Possibly high rf field.
The output of a RF generator is connected to the input of a RF power amplifier, and the output of the amplifier is connected to a dipole antenna. The generator is set around 370 megacycles. The radiation is detected with a second dipole antenna having two metal stubs connected by a flashlight bulb. The detector stubs are made so that they are telescoping in order to tune the detector to the output of the oscillator. when the detector antenna is parallel to the transmitter and to one side, there is a strong signal indicated by a bright glow of the bulb. If the detector is at the side, but with the antenna at 90 degrees to the transmitter, there is no signal. There is no signal for any orientation of the antenna if the detector is placed at the ends of the radiator. The signal on the sides falls off inversely as the square of the distance between the transmitter and receiver.