results will be shown in presentation) to investigate the effects of the periodic loading to the antenna size and bandwidth.

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Antenna and MEBG Configuration The proposed proximity-coupled feed microstrip antenna with a MEBG

substrate configuration is shown in Fig. 1. The antenna structures includes two sandwiched dielectric FR4 substrate layers 31 mil (0.7875 mm) thick each, with a dielectric constant 4.6 and loss tangent 0.01. The patch antenna is on the top layer interface and the MEBG elements and a microstrip line is lower layer surface.

The initial design is for an EMC patch without MEBG loadings. The patch antenna dimensions are L= 22.14mm and W=31.59mm and are designed to resonate at 3GHz. A 50 Ohm microstrip line with 1.2 mm width is in between two dielectric substrate layers, and is proximity-coupled to the top radiation patch element with inset length Loffset to the center of the patch. The MEBG square patches are also etched on the same inner metallic layer as the feeding microstrip line. A ground plane is located on the bottom of the second FR4 layer, and is electrically connected to MEBG patches by plate-through vias. The MEBG substrate unit cells structure is depicted in Fig. 2. The square patches with width B mm and unit cell with periodicity A mm, and therefore the spacing between two adjunct patches is S=A-B mm. Vias with radius R mm are plated-through from the inner layer square patches to the bottom ground plane. In order to compare the antenna performance, a conventional PC feed microstrip antenna with the same patch size is also constructed and analyzed.

Analysis and Discussion Several cases of MEBG loadings are used to investigate their effects on a

proximity-coupled microstrip patch antenna. Six sets of A and S shown in Table I for the same antenna are used for comparison. For each case, the feeding line

Figure 1. Geometry of proximity-coupled patch antenna with a MEBG substrate.

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inset length has to readjust to obtain best matching. All simulations are done by commercial EM package Zeland IE3D based on a method of moment (MOM) technique.

The MEBG unit cell periodity A and patch size B are varied in order to find the reduction rate for the resonance frequency, with adjacent patches spacing S invariant. The return loss comparisons for the three cases, A=7, 6, and 5mm with S=1.5mm are shown in Table I. The results show that with the increase of the MEBG periodity, the resonant frequency decreases. This can be explained by the fact that the MEBG sheet capacitance becomes larger with increasing EBG patch size, and therefore the antenna resonant frequency will decrease. A 23.3% frequency reduction is observed with A=7mm, B=5.5mm, and S=1.5mm. In such a case, the patch antenna length is reduced to 0.376λ from close to 0.5λ.

The MEBG adjacent spacing S is also varied to find the trend of frequency reduction. By comparing the three cases, S=2.5, 2, and 1.5mm with periodity A=7mm fixed, the resonant frequency varied from 2.5GHz to 2.3GHz. Since the MEBG capacitance also increases while adjacent patch spacing reduces, the size reduction is expected.

It is observed that the antenna impedance varies by adding MEBG substrate. In order to achieve the optimal radiation performance, fine-tuning the feed line inset is necessary. Loffset is defined as the offset of feed line to the center point of radiation patch. The larger Loffset, the larger coupling capacitance at feeding point and the smaller series reactance is, and the impedance locus moves clockwise in Smith Chart. This effect is investigate with A=7mm and B=4.5mm, and optimal matching is obtained with Loffset = -1.75mm. The results also demonstrate the resonant frequencies are basically determined by antenna and MEBG structures, and invariant with feeding offsets.

The radiation characteristics of the proximity-coupled patch antenna with different MEBG substrate profiles are summarized in Table 1. The analysis results show that the main beam is in the broadside direction and the MEBG substrate has little effect to it. The gain, directivity and radiation efficiency are at about the same level with a conventional patch antenna but with smaller bandwidth.

Conclusion Parametric study of MEBG profiles were performed for the case of proximity coupled in this paper. It is shown that the proximity-coupled microstrip patch antenna size can be reduced over 23% by using simple mushroom-like metal EBG substrate at the same layer as the feed line. The antenna's resonant frequency depends on the radiation patch size and MEBG profiles, and the return loss and bandwidth can be fine-tuned with feeding line insets. The results also show that radiation performance of patch antenna with MEBG substrate is comparable with conventional ones. Bandwidth enhancement due to a high-impedance surface is not observed. When the MEBG resonate and at the high-impedance condition, the patch antenna sees a localized metal plate underneath As a result the effective substrate thickness is reduced and the bandwidth drops. Further investigation on using MEBG with much reduced resonant size is ongoing.

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Table 1. Comparison of antenna parameters with different MEBG periodicity,

spacing for optimal tuning

TYPE Resonant

Freq. (GHz)

Optimal Offset(mm)

Bandwidth(MHz)

Gain(dBi)

Directivity (dBi) Efficiency

Conventional 3 0 60.3 3.875 6.420 55.7% A=7mm 2.31 1.25 28.0 2.924 5.672 53.1% A=6mm 2.34 1.5 28.0 3.013 5.733 53.5%

Different periodicity and patch size, same

spacing (S=1.5mm)

A=5mm 2.47 0 32.6 3.449 5.822 57.9%

S=1.5mm 2.31 1.25 28.0 2.924 5.672 53.1% S=2mm 2.4 0 30.8 3.205 5.750 55.65%

Different spacing,

same periodicity (A=7mm)

S=2.5mm 2.49 -1.75 33.6 3.512 5.834 58.59%

Reference

[1]. J.R. James, P.S. Hall, and C. Wood, Microstrip Antennas, Peter Peregrinus, 1981. [2]. D. Sievenpiper, L. Zhang, R.F. Broas, N.G. Alexopoulos, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Transaction on Microwave Theory and Tech., vol. 47, no. 11, pp. 2059-2074, Nov. 1999. [3]. Ermutlu, M.E.; Simovski, C.R.; Karkkainen, M.K.; Ikonen, P.; Tretyakov, S.A.; Sochava, A.A., "Miniaturization of patch antennas with new artificial magnetic layers," Antenna Technology: Small Antennas and Novel Metamaterials,. IWAT 2005. IEEE Int. Workshop, pp. 87- 90, 7-9 March 2005. [4] Mosallaei, H.; Sarabandi, K., "Antenna miniaturization and bandwidth enhancement using a reactive impedance substrate," IEEE Trans. on Antennas and Propagation, col. 52, no. 9, pp. 2403-2414, Sep. 2004.

Figure 2. Dimension of MEBG unit cell parameters.

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