## 3 Archimedean Spiral Antenna
### Harvested Power  
Once it has been demonstrated that the antenna is able to properly operate as an ultrawideband harvesting element, a realistic measurement of the harvested power must be done. The components  
Figure 6: 3D farfield radiation pattern at 830 MHz.  
Figure 7: Prototype of the manufactured Archimedean spiral antenna.  
of the conditioning circuit (nonlinear circuit) are subsequently estimated according to the frequency bands with the highest contribution of spectral power. Thus, the spiral antenna is connected to a N9020A MXA signal analyzer and tested in two different scenarios: indoor and outdoor. Both scenarios take place in the facilities of the Telecommunication Engineering School, Universidad Politecnica de Madrid (UPM), Madrid, Spain.  
Figure 8 presents the spectral power measured indoors and outdoors, and the measurement setup for the Archimedean spiral antenna. The frequency bands with major power contributions in the radio spectrum are the mobile assignment bands (marks 3,4,5,6,8 in Figure 8). Concretely, this accounts for 98% of the total power outdoors, as Figure 9 shows. It can be seen that the remaining 2% is contributed by the FM, WiFi, DTT and WiMAX bands. Thus, the power that comes from WiFi bands (marks 7 and 9 in Figure 8) is almost negligible in this particular case. It is very useful to know that 82% of the harvested power is located in the 800 MHz and 900 MHz bands. As a result of this, future antennas can be designed to uniquely operate at these frequencies. Thus, no significant amount of power will be lost, and the design can be greatly simplified.  
From 5.2 GHz onwards, at present, only satellite and military applications can be found. However, the antenna gain is so reduced that no significant power contribution is obtained. As a result of this, a \(-65\) dBm noise level is captured over a large frequency range (5.2 GHz-16 GHz). Nevertheless, the contribution of the noise to the total harvested power is negligible. This fact indicates that it is not necessary to design an omnidirectional wideband antenna with an upper cutoff frequency above 5.2 GHz.  
Figure 8: Spectral power acquired by the Archimedean spiral antenna in outdoor and indoor scenarios (resolution bandwidth: 1 MHz). The most relevant frequency bands are remarked: 1—FM, 2—DTT, 3—LTE-800, 4—GSM-900, 5—GSM-1800, 6—LTE-2100, 7—WiFi, 8—LTE-2600, 9—WiFi.  
Figure 9: Distribution of the acquired spectral power outdoors.  
The measured spectral power is higher outdoors than indoors, as can be noticed in Figure 8. This is reasonable. The higher spectral contribution comes from outdoor radio sources (base stations), and the signals are attenuated by the walls, windows, and the buildings themselves when measuring indoors. That is why the power level of FM band is almost identical indoors and outdoors, since 100-MHz signals penetrate walls more easily than higher frequency signals. Hence, the attenuation suffered is lower.  
Finally, the total harvested power is calculated from Figure 8 by integrating the acquired spectral power (with a resolution bandwidth of 1 MHz) in the displayed bandwidth (0.1-6 GHz). This power is the actual available power between the arms of the antenna. Therefore, it is the power that is going to be delivered to the conditioning circuit. This is determined to be \(-3.19\) dBm (480 \(\mu\)W) indoors and 1.86 dBm (1535 \(\mu\)W) outdoors. Both values are sufficient to feed low-power sensors, which generally consume less than 30 \(\mu\)W [6]. Evidently, not all of the collected power will be available for the sensor. Part of it will be lost in the conditioning circuit.