диафрагмированные волноводные фильтры / caca18db-fe0b-41d5-b67b-233a720afc2f

Abstract — In this paper, a novel mode decoupling technique of a SIW dual mode square resonator is presented. It is then utilized to design a self diplexing radiator. A single corner cut square cavity with a triangular ring slot on top is used for dual mode excitation and mode decoupling. Both the electric and magnetic mode couplings are used simultaneously to improve the port isolation. Once the single square cavity is optimized for good port isolation, two extra TE110 mode cavities are further added to improve the isolation. Finally, 37 dB port isolation is achieved. The measured gains are 4.45 dBi and 4.20 dBi at 10.5 GHz and 11.7 GHz respectively. The proposed design can be used in a transceiver system without any circulator, switch or additional bandpass filter.

Index Terms — Diplexer, dual-band antenna, dual mode resonator, substrate integrated waveguide (SIW).

I. INTRODUCTION

In modern communication systems, compact, low profile, dual, and multiband antennas are in great demand as they minimize the number of antennas in a system. On the other hand, the substrate integrated waveguide (SIW) offers compact circuit size and lower loss [1]. Because of these advantages, SIW based antennas are becoming popular. In [2], cavity backed dual-band SIW antennas radiating at 9.4 GHz and 16 GHz with a dual-mode triangular ring slot are presented. In [3], a dual-band circularly polarized antenna is reported that uses a circular slot on top of a SIW cavity. SIW based various dual band antennas are reported in [4]-[6]. However, for all the dual-band antennas in [2], [4], and [6], port isolation is poor. In [7], a filtering radiator in multilayer SIW technology is presented for improved port isolation, operating at 2.0 GHz and 2.10 GHz. Measured isolation between the two ports is 15 dB. On the other hand, self-diplexing antennas offer good port isolation without other additional components. A dual frequency antenna centered at 2.45 GHz and 5.5 GHz with double T-stub loaded aperture is presented in [8]. Self-diplexing antennas with the feeds at two

Fig. 1. Layout of the proposed dual mode, dual frequency radiator.

different ports have been presented in [9] and [10]. In [11], half mode SIW based self diplexing antenna is presented operating at 8.97 GHz and 11.3 GHz. In [12]- [13], SIW based self diplexing antennas are presented for X-band applications. SIW based self triplexing antennas in which excitations at three isolated ports radiate at three different frequencies are presented in [14]-[15].

In this paper, a SIW based self-diplexing radiator is presented. The proposed structure is based on a single SIW square cavity with orthogonal feed lines for the two radiating frequencies. The dual frequencies and isolation between the two ports are achieved by a novel mode decoupling technique. The tuning of resonant frequencies, improvement of isolation, input matching and radiation characteristics are discussed in detail.

II. DUAL MODE SIW CAVITY RADIATOR

Fig. 1 shows the proposed dual mode SIW self diplexing radiator with a triangular slot. Port-1 excites TE210 resonant mode while port-2 excites TE120 mode. A 1.58 mm thick RT/Duroid 5880 substrate with εr = 2.2, tanδ = 0.0009, and the Ansoft’s HFSS full wave simulator are considered for all the studies.

A. Dual mode SIW square cavity

Without any perturbation, the modes inside a dual mode square cavity are orthogonal to each other, hence they are isolated [13]. A square SIW resonator that supports TE120 and TE210 modes at 11 GHz is designed first. The optimized SIW via diameter d, pitch p are p = 2d = 2.0 mm [1]. The cavity dimensions are W = L = 20.9 mm to set f210 = f120 = 11 GHz [16]. Initially, feed offsets x1 = x2 = 0 mm. The cavity is excited by two 50 Ω microstrip lines of width Wm = 4.58 mm. The feeding gaps xf1 and xf2 control the external quality factors, hence the input matching. The field plots in Fig. 2 show that an electric wall bisects the unused port symmetrically, which results negligible transmission between the two ports. As the resonator is square in shape and fed symmetrically, the two ports excite the resonator at same frequency. To design a self diplexing antenna, it is essential that the resonator supports two different resonant frequencies along with a good port isolation.

