Ku-band Transition with not Metalized Air-Vias between Microstrip Line and Substrate Integrated Waveguide

In this letter, a new transition between microstrip line and substrate integrated waveguide in Ku-band frequency is proposed. This transition composed of one row of not metallized air-vias drilled on both sides of the microstrip line with taper end. The electromagnetic analysis is carried out using a commercial software tool. The presented transition achieves return losses better than 41 dB in Ku-band frequency (12.4-18GHz). In order to validate the simulated results of the proposed concept, a back-toback transition prototype is designed, fabricated and measured. The measured results demonstrate a minimum return loss of 29.05 dB and maximum insertion loss of 0.685 dB over the entire Kuband frequency.


I. INTRODUCTION
The emerging technology of substrate integrated waveguide (SIW), a veritable paradigm, considering migration from metal walls waveguides to the planar technology.In its simplest form, its two cylinder rows between two metals plates which can confine the electromagnetic field.Since the microstrip line is very practical in many configurations, including active components connectivity, the trend is to merge it in the same substrate with SIW, geometrically irreconcilable structures.This requires the development of transitions, junctions and couplings, with high technical quality, and therefore implies performing a good calculation model.These transitions are very important for the matching of impedance and field between the SIW and planar circuits.D. Deslandes et al. proposed the first transition between microstrip and SIW with taper form [1]. H. Nam et al. proposed a transition microstrip-to-SIW in Ku-band frequency range [2], in this transition, the return loss is about 15 dB and bandwidth is 24%.In the past years, several works about the transition between SIW and microstrip line or coplanar waveguide have been developed ( [3][4][5][6][7][8]).In [9] another transition from microstrip-to-SIW is proposed.This transition are formed by placing two vias symmetrically at either side the microstrip taper, this vias and the SIW vias, have the same diameter.In Ku-band frequency, this transition gives return losses better than 35dB.

Ku-band Transition with not Metalized Air-Vias between Microstrip Line and Substrate Integrated Waveguide
Farouk Grine , Mohamed Taoufik Benhabiles and Mohamed Lahdi Riabi Laboratoire des électromagnétismes et télécommunication, Université de Constantine 1, Route d'Ain El Bey, Constantine 25000, Algerie Farouk Grine; faroukgrine@umc.edu.dzMohamed TaoufikBenhabiles: mt.benhabiles@umc.edu.dzMohamed LahdiRiabi:ml.riabi@yahoo.frimpedance.This width is generally selected to achieve a characteristic impedance of 50Ω and we calculate the ratio w 0 / h by the following formula [10]: Fig. 1. tapered transition between microstrip line and SIW Thus, as "h " is the substrate height, it becomes easy to calculate the value of w 0 .It remains now the value of two other transition parameters to calculate.The w must be calculated by equating the two right sides of the equations ( 4) and ( 5) and solving it (w e is the width of an equivalent waveguide) [11]: For the parameter l we can use a simpler method by choosing the median width between the microstrip line and the end of the transition, as follow: It suffices to calculate the wavelength for this width and set the length to aquarter of wavelength.We must first find the effective dielectric constant for this microstrip line width [11]: We must then calculate λ c with the following equation:

III. PROPOSED TRANSITION STRUCTURE AND DESIGN
The new transition we propose is achieved by drilling one row of not metallized air-vias on both sides of the microstrip line with taper form.Fig. 2 presents the transition between microstrip and SIW.In this figure the configuration ends with regular waveguide.Fig. 2(a) presents the transition without air-vias on both sides of the microstrip line.The new microstrip-SIW transition with not metallized air-vias is presented in Fig. 2(b) and (c).In this figure, a SIW is the SIW width, W equi is the equivalent width, d is the diameter of vias, p is the vias inter-distance, w t is the width of taper form, w m is the width of the microstrip line, and l t is the length of taper form.In this study, two structures have been investigated.In the first one, the substrate is made of RT/duroid 6002 with electric permittivity ε r =2.94, loss tangent of 0.0012.In the second structure, the dielectric is chosen as Rogers RO3003 with ε r =3, loss tangent of 0.0013, and the substrate height is h=0.508mm for both taper-via transitions and taper not metallized airvias transition.
Because the fields in the microstrip line are concentrated near the line, only one row of air-vias is needed on both sides of the microstrip.Comparing to the other microstrip transitions, the advantage of this new structure, is that there is better field matching between the SIW and micosrtip due to the confinement of the fields around the microstrip line.In order to validate this configuration we compare with taper transition without air-vias proposed in [9].The structure consists of SIW substrate with ten metallized vias and thirteen not metallized air-vias at the both sides of the microstrip line.The width of SIW is obtained directly from [12], and the effective width can be calculated as: Where c is the speed of light, f c is the cutoff frequency and ε r is the relative permittivity of the substrate.

