Presentación y descripción de datos

5.4.1

Antenna configuration

To feed the measured power into the receiver, discussed in the previous sec- tion, an antenna is needed. Because Multi-copters are small compared to the measured wavelength, antenna design becomes an important part of the sys- tem. A Multi-copter tends to use its entire platform to control its position. As a result, an antenna attached to the vehicle will need to either compen- sate for its movement or the logged data needs to be post-processed for each antenna position. The first option can be achieved by using an antenna gim- bal. However, changing the orientation of the antenna about the vehicle will change the antenna pattern. Given enough time it might have been possible to correct for each gimbal position. However, in the end it was decided to go for a much simpler static-antenna design. The problem however with a static antenna is its variability in certain orientations. This becomes a problem when the source is located through the null of the Multi-copter antenna. To counter this, a dual antenna design was chosen. Each antenna is placed at a 45-degree angle, 90-degrees rotated around the vehicle as seen in Fig. 5.10a.

Each antenna has its own receiver and will be continually logged. The an- tenna used in the final measurement is only selected in post-processing. This is based on the antenna pattern, vehicle orientation and relative direction to the source. Fig. 5.10b shows how the two antennas, measured individually, forms an almost isotropic antenna pattern. An added advantage of the static config- uration is that the antennas themselves can be embedded into the structure of the Multi-copter. Using 3D printing, the legs of the Multi-copter could be designed within a CEM package and directly exported and printed in the lab- oratory (See Fig 5.10a). This meant that the constructed antenna leg should behave very much like the simulated model.

(a) Antenna Layout (b) Antenna Pattern

Figure 5.10: Photo showing the layout of the dual antenna configuration. In (b), the isotropic antenna pattern formed by an idealised dual-configuration can be seen.

CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 76

Figure 5.11: A photo in-flight showing the grounded-copper shield enclosure. The compass and GPS due to the nature of their operation are still placed on the top of the cube. Photo was taken by Dr. P. G. Wiid.

The antenna pattern is a function of the wires and metallic structures on the vehicle. The complexity of these structures makes them almost impossi- ble to simulate in a CEM package. Added to this, after each battery change, the position of the wires might shift slightly which will in turn change the antenna pattern. Therefore, to maintain a stable antenna pattern throughout a measurement campaign, it became necessary to have a stable metallic envi- ronment. Consequently, it was decided to enclose most of the on-board wiring into a grounded copper-cube seen in Fig 5.11. This meant that perturbations to cabling inside the vehicle could be mostly isolated from the antennas. The global positioning system (GPS) and compass subsystems were still placed on the top of the shielding enclosure to ensure their operation.

5.4.2

Broadband antenna design

A drawback of the integrated antennas is that they are not easily exchanged. With limited time on campaigns, it was important to design a single antenna that would cover a reasonable amount of bandwidth. The bandwidth of the receiver is 260 MHz to 960 MHz. This is not achieved easily on a platform the size of a Multi-copter. However, with some compromise, a design was attain- able using a resistively-loaded monopole. This design works for the mentioned

CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 77 0 5 _{Length [cm]}10 15 20 0 500 1000 1500 2000 2500 3000 Resistance [Ohm] 82.37 110.8 160.6 257 494.2803.1 2142

Antenna Loading Profile (Wu-King)

(a) Loading Profile (b) Leg Antenna Model

Figure 5.12: The loading profile of the monopole can be seen in (a). In (b) a picture of the initial simulation model, used in CST MWS R

, can be seen. One half of the leg here is removed to reveal the antenna with its loaded elements. bandwidth with limited efficiency. In the interest of saving weight and main- taining a rigid landing structure for the Multi-copter, the integrated antennas were limited to a length of 20 cm. Using this length the monopole is discretely loaded with resistive elements, using a Wu-King [75][76][77] profile seen in Fig. 5.12a. To ensure the proper functioning of the design, a CEM model was constructed which can be seen in Fig. 5.12b. In this image, it is possible to illustrate how the Multi-copter arm was used as a ground plane. The antenna here is encapsulated in a PLA plastic that also functions as the Multi-copter landing gear. Encasing the element inside a dielectric would also help lower the minimum frequency of the antenna. After investigating the simulated model, it became apparent that the loaded antenna would be well matched to a 200 Ω system. A 4:1 impedance transformer was used to match the cur- rent 50 Ω system to the 200 Ω antenna. It was not possible to replicate the simulated performance. The characteristics of the transformer decreased the overall measured performance of the antenna. Because of this it was decided to use the antenna in its unmatched form. To compensate for this mismatch, as well as the inefficiency of the loaded monopole, a broadband low noise amplifier (LNA) (ZFL-1000LN+ see Appendix E.3) was added between the antenna and receiver. This simplified simulation expedited the optimisation and testing of the antenna to the point where the full vehicle could be simulated.

After confirming the operation of the antenna, it was simulated with the entire vehicle (see Fig. 5.13a). This included a copper shield enclosing the electronics. The simulated input reflection compared to its measurement can be seen in Fig. 5.13b. The agreement between the two datasets was deemed good for this complex structure. Only a slight deviation is noticed, which is less than 1 dB. This was attributed to non-modelled details as well as the loading effect of the environment during measurement. With confidence in

CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 78

θ φ

(a) Full Simulation Model

200 _{Frequency [MHz]}400 600 800 −10 −8 −6 −4 −2 0 S11 [dB]

Simulation and Measurement Comparison

Simulation

Measured Antenna

(b) Simulation and Measurement

Figure 5.13: Image of the full Multi-copter CEM model. Here the simulated input reflection of the antenna is compared to its measured equivalent.

0 50 100 150 200 250 300 350 Theta [Degrees] −35 −30 −25 −20 −15 −10 R ealised Gain [dB]

Const. Phi cut (260MHz) at 15 degrees

Antenna 1 Antenna 2 (a) 260 MHz 0 50 100 150 200 250 300 350 Theta [Degrees] −35 −30 −25 −20 −15 −10 R ealised Gain [dB]

Const. Phi cut (900MHz) at 60 degrees

Antenna 1 Antenna 2

(b) 900 MHz

Figure 5.14: Realised gain of Multi-copter antennas on a constant phi cut. Cut locations were chosen to align with the deepest nulls of each pattern. During measurement extraction, the data from the antenna with the highest realised gain will be used. Here the difference with increasing frequency can be seen. the CEM model of the antennas on the Multi-copter, it is now possible to simulate its spherical realised-gain patterns. For the measurements during the campaign, discussed in the next chapter, these patterns (see Appendix E.2) will be used to de-embed the data. These patterns become more dynamic at the higher frequency bands (900MHz). This causes the sizes of the nulls to increase leading to a less isotropic antenna pattern. This can be seen illustrated in Fig. 5.14. The superposition of the two antennas illustrates its benefit. The null point of each antenna is covered by the peak of the other. During post- processing, the data from the antenna with the best realised-gain will be used.

CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 79

θ

AUT

φ

Figure 5.15: Multi-copter mounted on the near-field scanning device. The highlighted (white line) is the active antenna during the measurement.