A self-triplexing antenna employs a slot backed with a circular substrate integrated waveguide (SIW) cavity, fed by three distinct feedlines to produce three different resonant frequencies in C-Band (4 to 8 GHz). By optimizing the antenna parameters, port-to-port isolation greater than 25 dB is obtained. The antenna operates at frequency bands around 5.6, 6.64 and 6.95 GHz, with gains of 4.48, 3.7 and 4.3 dBi, respectively. Its compact planar geometry is attractive for mobile applications.

With the unprecedented demand for compact handheld and mobile transceivers, the design of multi-band antennas with high transmit-to-receive channel isolation has attracted the attention of antenna designers.1, 2 Self-diplexing/triplexing antennas, where the antennas are fed by separate feedlines with intrinsic isolation among the input ports have recently been demonstrated.3-6 The use of a self-diplexing/triplexing antenna eliminates the need for a high-order decoupling network or frequency selective element, improving RF front-end compactness and efficiency. Cheong et al.4 introduced an integrated antenna-triplexer using a two-layered substrate, achieving good three-port isolation with a multi-mode excitation technique. Several groups developed cavity based triplexing slot antennas using SIW.5-8

In this work, a small SIW cavity-backed self-triplexing slot antenna with simultaneous three-port isolation is presented. This design uses a simple feed network of three discrete 50 Ω microstrip lines to excite the slot for three frequency operation. The SIW cavity-backed structure achieves high gain and unidirectional radiation while preserving its planar form. This makes it an attractive choice for radio navigation, radar and satellite applications.

Figure 1 shows the self-triplexing antenna layout. A circular SIW cavity is realized by embedding metallic vias in the dielectric substrate, with the via spacing and radius 1.5 and 0.5 mm, respectively.6 The theoretical cavity size (Rc) is chosen so that its dominant TM010 mode operates in the desired frequency band.9, 10 A radiating slot is etched on the top metallic layer of the cavity, splitting it into three unequal apertures. This produces three different resonant frequencies when each aperture is individually excited by inset microstrip lines.

Figure 1

Figure 1 Antenna layout, top and bottom.

The feedlines are located at the bottom of the substrate to minimize spurious effects on radiation performance. The transition between the SIW cavity and microstrip lines is achieved by adjusting the parameters g1, g2 and g3 of ports 1, 2 and 3, respectively. Their positions are also adjusted so impedance matching is achieved only at the desired frequencies.

The antenna resonates at ƒ1 = 5.6, ƒ2 = 6.64 or ƒ3 = 6.95 GHz when port 1, 2 or 3 is excited, respectively. An isolation greater than 25 dB is achieved between any two of the input ports to realize self-triplexing.


Three shorted metallic vias (v1, v2 and v3) are inserted nearby the radiating slot to increase intrinsic port isolation (see Figure 2). These vias are also useful in tuning the resonant frequencies (ƒ2 and ƒ3). The shorted via increases the difference in the magnitude and phase of the electric field on the opposite sides of the slot. As a result, it radiates into the free-space maximum through the aperture when excited at the corresponding input port. A negligible amount of the incident wave propagates to the adjoining input ports, enhancing port isolation (see Figure 3).

Figure 2

Figure 2 Simulated antenna S-parameters.

Figure 3

Figure 3 E-field isolines when each port is excited at its resonant frequency: port 1 (a), port 2 (b) and port 3 (c).