Design Investigation of a Laminated Waveguide Fed Multi-Band DRA for Military Applications

1 Introduction

Laminated Waveguide knew as Substrate Integrated Waveguide (SIW) or post wall waveguide. The DRAntenna have several eye-catching merits such as high radiation efficiency, small size, low cost, and low loss. All these advantages make DRA be an appropriate choice for wireless communications as an antenna element [1].

There are several conventional techniques for exciting the DRAntenna such as co-axial probe feed, aperture coupling, dielectric image guide, co-planar waveguide and perpendicular feed structures. In all these feeding schemes, microstrip line feeding technique is frequently used for DRAntenna due to straightforward fabrication and compatibility with planar microwave integrated circuits [2]–[4].

Microstrip-line fed DRAntenna, will show conductor loss and dielectric loss at the millimeter wave range frequencies. The structure will also produce back radiation and parasitic radiation at higher frequencies which will affect the DRA’s radiation pattern [3]–[5]. Since the Substrate Integrated Waveguide is enclosed by metal plates and feeding fields are restricted in the substrate, it has an exceptionally good shielding between exterior and interior, thus avoiding radiation loss and parasite radiation of the feeding structure [6].

For zero electromagnetic wave leakage, through hole pitch should be smaller than a quarter wavelength (λ/4) [7]. The Substrate Integrated Waveguide (SIW) technology exemplifies an evolving approach for the implementation and integration of microwave, millimeter components [4]–[6]. SIW permits to comprehend traditional rectangular waveguide components in the planar form. It is compatible with planar processing techniques like low-temperature co-fired ceramic (LTCC) or standard printed circuit board (PCB) technology [6, 8]. High-density integration of components is possible in Substrate Integrated Waveguide. SIW technology permits integration of active, passive components; and antennas in a single substrate. This reduces the losses and parasitic effects (8).

2 Antenna structure

The geometry of the proposed antenna is shown in Figure 1. It consists of an SIW cavity on arlon cu clad 217(tm) of permittivity 2.17 and tanδ=0.0009 with a 50 Ω Coaxial feed line. Rogers TMM10i material (εr=9.8,tanδ=0.002) is used for DRA which is excited by two asymmetric parallel slots, which are equal in length (6 mm) but different in widths (1.3 mm & 0.8 mm). Single slot can be used to get a dual operating band. Dimensional details are mentioned in Table 1.

Figure 1:

Configuration for the proposed SIW fed DRA (a) 3D view (b) Top view.

3 Theory and design

Size of the SIW cavity is determined with the help of resonance frequency –

In case of Substrate Integrated Waveguide (SIW), the dominant mode is same as standard size rectangular waveguide, hence for TE₁₀ mode –

Where wSIW and LSIW are the effective width and length of the SIW cavity respectively. The WSIW and LSIW can be defined as by the following equations-

W1 and L₁ are the actual width and length of the SIW cavity. The diameter of the metal via is d₁ and p₁ is the distance between adjacent via holes as shown in Figure 2.

Figure 2:

The diameter (d₁) of via and pitch (p₁).

There are following conditions that are required for SIW design

There are three type of loss mechanisms present in the SIW structure i. e. conductor, dielectric and radiation loss. Conductor and dielectric losses are due to the finite conductivity of conductor and dielectric material. An appropriate choice of conductor and the dielectric material can reduce the involvement of first two losses [9]. Conductor, Dielectric, and Radiation losses are empirically formulated as [9]–[10] –

Where, α_c=conductor loss, α_d=dielectric loss, β=phase constant, k=free space wave number, h=height of substrate, tan δ=dielectric loss tangent, αR=radiation loss, λg=guided-wavelength, f=operating frequency, c=velocity of light, σ=conductivity, ε=permittivity, ɳ=intrinsic impedance of the medium, Rs=surface resistivity of the conductors.

The experimental proof shows that the attenuation constant increases in proportion to the diameter of the via. In order to ensure that the synthesized waveguide section becomes radiation less or free from leakage loss, we have used radiation loss empirical formula for showing the effect of p1d1 ratio on radiation loss of given SIW structure at 10 GHz. To simplify the loss analysis of SIW, dielectric and conductor losses are not considered, the loss solely comes from radiation.

