Array bandwidth

In section 1.2.1 the concept of bandwidth applied to a single antenna was introduced. It is important to clarify that the definition of the bandwidth can depend on a single parameter or a combination of parameters. The parameters used in defining the bandwidth of the antenna (e.g. S11 and XpolR) are also used for the bandwidth of the array. In the case of an array, the grating lobes presence, secondary lobes level, gain and dual polarization capabilities are among the parameters that can also be useful. As an example, Fig. 1.30 depicts a common scenario of an array using spiral antennas. Lines represent bandwidths where the parameters respect certain range of values. The intersection of the bandwidths is generally much less than the individual bandwidths.

In this work we are interested in the intersection of the bandwidths of the S11, XpolR and RSLL, while keeping dual polarization capabilities and using a ground plane.

1.4.6.1 Design of large bandwidth arrays

We have shown that the lattice of the array plays an important role in the presence of the grating lobes and the sidelobe levels. Usually, this will set the higher limit of the bandwidth of the array. In order to obtain large bandwidths, we can also improve its lower limit. There are two main trends, or “paradigms” (as presented in (Munk et al., 2003)), while designing wideband arrays. The first one is to take antennas that already have a wide bandwidth. The second one

Figure 1.30: Example of bandwidths for different parameters in an array of spiral antennas.

relies on the strong interaction between the elements to achieve very broad bandwidths in order to obtain an idealized current distribution as proposed by Wheeler (Wheeler, 1965).

(a) Antenna array using Vivaldi antennas (Hong et al., 2006).

(b) Antenna array using BOR antennas (Holter, 2007).

Figure 1.31: Examples of wideband arrays using wideband elements.

Arrays composed of wideband elements

To this group belong the arrays composed of wideband antennas such as Vivaldi antennas (Hong et al., 2006), spiral antennas (Stutzman and Buxton, 2000), and the Wideband Array with Vari- able Element Sizes (Caswell, 2001), for the mono polarized case. Examples of dual polarized arrays are found in (Guinvarc’h and Haupt, 2010), using spiral antennas, and in (Holter, 2007), using body of revolution antennas. In these cases, the coupling between the elements can increase or degrade the bandwidth of the array. Fig. 1.31 shows some examples of arrays that follow this design trend.

Wideband array with highly coupled or connected elements

To this group belong the long slots array (Lee et al., 2008), the fragmented aperture array (Maloney et al., 2011) and PUMA array, Planar Ultrawideband Modular Antenna (Holland and Vouvakis, 2012). All of them are linearly dual polarized. Spiral antennas can also be connected using resistors, as in the dual polarized array of connected spirals (Guinvarc’h and Haupt, 2011);

or highly coupled, as in the interwoven mono polarized spiral array with a bandwidth of 10:1

(Tzanidis et al., 2011), although the latter presents a low XpolR (7 dB). Fig. 1.32 presents some arrays that were designed using this trend.

(a) PUMA array (Holland and Vou- vakis, 2012).

(b) Antenna array using connected spirals (Guinvarc’h and Haupt, 2011).

Figure 1.32: Examples of wideband arrays using connected elements.

Discussion about the arrays

Tab. 1.5 shows a summary of the most important antenna arrays mentioned before. It presents the trends used in the design, polarization cabilities, S11 and XpolR bandwidths with their respective maximum levels, the RSLL bandwidth where the side lobe level is kept below a certain value stated by the author, the frequency at which the grating lobes (fGL) will appear (computed from the distance between the elements and the maximum scan angle), maximum scan angle from broadside, intersection of bandwidths and presence of ground plane.

It can be seen that the arrays might have different bandwidths for each parameter. Usually, it is the S11 which has the largest bandwidth and exhibits the lowest cutoff frequency among the other parameters. At higher frequencies, the presence of grating lobes is the most common limitation.

Some applications need the use of planar arrays. In these cases the only options are the PUMA array and the spirals array. Vivaldi antennas can be very bulky. If a ground plane is considered, sometimes it is beneficial to have a low profile array, as in the case of the BOR antenna array and PUMA Array. A linear array of spiral antennas can be backed by a ground plane or a cavity (cf. section 1.3.3). It will be seen later that a variant of this idea is also applicable to a planar array.

Considering the design process, the most complex case is the PUMA array, which considers the design of the element, solving issues with the feeding system and scan blindness problems due to the configuration of the array. But its main advantage is its great bandwidth. The simplest design can be the spiral arrays, although having not so wide a bandwidth. Furthermore, additional issues can appear when adding the ground plane. Later it will be shown how to increase this bandwidth.

Array Design Polar. BWS11 BWXpolR BWRSLL fGL Scan ∩ BWs GND

trend level level level θ ratio plane

Vivaldi antenna Wideband Mono 8-35 GHz 10-35 GHz 8-35 GHz 35 GHz 25^◦ 10-35 GHz Yes

(Hong et al., 2006) element linear -10 dB 20 dB -6 dB 3.5:1

BOR antenna Wideband Dual 4-18 GHz 6-18 GHz No spec. 20 GHz 45^◦ 6-18 GHz Yes

(Holter, 2007) element linear -10 dB 15 dB 3:1

PUMA array Strong Dual 1.1-5.3 GHz 1.1-5.3 GHz No spec. 7.2 GHz 45^◦ 1.1-5.3 GHz Yes

(Holland and Vouvakis, 2012) interaction linear -6 dB 15 dB 5:1

Connected spirals Strong Dual 2-5 GHz 2.2-5 GHz 2-2.9 GHz 2.9 GHz 30^◦ 2.2-2.9 GHz No

(Guinvarc’h and Haupt, 2011) interaction circular -10 dB 15 dB -10 dB 1.3:1

Interleaved spirals Wideband Dual 2-7 GHz 3-6.5 GHz 2.7-5.9 GHz 5.9 GHz 30^◦ 3-5.9 GHz No

(Guinvarc’h and Haupt, 2010) element circular -10 dB 15 dB -10 dB 2:1

Table 1.5: Summary of wideband antenna arrays.

31