This paper starts by calculating the electric field of an infinite sheet of cophased current and also the field of a flat grating ofNequally spaced wires of infinite length. It shows that the field of the grating approximates closely to that of an infinite sheet at near-by points and approximates to that ofNconcentrated currents at distant points: thus the wave front starts nearly plane and merges gradually into a cylinder. Section 4 shows that the field close to a single Franklin aerial approaches rapidly to that of a filament of infinite length and Section 5 shows that the field close to a “broad-side curtain array” approaches rapidly to that of a grating of filaments of infinite length.Having established that a broad-side array, having the dimensions commonly used in practice, can be validly represented by an idealized grating the author shows that the total output of such an array can be measured by means of a single monitor aerial. The monitor aerial may be placed at any distance less than a wavelength from the plane of the array, but it should be near the ground and near the middle of the panel and in a plane mid-way between two consecutive wires.Then the problem is considered of using a monitor aerial to test whether successive members of the curtain are carrying similar currents: it is shown that the response of a monitor aerial is unduly sensitive to phase and insufficiently sensitive to the magnitude of the current in the member to which it is closest. It is shown that equality of loading should be tested by a monitor loop and not by a monitor aerial, though it is recognized that the use of a loop involves serious instrumental difficulties at wavelengths less than, say, 5 metres. An example is given to show how maladjustments of phase can be located by systematic analysis of the inequalities in response of a monitor aerial which is moved parallel to the plane of a curtain array. In short, a monitor loop is a suitable device with which to test equality of loading, and a monitor aerial to test equality of phase.