Short Range Wireless Power Transfer (WPT) for UAV/UAS Battery Charging

Editor's note: Reprinted with permission from the Naval Postgraduate School's CRUSER News.

by Professor David Jenn, NPS Electrical Engineering Faculty, jenn(at)

There are numerous advantages of wireless power transfer (WPT) for many remote energy source and battery charging applications. In a WPT system, power is transmitted wirelessly from a base station to a client. The concept was first demonstrated for vehicle propulsion in the mid 1960s. More recently, WPT has been used for charging wireless devices, and commercial WPT charging technologies have appeared on the market under the names Witricity and Energous.

Measurement setup used to obtain efficiency and coil installed in a UAV hull
(photo courtesy NPS).
In a typical WPT system, prime power is provided by the base station, converted to radio frequency, and then transferred through space to a receiving coil or antenna. On the client side the received power is filtered, transformed in voltage, and subsequently delivered to the battery or power plant.

Inductive systems use two coils with one located in the charging station and the second in the device. Energy is transferred by the magnetic fields linking the coils. At the receiving coil, circuits are required to rectify and condition the output voltage for charging the battery. Inductive systems generally operate at low frequencies (< 10 MHz). Efficiencies greater than 95% have been achieved, but only at very short distances (a maximum of several cm) and alignment of the coils is critical.

Radiative WPT systems use two antennas rather than coils, and the energy is transferred by a propagating wave. The receiving antenna has an integrated rectifier, and is called a rectenna. Radiative systems operate at higher frequencies than inductive systems (> 1 GHz), and suffer a (1/distance)2 propagation loss. High gain antennas can be used to increase the received power, but they become physically large. The use of antennas has advantages though. Solid state arrays allow full control of the antenna excitation, which permits scanning and focusing of the beam. This capability relaxes the alignment requirements between the two antennas. Radiative systems can be designed to operate at distances of tens of meters or more.

A disadvantage of radiative systems is that it they are more susceptible to environmental conditions. To minimize loss, a clear line-of-sight in air is desirable. Therefore, this approach cannot be used for
vehicles submerged in water or buried in wet ground. However it can be used for ground vehicles, air vehicles on the ground, or even warfighter packed equipment.

Other issues that must be considered when using electromagnetic energy are safety and interference. Because of the short ranges and relatively low power involved, safety should not be an issue and the interference introduced by a practical WPT system will be limited to same platform (self) interference.

In Phase I of the study (completed in FY14) both approaches were simulated using commercial software. For the inductive case, working at 100 kHz, efficiencies over 90% were achieved at short ranges (less than 30 mm). A frequency of 100 kHz was used to allow the system to operate in seawater without suffering decreased efficiency due to the water resistance. For the radiative approach, the transmission loss between antennas was less than 1 dB at ranges less than 3 m when near field focusing was employed. The results are important because they demonstrate that efficient transmission of energy can take place between the WPT ground station and a client for both approaches.

The next phase in the research is to demonstrate efficient rectifying and battery charging circuits. It includes the design of a practical interface between the coils, and optimization of the rectifying and charging circuit. The demonstration of an inductive system is planned in FY15.
Full Report available at  


  1. Lighthouse DEV based in Maryland as delivered an “eye-safe” based laser WPT demo system to the U.S, Army that can transmit power for several miles very much like the system shown in this BBC Video

    Bert Murray


Post a Comment

Popular posts from this blog

2013 Naval Drones Survey

Inspector Gadgets: Drones in the Hangar