Microwave Energy Transmission for Aircraft

Sunday, August 01 2010

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.

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.