**6.6. Optimized antenna design for SLAM**

To further optimize the results of the SLAM algorithm described in this section, antennas with a broader 3 dB beam-width (>60◦) than for the object recognition in section 7 are needed.

Apart from the broad frequency band of 3.5 to 10.5 GHz in order to meet further conditions the antenna also has to be dual-orthogonally polarized. The radiation phase center should be constant over frequency, and the two polarizations should have identical radiation conditions. In literature several types of UWB antennas can be found. Most of them are either biconical

**Figure 26.** Outline of the measurement scenario.

**Figure 28.** Schematic illustration of the dual polarized antenna element and the feeding networks, all

Cooperative Localization and Object Recognition in Autonomous UWB Sensor Networks 209

The feeding networks themselves are placed orthogonally to the radiating element, see Fig. 29 (left). Similar to the Vivaldi structure, Fig. 32, a balun is used for microstrip to slotline transition. The slotline is then split up and used to feed the monopoles. The two polarizations are realized by shifting the two orthogonal feeding elements into each other as shown in Fig. 28. Therefore, a slot has to be cut into both (feeding) networks. The gaps in the metallic structures have to be closed again. This is realized by soldering through vias in the

**Figure 29.** Schematic illustration (left) and photograph (right) of the 4-elliptical antenna

units in mm).

respectively orthogonal feeding network.

(a) Reconstruction using 1800 measurements (b) Reconstruction using 120 measurements

**Figure 27.** Reconstructions of an L-shaped corridor using different numbers of measurements. Dotted lines show actual room outline.

structures [5], traveling wave radiators [16], or even a combination of both [6]. The requirement of a common phase center (over frequency) limits the possible solutions. For this purpose a planar solution, a broad-side radiating antenna is chosen and optimized.

The antenna consists of two elliptically shaped dipoles surrounded by a metallic ground plane as shown in Fig. refschematic . The ellipses for each polarization (vertical and horizontal) are orthogonal to each other. Contrary to normal dipoles the feeding is separated and placed between the ground plane and the single ellipses. This is outlined by the arrows in the radiating element shown in Fig. 28. This type of feeding allows a separate feeding for each polarization and helps to keep the current distribution in the radiation zone symmetrical resulting in a constant phase center (of each polarization) exactly in the middle of the elliptical dipoles (two monopoles) [1].

30 Will-be-set-by-IN-TECH

station 12

station 13

station 14 station 15

−18 −16 −14 −12 −10 −8 −6 −4 −2 0 2

Distance [m]

**Figure 27.** Reconstructions of an L-shaped corridor using different numbers of measurements. Dotted

structures [5], traveling wave radiators [16], or even a combination of both [6]. The requirement of a common phase center (over frequency) limits the possible solutions. For this purpose a planar solution, a broad-side radiating antenna is chosen and optimized.

The antenna consists of two elliptically shaped dipoles surrounded by a metallic ground plane as shown in Fig. refschematic . The ellipses for each polarization (vertical and horizontal) are orthogonal to each other. Contrary to normal dipoles the feeding is separated and placed between the ground plane and the single ellipses. This is outlined by the arrows in the radiating element shown in Fig. 28. This type of feeding allows a separate feeding for each polarization and helps to keep the current distribution in the radiation zone symmetrical resulting in a constant phase center (of each polarization) exactly in the middle of the elliptical

Distance [m]

station 1 station 2 station 3

−15 −10 −5 0

Distance [m]

(b) Reconstruction using 120 measurements

station 4 station 5 station 6 station 7

station 8

station 9

path of the robot

station 10

station 11

−2

lines show actual room outline.

dipoles (two monopoles) [1].

Distance [m]

**Figure 26.** Outline of the measurement scenario.

path of the robot

−15 −10 −5 0

Distance [m]

(a) Reconstruction using 1800 measurements

0

2

4

6

Distance [m]

8

10

12

**Figure 28.** Schematic illustration of the dual polarized antenna element and the feeding networks, all units in mm).

The feeding networks themselves are placed orthogonally to the radiating element, see Fig. 29 (left). Similar to the Vivaldi structure, Fig. 32, a balun is used for microstrip to slotline transition. The slotline is then split up and used to feed the monopoles. The two polarizations are realized by shifting the two orthogonal feeding elements into each other as shown in Fig. 28. Therefore, a slot has to be cut into both (feeding) networks. The gaps in the metallic structures have to be closed again. This is realized by soldering through vias in the respectively orthogonal feeding network.

