Apparatus comprising: a rectifying circuit arranged to convert a variable electrical signal received at an input from an associated antenna into a direct current electrical signal for supplying to an electrical energy storage unit, the antenna for use in harvesting ambient radio frequency, RF, energy; and a power management module (105, 300) having an input arranged to receive the direct current and control supply of the direct current to the electrical energy storage unit; wherein the rectifying circuit comprises a plurality of transmission lines, wherein the input of the rectifying circuit and the input of the power management module are connected via the plurality of transmission lines; and the power management module (105, 300) is arranged at least partially within a boundary defined by the plurality of transmission lines; characterised in that: the plurality of transmission lines form first and second arms, the first arm connecting the input of the rectifying circuit to the input of the power management module (105, 300) via a forward biased rectifying device, and the second arm connecting the input of the rectifying circuit to the input of the power management module (105, 300) via a reverse biased rectifying device; the first arm comprises a first transmission line that connects the input of the rectifying circuit to the forward biased rectifying device, and a second transmission line that connects the forward biased rectifying device to the input of the power management module (105, 300); the power management module (105, 300) is connected at a first end to the output of the rectifying circuit; a distance between a second end of the power management module (105, 300) and the input of the rectifying circuit is between λ g /2.5 and λ g /5; a distance between a first side of the power management module (105, 300) is between λ g /6.5 and λ g /12 from an edge of the first arm; and a distance between a second side of the power management module (105, 300) is between λ g /9 and λ g /16.4 from an edge of the second arm; wherein λ g is a wavelength at a centre frequency of a respective frequency band arranged to be received by the antenna. a rectifying circuit arranged to convert a variable electrical signal received at an input from an associated antenna into a direct current electrical signal for supplying to an electrical energy storage unit, the antenna for use in harvesting ambient radio frequency, RF, energy; and a power management module (105, 300) having an input arranged to receive the direct current and control supply of the direct current to the electrical energy storage unit; the rectifying circuit comprises a plurality of transmission lines, wherein the input of the rectifying circuit and the input of the power management module are connected via the plurality of transmission lines; and the power management module (105, 300) is arranged at least partially within a boundary defined by the plurality of transmission lines; the plurality of transmission lines form first and second arms, the first arm connecting the input of the rectifying circuit to the input of the power management module (105, 300) via a forward biased rectifying device, and the second arm connecting the input of the rectifying circuit to the input of the power management module (105, 300) via a reverse biased rectifying device; the first arm comprises a first transmission line that connects the input of the rectifying circuit to the forward biased rectifying device, and a second transmission line that connects the forward biased rectifying device to the input of the power management module (105, 300); the power management module (105, 300) is connected at a first end to the output of the rectifying circuit; a distance between a second end of the power management module (105, 300) and the input of the rectifying circuit is between λ g /2.5 and λ g /5; a distance between a first side of the power management module (105, 300) is between λ g /6.5 and λ g /12 from an edge of the first arm; and a distance between a second side of the power management module (105, 300) is between λ g /9 and λ g /16.4 from an edge of the second arm;