This paper describes a high-frequency radio apparatus capable of measuring in absolute units the intensity of the electromagnetic field over the frequency band of 5 to 12 megacycles per sec. It is easily portable, since the total weight without batteries is only 60 lb. An untuned antenna, which is coupled to the grid of the detector valve in an unusual manner, is used to pick up the energy to be measured. Sensitivity is not seriously affected since the range of intensities which can be measured is from 10 microvolts per metre to 10 000 microvolts per metre. The screening difficulty is lessened by using a resistance attenuator and toroidal inductances, and by totally disconnecting the aerial during calibration. The set consists of a detector valve with which is associated a control valve. One stage of audio-frequency amplification follows the detector, and a diode used as a second detector allows a galvanometer to be used for the signal comparison. A separate heterodyne is used to obtain the audio frequency. A resistance method is used to attenuate the current from the local source. The complete apparatus has been subjected to a series of tests in self-consistency and absolute value, and these are described in the paper.