For Zero radiation loss p₁ and d₁ should be equal, this condition corresponds to conventional rectangular waveguide. In the case of SIW, the loss tends to decrease as the vias get smaller for a constant d₁/p₁ ratio, which is conditioned by the fabrication process. The condition on the dimension of the diameter of vias and pitch length are sufficient but not always necessary, a diameter larger than one fifth of guided wavelength or a pitch larger than two diameters can be used but carefully. These two conditions on p₁ and d₁ guarantee that the radiation loss is kept at a negligible level. When we follow the two above-mentioned conditions, the mapping from the SIW to rectangular Waveguide is nearly perfect in all the single-mode bandwidth [11]. We can see from Figure 3, the radiation loss almost tends to zero up to p1d1 is 2, after that it increases exponentially.

Figure 3:

Effect of p1d1 ratio on radiation loss.

Hence the pitch (p₁) must be kept small to reduce the leakage (radiation) loss. The TM₁₁₀ mode resonator frequency of a cylindrical DRA can be calculated using [12]

Where c is the velocity of light in vacuum, H and d are the height and diameter of dielectric resonator, respectively.

In cylindrical DRAs, Q-factor and the resonant frequency are determined by the radius to height ratio (i. e. aspect ratio of the antenna). While changing the aspect ratio of the Dielectric resonator Antenna, We can maintain the same resonant frequency (but not necessarily the same bandwidth) for a given dielectric constant [13].

4 Parametric study

The most important parameters for the design of the proposed antenna is the width of the slot, offset of the slot (Xs, Ys) and the gap between two slots. These parameters control the number of operating bands and an amount of the coupling energy between the SIW and the Dielectric Resonator. Obviously, the dimension and material of Dielectric resonator influence the coupling too, their size is given by the equation [12]. The influence of offset (Xs) position of the solo slot on the behavior of the return loss and operating band is shown in Figure 4 (a) & (b). By seeing the Figure 4 (a) & (b), we can conclude that the resonating frequency of DRA depends on type & position of excitations. The same dimension of DRA resonates at different frequencies due to its hybrid mode nature by using different excitations. When we provide an offset in the position of the slot under DRA it excites the DRA with different field variations in present mode or it will start operating in different modes, that’s why variation in operating bands occur. Return loss and bandwidth depend on coupled power from slot to dielectric resonator. Due to variation in slot positions, there is a mismatch of impedance between the SIW and dielectric resonator through the slot, which results in nonlinear variation of return loss and bandwidth. The offset of the slots primarily controls the amount of the coupling, and minor shifts in the resonance frequency (a, b) as well as it increases the number of operating bands (b). The effect of slot variation is summarized in Tables 2 and 3.

Figure 4:

Effect of single slot position offset on operating band.

It is very challenging to achieve multi-band with one slot. We have achieved dual band with single slot but by using two asymmetric slots with small gaping between them three bands has been achieved. This third band comes into existence mainly due to mutual coupling between two asymmetric slots. It is not possible to achieve a different band by mutual coupling if the slots are symmetric.

Here we also studied the parametric variation (Figure 5) of the gap between slots which are a very sensitive parameter to control operating band. Here we varied gap from 0.5 mm to 0.1 mm when the gap is 0.5 mm lower order band has return loss 10 dB only. At g= 0.4 mm lower order band has return loss 15 dB while other two bands are return loss 21 dB and 20 dB respectively. But at g= 0.3 mm three operating bands has achieved with return loss 15.6 dB, 37.8 dB, and 41 dB respectively. When the further gap is reduced only two bands achieved. At g= 0.1 mm the only band appear at 10.14 GHz with return loss 24 dB.

Figure 5:

Effect of gap between slots on operating band.

5 SIW and DRA fabrication technique

The technological aspects are an important point for the implementation and development of Substrate Integrated Waveguide structures, especially for applications in the microwave frequency range. Conventional PCB techniques have been widely used to realize SIW structures, due to the reduced manufacturing cost and the great design flexibility. In this case, the conductive holes are created either by micro-drilling or by laser cutting and metallization of vias inner walls are performed by using a conductive paste or metal filling. The PCB technique exhibits an extra advantage because it allows the integration of the complete systems on the same substrate with the same fabrication technique. At higher frequencies, radiation issues can arise, due to fabrication constraints which prevent the longitudinal spacing between metal vias going below a certain value. A possible solution to this problem can be resolved by the via holes are replaced by metallized slots in a circuit by the electroless chemical process. LTCC technology has also been used in SIW implementation. Here the fabrication process of proposed design has done by the following steps: Firstly creation of SIW cavity on Arlon Cu clad 217(tm) (εr=9.8,tanδ=0.002) by drilling as design consideration (holes diameter d₁ and distance between holes p₁) and electroless chemical process for metallization. Conventional PCB fabrication process is used for creation of slots and feed point. Next, proposed Dielectric Resonator is cutout of required dimension in a cylindrical shape by laser cutting techniques from Rogers TMM10i (εr=9.8,tanδ=0.002) material. It is fixed over the substrate at appropriate location with Eco stock paste (εr = 10).

