Unmanned aerial vehicles, or UAVs, are used in many applications to gather intelligence without risking human lives. These aircraft, however, have limited flight time because of their reconnaissance payload requirements coupled with their limited scale. A microwave-powered flight vehicle would be able to perform a reconnaissance mission continuously.

Figure 1. The assembled array prototype is approximately 3x4

Using beamed microwave energy from a remote source on the ground, the airplane gathers energy using onboard antennas. A rectifying antenna, or rectenna, harvests power and rectifies it into a form usable by an onboard electric motor that drives the propeller, providing thrust. Using a rectenna array affixed to the underside of the aircraft, the power needed to maintain flight can be remotely transmitted.

The idea of a fuel-less flight vehicle, or an aircraft that does not carry its own fuel, has been pursued in few different forms over the past decades. There are many different approaches for how to power these vehicles; however, the common theme is that power must be transmitted from a source remote to the aircraft. Some of the possibilities for power transmission include solar power, the heating of air underneath the aircraft to cause thrust, and using antennas to convert microwave radiation into electrical power.

The goal of this project was to design and build a rectenna to receive microwave energy and convert it to usable DC power for propulsion. This required a flexible substrate in order to conform to the aircraft exterior, and an efficient antenna design, both with respect to power and to area and mass required. To this end, a prototype rectenna was designed and experimentally tested under controlled microwave radiation. The efficiency of power conversion and storage has been characterized for this system.

Design Background

A patch antenna design was chosen for the antenna array in order to simplify the design and manufacturing. Other designs considered include dipole antennas with discrete filter elements and dipole antennas with microstrip filter elements. The dipole antenna with filter elements is simple to manufacture, but is highly polarized and thus sensitive to the orientation of incoming radiation.

Microstrip filter elements have proven to be difficult to design with the constraints on manufacturing capability. Traditional PCB manufacturing techniques have a minimum line/space width of 0.006"; this constraint sets the minimum spacing for an interdigital capacitor design. The capacitor design would be a significant fraction of the antenna surface area and would likely substantially interfere with efficient operation. The remaining design option, patch antennas, has proven simpler to design.

Design Methodology

The basic patch design utilizes a square antenna sized to match the frequency and reflective plane spacing. The basic square patch antenna side should be a half-wavelength. This does not take into account the fringing that occurs when the patch is placed over the conductive reflecting plane. Matlab code was used to solve the equations for the ideal patch dimensions. The Matlab program uses the microwave frequency, gap dimensions, dielectric constants of the materials, and various physical constants to determine the ideal patch antenna dimensions.

For a PCB made of FR4 that is 0.031" thick with an air gap of 0.125", the ideal patch antenna dimension is 7.34 mm. The prototype board is manufactured with antennas with a 7.3-mm side length. The ideal spacing of each of the elements is estimated as a half-wavelength, so each antenna element is 7.3 mm away from its neighboring elements.

A low-pass filter was designed and placed between each of the antenna elements to try to improve its power conversion efficiency by eliminating higher harmonics. The filter is designed to prevent the high-frequency signal from the microwave from propagating to the next element and potentially becoming out of phase. If the signal is out of phase, it can decrease the efficiency of the element through destructive interference and cause a decrease in power output. By putting a low-pass filter in, the voltage experienced at the next element should be relatively constant. The filter consisted of two surface-mount parts — a 2.7-nH chip inductor, and a 100-pF chip capacitor, arranged as a typical LC low pass filter. Avago Technologies HSMS-8101 Surface Mount Microwave Schottky Mixer Diodes were utilized for each of the rectifying elements. They were designed for use in the X/Ku band, which is 10-14 GHz. Since the transmitter used for testing uses a frequency of 10.5 GHz, this diode matched perfectly. The diode has a maximum 350 mV forward voltage loss at 1 mA. This allowed the diode to conduct even when there was a minimal voltage potential created by the antenna elements.

The antenna layout was designed to successfully integrate the parts. The antenna feed points were chosen just to allow straight connections between the patch elements. The traces were sized to match the component pads and were kept as short as possible to minimize pickup and re-radiation.

Antenna Prototype

Figure 2. This plot shows the measured voltage difference across each of the antenna elements while the antenna is loaded by a 4700Ω resistor. This shows how the output power of the emitter varies as it passes through the plane of the antenna.

The completed prototype was approximately 3×4" and contains 35 patch antenna elements in an array of five elements in series paralleled as seven strings (Figure 1). The prototype was tested using a 10.5-GHz, 15-mW microwave transmitter that had been fitted with a horn in order to increase the focus and directionality of the emitted radiation. Included with the antenna was a receiver that used a panel meter to display relative power received. By adjusting the gain on the receiver to read 100% without the antenna in the beam path, the power absorbed by the antenna array was estimated. This, combined with the power output from the antenna array, and the specified 15-mW output from the transmitter, provided an estimate of the efficiency.