IV. RESULTS AND DISCUSSION
The simulations were carried out using commercial electromagnetic simulator: Ansoft HFSS.In the first structure, the final parameters of the simulated proposed transition are presented in table.1.Theoritically, The transition between microstrip line and SIW is much better when the return loss is low; which is achieved by having a good match between the microstrip and SIW in the fields and impedance.In order to do that, we have to maintain the greatest value of the fields on both sides of the microstrip.Fig. 3.
Shows E and H fields distribution in the transition for (a) the proposed taper air-via transition and (b) the conventional taper via transition, where the incident wave has been chosen with the same phase.From this comparaison, we notice that the fields are better confined when adding the air-vias on both sides of the microstrip line which lead to a best transition with SIW.Fig. 4 shows the comparison between the simulated S parameters of the proposed transition and those from [9] in Ku-band frequency.
After the optimization in the parameters of the transition, in the simulation results we can see that the insertion loss is lower than 0.16 dB for taper-vias transition [9] and lower than 0.15 for the proposed transition in the entire Ku-band (12.4 -18 GHz), from Fig. 4(a).About the return loss in fig.4(b), the results show that the return loss is higher than 35 dB for taper-vias transition [9] and higher 41 dB for the proposed air-vias transition.In the second structure, some parameters have changed and presented in the table.2.Two holes from both sides of the microstrip that have a distance w 4 between them were removed, (Fig. 2.c) and the substrate is chosen as Rogers RO3003 with ε r =3.Fig. 5 shows the comparison between the simulated S parameters of the proposed transition and the taper-via transition in Ku-band frequency.From Fig. 5(a), we can see that the insertion loss is lower than 0.17 dB for taper-via transition and lower than 0.16 for the proposed transition. in Fig. 5(b), the return loss is higher than 33 dB for taper-via transition, and higher than 40 dB for the proposed air-via transition.In this structure, back-to-back transition is simulated for conventional taper-via and taper with air-vias.Fig. 6 presents the comparison between the simulated S parameters of the two transitions.We can see that the return loss is higher than 31 dB for taper-via transition, and higher than 35 dB for the proposed air-via transition, in the entire Ku-band .
In order to validate the simulated results, an experimental prototype of the proposed back-to-back transition with air-vias was fabricated (Fig. 7), where the substrate made of RT/duroid 6002 and the parameters used are presented in table 1.Because of drill size restrictions, the simulation via diameter (cf. Table 1) was changed for d 1 and d 2 to 0.95 mm.Fig. 8 presents the comparison between the simulated and measured results of the new transition with not metalized air-vias and we can see that the simulation results are in good agreement with the measurements.From Fig. 8 (a), in the Ku-band frequency , the proposed transition shows that the return loss is higher than 35 dB for the simulated results

Fig. 2 .
Fig. 2.Structural parameters of the proposed transition between microstrip and SIW (a) Structure parameters of taper transition without Air-vias.(b), (c) Structure parameters of taper transition with Air-vias.

Fig. 3 .
Fig. 3. E and H fields distribution in the transitions : (a) proposed taper air-via transition; (b) conventional taper via transition.

Fig. 5 .Fig. 6 .
Fig. 5. S-parameters of the Transition micorstiop to SIW. (a)Transmission coefficients ; (b) Reflection coefficients -vias S11 without Air-vias and higher than 29 dB for the measured results whereas in Fig.8(b) the simulated results of the insertion loss is lower than 0.68 dB and about 0.685 for the measured results.

Table 1 :
Parameters dimensions for the first structure.

Table 2 :
Parameters dimensions for the second structure