B. Mode decoupling

At first, for radiation, some popular radiating slots like square, circular, and triangular ring slots having same outer peripheral length and width of 30 mm and 1.0 mm, respectively, are placed on the top plane of the SIW cavity. To minimize the loading of the slots on the feeding, the slots are placed at the furthest corner of the cavity as shown in Fig. 1. The peripheral length of 30 mm of the test slots support radiation near 11 GHz as it is nearly equal to one wavelength at the mentioned frequency. Fig. 3 shows the |S|-parameters for the three types of slots. The triangular slot is finalized since it provides highest asymmetry hence maximum mode separation resulting at least 12 dB of port isolation at f210 = 10.4 GHz, and f120 = 11.62 GHz. Initially, the triangular ring slot has a = (W/2 - 0.75) mm and ws = 1.0 mm to avoid any fabrication difficulties. Retuned values are W = L = 19.86 mm to keep (f120 + f210)/2 ≈ 11 GHz.

The vector H- field distributions in Fig. 4 shows that with the slot, the modes are not orthogonal and at lower resonant frequency, fields have nearly odd symmetry

(a) (b)

Fig. 2. (a) |S|-parameters of a square cavity without any perturbation. H-field isolines for (b) port-1 and (c) port-2 excitations (f210 = f120 = 11.0 GHz).

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Fig. 3. |S|-parameters for different types of slots.

(a) (b)

Fig. 4. Vector H-field distribution for a = 9.2 mm at (a) f210 = 10.4 GHz and

(b) f120 = 11.62 GHz.

hence they are electrically coupled [17]. This coupling degrades the port isolation. To nullify this coupling, the corner perturbation of dimension t is introduced, which offers magnetic coupling [18]. Corner perturbations are widely used to implement dual mode filters. The coupling and creation of transmission zeroes (TZ) are well described in [18]. As the electric and magnetic couplings are opposite in phase, they cancels each other [17]. Fig. 5(a) shows that the port isolation is considerably improved and two dips (TZs) in |S21| are present whereas the even symmetry of vector H-field plot at lower resonant frequency as of Fig. 5(b) confirms that the corner perturbation results magnetic coupling.

However, t decreases the parallel inductance that shifts f210 and f120 to higher frequencies [18]. Therefore, W and L are further tuned to bring back the original f120 and f210. With W = L = 20 mm and a = 9.2 mm, f210 = 10.45 GHz and f120 = 11.65 GHz.

Next, a, ∆1, ∆2, x1, x2, and t are optimized for best isolation. The study starts with W = L = 20 mm, a = 9.25 mm, slot offsets ∆1 = ∆2 = 0 mm, x1 = x2 = 0 mm and the slot width ws = 1.0 mm. The initial values of a, ∆1 and ∆2 are chosen to keep at least 0.25 mm gap from slot edge to side wall vias to avoid any fabrication difficulty. Fig. 6(a) shows that a = 9.25 mm results best isolation at two frequencies. Larger a increases the patch area inside the slot which increases the capacitance hence f120 and f210 decreases. Fig. 6(b) and (c) show that the optimum values of ∆1 = 0.0 mm and ∆2 = -0.5 mm.

(a)

(b) (c)

Fig. 5. (a) |S|-parameters with and without perturbation t in presence of the triangular slot. (b)Vector H-field distributions inside the substrate without any slot with t = 5.0 mm at (b) f120 = 10.9 GHz and (c) f210 = 11.75 GHz.

