antenna. The charger stops at optimal charging spots to charge all sensor nodes uniformly up to a target energy level; however, time elapsed during transitions between two consecutive spots is not considered in total charging time. In addition, computational complexity increases quadratically with linearly increasing number of sensor nodes. [PERSON] et al. (2014) addressed a charging scheme for wireless sensor networks by making use of ideal magnetic resonance coils for energy transfer. [PERSON] et al. (2015) proposed a similar charging procedure using a vehicle capable of wireless power transfer through magnetic coupling to wireless sensor networks. They attempted to maximize the ratio of the designated vacation time, which is spent when the charging vehicle is at the service station, to the cycle time by using a time varying data routing algorithm. Linearization techniques were considered in this work to solve the problem. [PERSON] et al. (2016) considered a mobile charging scheme and split the service area into hexagonal cells. The charging was initiated only when the charger is located at the center of those cells. In [PERSON] et al. (2017), experimental charging efficiency of a radio frequency (RF) power transmitter was investigated. [PERSON] et al. (2017) inspected a dual antenna configuration for sensor nodes to enable energy harvesting and signal transmission simultaneously. [PERSON] et al. (2016) proposed an energy harvesting scheme, in which a directional antenna was used for harvesting electromagnetic energy, and served as a reflector for a photovoltaic film. [PERSON] et al. (2017) investigated the use of a sink node with a directional antenna to prolong the lifetime of wireless sensor networks. For some remote sensor nodes, transferring energy from overcharged nodes to undercharged nodes has been considered. Here, overcharged and undercharged sensor nodes, respectively, mean sensor nodes charged in excess and insufficiency of a target energy level. The main goal of energy trading between the sensor nodes is to achieve a balanced energy level among all the sensor nodes, to prolong the lifetime of the entire network. [PERSON] et al. (2018) investigated an energy trading scheme based on a stochastic game theory between nodes. In their work, a sensor node is classified into either a seller or a buyer node, depending on the amount of energy it can harvest from the surrounding environment. In their paper, a seller sensor node is any overcharged sensor node, whereas a buyer sensor node is any undercharged node. The surplus energy of the seller nodes is transferred to a buyer node by many-to-one correspondence. In the present paper, a hybrid RF charging scheme is proposed, making use of a directional antenna. It is hybrid in the sense that a major proportion of the distributed sensor nodes are overcharged by the mobile charger, using a directional antenna, and the remaining sensor nodes are then charged up to the target level through energy trading. The total charging time is the sum of the time taken for charging by the mobile charger in the first stage and the time taken for energy trading in the second stage. As it is computationally complex to consider all the distributed sensor nodes simultaneously when determining the shortest path for charging, clustering of the sensor nodes is performed by the charger prior to charging. Each cluster obtained from the clustering process consists of one cluster head and member nodes. Selection of the cluster head can be performed based on geographical consideration or energy efficiency in operation of sensor network ([PERSON] et al., 2015). Charging by the mobile charger is executed only for the cluster heads. This charging continues until all of the cluster heads become overcharged. Concurrently, most sensor nodes in the vicinity of the cluster heads also become overcharged. The proportion of overcharged member nodes depends significantly on the antenna gain. In the second stage, energy trading is conducted between the overcharged and the undercharged sensor nodes.