## 2 Methodology
### Experimental--Electromagnetics
#### 2.1.2 Effects on the Signals  
Some causes of error on the measurements and the precautions employed to counter their effects on the signal of interest are listed below:  
(a) Noises from the power supply: the power supply, which is required to be a 5 V DC, may present ripples, spikes and transients that induce noises on the synthesized signal. For instance, the presence of spurious signals (spurs) [12] may arise due to the power supply ripple modulating the output signal. In order to minimize these overall effects, we used a 12 V battery with integrated circuit regulation to 5 V. This integrated circuit was connected to a heat sink with water cooling to avoid heating. The Rx was connected to a laptop computer, which was powered only by its battery during the measurements. Moreover, ferrite beads were also used in the power and data cables in order to reduce the chance of electromagnetic interference. The experimental setup is shown in Figure 2.  
(b) Non-shielded Tx: as the adopted synthesizer was in a low-cost prototyping board with not much protection against incoming or generated electromagnetic interference, an aluminum case was used to better isolate it from the environment (see Figure 3). The Tx circuitry case was placed as far as possible from the Tx antenna to minimize interference. Additionally, as it was elongated, it was positioned perpendicularly to the polarization of the antenna. A 20 cm RG316 coaxial cable was used to connect them, as in Figure 4.  
Figure 2: The free-space setup. The antennae and material are held by low permittivity blocks (extruded polystyrene foam) and small plastic pieces. From top to bottom, we have the battery for powering the transmitter (Tx), the Tx itself, the slab of the material in the center of the setup, the receiver (Rx), and the computer that controls the radios and measurements. In the photo, there is metal foil on the surface of the MUT; thus, this is the MET configuration.  
(c) Phase noise and thermal drift: the temperature variation, which is a source of a lot of noise, was reduced by the use of custom heatsinks. For this, an aluminum heatsink placed the transmitter's oscillator crystal in contact with the inside of the case, which is a good thermal conductor and has sufficient mass and area for heat dissipation.  
The receiver is already commercially available in a well-finished aluminum case, which is designed for some heat dissipation. However, due to its compactness and the demanding streaming processes of the captures, the radio was still warming up initially. Therefore, we connected its case to two external aluminum heatsinks. They were assembled vertically in order to maximize the contact area for heat transfer and to cause less interference on the radiation patterns of the nearby horizontally polarized antenna--see Figure 5. Wherever applicable, thermal paste was used to increase the heat transfer between the metallic surfaces.  
In order to have more compactness and mechanical stability, and to avoid delay and loss in the received signal due to cables, the receiving antenna was directly connected to the Rx SDR, using only an adapter to match the gender of the connectors, as shown in Figure 5.  
Figure 4: The Tx radio is connected to the transmitting antenna by an SMA RG316 coaxial cable. On the left is the connection to the power supply (battery is partially shown); on the right is the USB cable going to the computer (during the tests, its connector was placed inside the case to improve shielding).  
Figure 3: Detail of the transmitter board inside the aluminum case. The USB connector (universal serial bus), used for receiving configurations from the computer, and two RF SMA connectors [2] (only one was used and the other was turned off during the tests) are shown. The power connector is on the right.