**Figure 29.** Schematic illustration (left) and photograph (right) of the 4-elliptical antenna

#### 32 Will-be-set-by-IN-TECH 210 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications Cooperative Localization and Object Recognition in Autonomous UWB Sensor Networks <sup>33</sup>

As this type of antenna is radiating broad-side in both directions (forward and backward) and should be used as (mono-) directional radiating antenna, the backward radiation (illuminating towards the feeding network) must be absorbed by a carbon fiber housing as shown in Fig. 29. This results in a reduction of the radiation efficiency. An alternative would be to use a reflector, which, however, would limit the bandwidth of the antenna.

A common antenna for such an application is the so-called Vivaldi antenna. The radiation mechanism is based on exponentially tapered slots and the traveling wave principle [3]. This type of antenna has a convenient time domain behavior as shown in [59] and a relatively stable radiation pattern in the whole frequency range. In CoLOR the frequency range for the final demonstrator covers 3.5 to 10.5 GHz. A second band from 4.5 to 13.5 GHz was also used during the development process. Therefore, the objective of the antenna design is to cover the

Cooperative Localization and Object Recognition in Autonomous UWB Sensor Networks 211

A 3D illustration (left) and a photograph (right) of the fabricated antenna is shown in Fig. 31, see also [45]. Combining the integration of two tapered slot line antennas on a single substrate with embedding them into Polytetrafluoroethylene (PTFE) allows the total bandwidth to be covered. The integration of two radiation elements per polarization yields a higher gain and saves space compared with an array of two separated antennas. Furthermore, it is less cost intensive and easier to manufacture. The possibility of varying the tapering of the inner and

Both radiating structures are fed by a network shown in Fig. 32 (left), and for the second polarization in Fig. 32 (right), respectively. Starting with microstrip, where the connector is soldered on, an aperture coupling transforms to slotline which finally divides the power and

As already mentioned, the integration of the antenna elements into a dielectric reduces the effective wavelength. This affects several advantages compared to an antenna in free space. The antenna is capable of radiating a lower frequency, the far-field conditions are fulfilled in a closer distance and the shaping of the dielectric can (also) be used to focus the beam and for sidelobe suppression. For this work PTFE is chosen as dielectric. Its permittivity of *<sup>r</sup>* = 2.1 is similar to that of the substrate used (Rogers Duroid 5880 with *<sup>r</sup>* = 2.2). Furthermore, PTFE has low losses and can be easily shaped to adapt to the antenna design. The shaping and the dimensions of the PTFE structure are given in [45]. The conically shaped rod (in radiation

The two polarizations are realized by shifting two orthogonal elements into each other as shown in Fig. 31 left. For doing this, a slot has to be cut into both elements. Thus, the metalized structures are interrupted, see Fig. 32. They have to be galvanically connected again. This is

direction) allows a smoother transition of the guided wave into free space.

realized by introducing vias in the orthogonal antenna.

hole frequency range from 3.5 GHz to 13.5 GHz.

**Figure 31.** Antenna, embedded in the dielectric.

feeds it to the two elements.

outer structure, see Fig. 32, can be used to focus the beam.

The antenna characteristics are measured with a vector network analyzer in an anechoic chamber. The input impedance matching is around -10 dB between 3.5 and 10.5 GHz and the decoupling of the two ports is approximately 20 dB.

Figure 30 shows the 2D gain over frequency and angle for both planes (E- and H-plane) of one polarization in co-polarization arrangement. The measured gain of the second polarization is very similar and not specifically shown.

**Figure 30.** Measured gain over frequency and angle in the E-plane (left) and H-plane (right).

This solution of a planar, dual-polarized UWB antenna covers the frequency range between 3.5 and 10.5 GHz. In this frequency range, the radiation pattern of the antenna remains stable and directive. Both polarizations have the same radiation phase center, which is frequency-independent.
