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The paper describes a method for the calibration of shortwave field-strength measuring sets by radiation using a loop transmitter in the horizontal plane. It is shown experimentally that in the case of vertically polarized waves the simple ray theory does not apply-unless the transmitter and receiver are both elevated to considerable heights above the ground. With horizontally polarized radiation, however, the simple ray theory holds on short waves for practically all heights of transmitter and receiver. This distinction between the propagation characteristics of the two types of radiation suggests the use of horizontally polarized waves for fieldstrength calibrations on short waves. The advantages ensuing from the use of horizontally polarized waves for this purpose may be summarized as follows:Â¿ (a) The heights of transmitter and receiver may be reduced practically to any extent, so that high masts are no longer required. (b) For low heights of transmitter and receiver the reflection coefficient of the ground is sensibly unity for all types of the earth's surface, so that little error can be introduced in the analysis due to wrong assumptions as to the electrical properties of the ground. The analysis of the results is considerably simplified when the reflection coefficient of the ground is approximately unity. (d) With no masts at the transmitter or receiver it is an easy matter to take an attenuation run, so that the calibration is not dependent on one observation. (e) It is particularly suitable for the calibration of receivers incorporating rectilinear antennae, as there is no error in calibration due to varying height of the aerial above ground.