The prototype antenna successfully received and rectified 10.5-GHz microwaves for immediate use for powering DC devices or storage for later use. The output power from the antenna array was measured up to 9 mW with an appropriately placed reflecting plane. Without a reflecting plane, the power output was reduced by approximately 50%. This seems to correspond well with theory and the reason for a reflecting plane. With a 9-mW power output, the power density is 1.16 W/m2 (116μW/ cm2).

This seems reasonable for the simple antenna design and the transmitter. When the antenna was placed in front of the receiver, the power meter dropped to between 50 and 60% of the original value. This, combined with the specified power output of 15 mW, and the received power of approximately 8 mW under typical conditions, lead to approximately 50% efficiency of the antenna. Of the microwave energy that was irradiant, approximately half of the power was converted into usable DC energy.

A resistive load was placed across the antenna to measure the power output. A capacitor was placed in parallel with the load in order to smooth the power output from the antenna. The transmitter pulsed at 60 Hz, which means the antenna only produced short bursts of output at 60 Hz. By placing a capacitor across the antenna, the output was smoothed to a steady DC voltage. The resistance of the load and the voltage across the load were measured using a Fluke 117 digital multimeter with 0.9% and 0.5% accuracy specification, respectively. The power was calculated using Ohm’s law, yielding to P = V2/R, where V is the measured voltage across the resistor and R is the measured resistance. This assumes that no power is lost across the capacitor. Since the capacitor had a finite leakage current, this is not a valid assumption, but it can be assumed that the leakage current was negligible compared to the current through the resistor, and so it was ignored.

By varying the resistance of the load with a potentiometer, and measuring the power output of the antenna, the impedance of the antenna was estimated. From this analysis, the antenna had an impedance of approximately 2500Ω. A more thorough evaluation would provide more information on the actual impedance under various conditions. As the environment surrounding an antenna changed, the performance of the antenna also varied, which means the impedance can also change.

The distance between the antenna and the emitter horn was also varied and the power output determined at various locations. A 4700Ω resistor and a 1000μF capacitor were placed in parallel across the antenna in order to provide a DC output and to provide a load for power dissipation. The power output decreased as the distance increased. This is consistent with theory of electromagnetic wave propagation. Additionally, the effect of nodes was visible, as a dip in output power was observed every one-half wavelength. Between each of the nodes, a peak in power output was observed. This was also consistent with electromagnetic standing wave theories. Clearly, the emitter was generating the microwaves consistently, so it was producing a standing wave field. In an airborne application, the fluctuations would occur quickly, and would appear as an AC ripple in the output voltage.

Capacitors were charged from a discharged state and the power produced by the antenna was monitored throughout the process. The capacitors were connected directly across the antenna with no resistors or other components. The antenna was easily capable of charging a capacitor with a significant amount of energy. For each of the experiments, there was a peak in power output shortly after the charging process began. This peak reached ~3mW among all the tests. The power output then began to drop until it reached a steady-state value, likely due to leakage in the capacitor.

In order to try and capitalize on the ability for the antenna to produce a small amount of power as long as necessary, an energy storage circuit was built utilizing a low-power 555 timer, Texas Instruments TLC551. The TLC551 is capable of running with a supply voltage of only one volt with a supply current of only 15μA. With a low operating voltage and current, the timer was able to run on the power from the antenna easily. The circuit was designed to charge a capacitor, then discharge it through a load; in this case, an LED. This discharge only occurred once a sufficient charge had built up across the capacitor, which means that the load used more energy for a short period than would otherwise be available if the load were powered directly by the antenna. This can be used for intermittent data collection or other such tasks that can use power for a brief period and then enter a low power, or sleep, state.

Conclusions

Figure 3. The antenna under powered testing conditions.

The prototype antenna demonstrated that a 10.5-GHz rectifying antenna is feasible for beamed energy harvesting. The antenna is versatile and can be used to power small circuits, up to several milliwatts, under excellent conditions. The antenna could be made more useful by manufacturing it on a flexible substrate with an integrated reflecting plane. That would ensure that the reflecting plane was at the right distance from the antenna, and would allow the antenna to be shaped to fit the contour of an airplane wing. Additional antenna designs could allow for more reception of microwaves with varying frequency and polarization.

This article was written by Ephrahim Garcia of the Sibley School of Mechanical and Aerospace Engineering at Cornell University, Ithaca, NY. For more information, Click Here