6 Result analysis and discussion

There is a slight variation in band position between simulated and measured results. Comparison of the Measured and simulated return loss results are provided in Figure 6.

Figure 6:

The simulated and measured return loss for proposed antenna.

Figure 6 clearly shows that the measured results are in close agreement with simulated results. At all the resonating frequency bands i. e. 9.76 GHz, 10.53 GHz and 11.8 GHz, the antenna shows 56 MHz, 160 MHz, and 250 MHz impedance bandwidth (for S₁₁< –10 dB) with 6.3 dBi, 6.3 dBi, and 7 dBi gain respectively.

The photograph for the fabricated SIW-DRAntenna is presented in Figure 7, overall structure dimension is very compact. SIW cavity is made by PTH technology, but in the simulating structure, we took conducting cylinder filled in the cavity. That’s why, measured return loss is better than simulated result with slight shift in resonance frequencies.

Figure 7:

Photograph for the proposed SIW-DRAntenna.

Figures 8–10 shows antenna patterns at the resonant frequencies 9.76 GHz, 10.53 GHz, and 11.8 GHz. It can be seen that the difference between co-polar level (Gθ) and cross polar level (Gφ) are very high in both the vertical planes (φ=0° and φ=90°). A cross-polar level of 29 dB lower than that of the co-polar level at 9.76 GHz, the cross-polar level of 32 dB lower than to that of the co-polar level at 10.5 GHz, a cross-polar level of 30 dB lower than that of the co-polar level at 11.8 GHz in XZ plane. But in YZ plane we can see the level difference is comparatively less but more than 18 dB. For clear visualization of excited mode and gain pattern of Proposed structure, simulated 3-D gain pattern at three operating frequencies as well as E-field, H-field, and current distribution for the proposed antenna at highest operating band center frequency are shown in Figure 11.

Figure 8:

2-D radiation pattern (co-polarization & cross-polarization) of SIW DRA at frequency 9.76 GHz in (a) Phi=0° (X-Z Plane) and (b) phi=90° (Y-Z Plane) simulated and measured.

Figure 9:

2-D radiation pattern (co-polarization & cross-polarization) of SIW DRA at frequency 10.53 GHz in (a) Phi=0° (X-Z Plane) and (b) phi=90° (Y-Z Plane) simulated and measured.

Figure 10:

2-D radiation pattern (co-polarization & cross-polarization) of SIW DRA at frequency 11.8 GHz in (a) Phi=0° (X-Z Plane) and (b) phi=90° (Y-Z Plane) simulated and measured.

Figure 11:

(a) 3-D far field pattern of total gain (simulated); (b) E-field, H-field and current distribution (simulated) at 11.8 GHz.

It is clearly shown from Figure 11(b) that TE_11δ mode is excited, so we can say that at lower operating frequencies same mode will excite because it is dominant mode for cylindrical structures.

Figure 12:

Measured peak gain vs frequency & estimated radiation efficiency vs frequency plot.

We can see here much resemblance in both of the plots in Figure 12, where Radiation efficiency is computed by measured peak gain and simulated directivity, With the relation G=η*D.

Two important parameters for Dielectric Resonators are-(1) material loss should be very less (2) It should have a high-quality factor. But in DRA moderate Q-factor is used due to enhancing of its radiation characteristics. For moderate quality factor, its permittivity should be in the range of 5–15. Variation of a Loaded quality factor with frequency is computed by CST MW studio is shown in Figure 13. Overall antenna performance has been summarized in Tables 4 and 5 shows a comparison of the proposed antenna with Moosaei et al. paper mentioned in reference no. 1

Figure 13:

Estimated Q-factor.

7 Conclusion

A linearly polarized dual slot fed multi-band DRAntenna has been successfully implemented. Antenna has very good return loss, narrow impedance bandwidth (VSWR < 2), more than 90 % efficiency and stable radiation pattern over its operating band. SIW feeding technique is used to enhance the power handling capability of the planar feed line. This antenna offers multi-band with moderate gain and narrow bandwidth, which makes it highly selective and suitable for secure military communication beyond LOS. It is most suited for defense applications due to its operating band (X-band) characteristics, which include Interference Resilience, Rain Resilience, and Remote coverage It can be used in many civil applications like weather monitoring, air traffic control etc.