Design Aspects of a Broadband Beam-Reconfigurable Leaky-Wave Antenna E. Abdo-S´ anchez, D. Palacios, C. Fr´ ıas, F.Y. Ng-Molina, T.M. Mart´ ın-Guerrero, and C. Camacho-Pe˜ nalosa Dpto. Ingenier´ ıa de Comunicaciones, E.T.S.I. Telecomunicaci´ on, Universidad de M´ alaga, Andaluc´ ıa Tech, E-29071 M´ alaga, Spain. Email: elenaabdo@ic.uma.es Abstract—Design aspects of a novel beam-reconfigurable pla- nar series-fed array are addressed to achieve beam steering with frequency tunability over a relatively broad bandwidth. The design is possible thanks to the use of the complementary strip- slot, which is an innovative broadly matched microstrip radiator, and the careful selection of the phase shifter parameters. I. I NTRODUCTION In order to design a series-fed array with beam recon- figuration over a broad bandwidth, phase shifters can be introduced between two adjacent radiating elements to control the phase and, then, the pointing angle. However, doing that over a broad frequency band requires a non-resonant radiating element. In [1], the authors proposed a radiating element with broad impedance bandwidth, unlike most of the radiators in microstrip technology. This element allowed the design of a novel reconfigurable series-fed array with two functionalities: beam steering and fixed-beam frequency tunability over the LTE band from 1.71 to 2.17GHz [2]. In this contribution, some aspects of the design of a series-fed array with beam reconfiguration based on a broadband element are addressed. II. DESIGN ASPECTS The proposed antenna is shown in Fig. 1. It is a microstrip series-fed array with phase shifters between the radiating elements. The radiator is a complementary strip-slot [1], which consist of a slot etched on the ground plane of a microstrip line and its complementary stub (or strip) superimposed to it in the microstrip layer. The phase shifter is based on a high- impedance microstrip line periodically loaded with lumped elements (varactors), as proposed in [3]. varactors phase shifter slot strip Fig. 1: Layout of the proposed antenna. A. Effect of the varactor polarization in the distribution of the spatial harmonics Although the Floquet’s Theorem is stated for infinite pe- riodic structures, it provides accurate information about the radiation frequency bands even for arrays with a few elements. The reverse voltage applied to the varactors modifies the phase shift introduced between the elements and, then, the phase constant of the spatial harmonics when Floquet’s Theory is applied. This effect is illustrated in Fig. 2 for a specific geom- etry (given a distance between elements, a phase shifter and a radiating element). As can be observed, the reverse voltage modifies not only the location of the phase curves with respect to the radiation cone (delimited by the red lines) but also their slope. Therefore, the design must pursue the location of a spatial harmonic inside the radiation cone at each frequency for the design bandwidth, in order to ensure radiation. This is a quite difficult task and does not have a guaranteed solution, because, as the antenna must be electronically reconfigurable, it must be achieved for each reverse voltage. Anyway, the gaps of no radiation, such as the gap between 1.9 and 1.95 GHz in the blue curve of Fig. 2, must be minimised. As the slope of the curves increases, which happens for lower reverse voltages, the appearance of radiation gaps is easier. On the other hand, for low slopes, it can happen that two spatial harmonics are inside the radiation cone at the same frequency and voltage, which would lead to two beams in the radiation pattern. Therefore, there is a trade-off in the phase parameter controlled by the distance between elements, the design of the phase shifter and the geometry of the radiating element. With these three design criteria, unique-beam radiation at all the frequencies and over the maximum range of reverse voltages should be achieved. 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 2.15 -10 -5 0 5 10 15 Frequency [GHz] β n p [rad] air line 0.2V 2V n=-1 n=-2 n=-3 Fig. 2: Spatial harmonics for two different reverse voltages. B. Influence of the number of stages of the phase shifter on the array performance In order to study the influence of the number of cells of the phase shifter on the array performance, the total length of the phase shifter is fixed (55 mm) and the array behaviour