The ∆1 and ∆2 are positive when the slot moves along the positive X and Y directions. Next, the feeding offsets x1 and x2 are optimized. Both x1 and x2 are considered positive when the offsets are in the direction of positive Y and X axes, respectively. Positive x1 and negative x2 brings the two ports closer which results stronger source to load coupling. For a fixed value of t, proper strength of source-load coupling will result maximum isolation. Fig. 7 shows the variation of |S|-parameters with x2 and x1. The parametric studies suggest that x2 = -0.5 and x1 = 0 mm provide optimum source to load coupling for best isolation. Finally, t is tuned for optimum magnetic coupling to cancel the effect of electric mode coupling due to the slot, hence improves isolation. Fig. 8(a) shows the responses with t. Fig. 8(b) shows the simulated optimum and measured response for W = L = 20 mm, p = 2d = 2.0 mm, a = 9.25 mm, ∆1 = 0.0 mm, ∆2 = -0.5 mm, x1 = 0.0 mm, x2 = -0.5 mm, and t = 5.62 mm. The minimum port isolation at resonant frequencies of f1 =

f210 = 10.4 GHz and f2 = f120 = 11.6 GHz is 26 dB. From all the studies, it can be noted that further fine tuning of frequencies is possible by tuning W, L, and t for fixed a.

C. Improvement of isolation

To further improve the port isolation, as shown in Fig. 9(a), two extra resonators R1 and R2 are added at port 1 and port 2, respectively. The R1 and R2 resonate in TE110 mode at 10.4 GHz and 11.6 GHz, respectively. The 10 dB input matching bandwidths are now 3% and 2.2% at f1 = 10.5 GHz and f2 = 11.7 GHz, respectively. The shifts of mid-band frequencies are due to the loading of R1 and R2. The optimized parameters of the modified structure

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(a)

(b) (c)

Fig. 6. (a) Variation of |S|-parameters with a, (b) ∆1 (∆2 = 0 mm), and (b) with ∆2 (∆1 = 0 mm) for W = L = 20.0 mm.

(a) (b)

Fig. 7. Variation of |S|- parameters (a) with x2 and (b) with x1 (thick lines represent the optimized values).

(a) (b)

Fig. 8. Variation of |S|- parameters (a) with t and (b) the optimized response (thick and continuous lines denote the measured response of the single cavity prototype diplexing antenna, photograph shown in inset).

are L1 = 12.95 mm, W1 = 13.71 mm, L2 = 11.43 mm, W2 = 12.70 mm, Wf1 = 7.21 mm, Wf2 = 5.99 mm, xf1 = 6.10 mm, xf2 = 4.10 mm. Other parameters are unchanged.

III. FABRICATION AND MEASUREMENT

Finally, the self diplexing radiator with extra cavities is fabricated using 1.58 mm thick RT/Duroid 5880 substrate. The two radiating frequencies are f1 = 10.5 GHz and f2 = 11.7 GHz. Measured peak gains are 4.45 dBi and 4.2 dBi at f1 and f2, respectively. On both E and H-planes, cross-pol levels are better than 16.5 dB at f1.

(a) (b)

Fig. 9.(a) Proposed self diplexing antenna and (b) simulated and measured |S|- parameters of the modified diplexing antenna (photograph is shown in inset).

Fig. 10. Simulated and measured radiation pattern at f1 = 10.5 GHz on (a) E- plane, (b) H-plane and at f2 = 11.7 GHz on (c) E-plane, (d) H-plane.

At f2, it is better than 16 dB on H-plane and 13.5 dB on E-plane. The port isolation is improved to at least 37 dB. Fig. 9(b) and Fig. 10 show the measured results.

IV. CONCLUSIONS

TABLE I

COMPARISON WITH OTHER REPORTED SIW DIPLEXING ANTENNAS

Here the presented work is the design of a self diplexing antenna using mode de-coupling technique where electric and magnetic couplings inside a dual mode cavity are simultaneously used to improve port isolation. It offers linear, orthogonal polarization at two frequencies. Unlike [2], here the slot is used only for radiation whereas two cavity modes of a dual mode SIW

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cavity are used for dual frequency operation. In [13], similar cavity modes are used but no mode decoupling technique is proposed. Table-I shows the performance comparison of the proposed antenna with other reported SIW based self diplexing antennas.

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