The paper deals with the experiments carried out in 1943–44 to investigate the use of an earth screen or mat to reduce the polarization errors of the U-type Adcock direction-finder. These errors arise from the introduction of signal voltages into the system by the horizontally polarized component of the incident waves, when the aerial feeders are laid on, or buried in, ground of poor conductivity. The polarization error of a standard form of direction-finder, erected in the normal manner on highly conductive ground (3 × 108e.s.u., or about 3 × 103ohm-cm resistivity) at the Radio Research Station, Slough, was measured over the frequency range 3–10 Mc/s, with the aid of a local transmitter which radiated at controllable angles of incidence and polarization. An earth mat, about 31 m in diameter and of a square mesh between 0.3 and 1 m, was then installed at the direction-finder, and the measurements were repeated for various experimental arrangements of this installation.When the mat was raised above and insulated from the ground, the polarization error of the direction-finder was seriously increased; but when the mat was effectively earthed (both around its perimeter and at an intermediate radius) the error was slightly reduced. In place of the direct earth connection, a similar result was achieved by attaching radial wires to the perimeter of the mat, the length of the wires being selected to resonate at the frequency in use. By using a combination of 108 wires of 11, 15 and 25 m lengths, arranged so that there were 36 of each length, the desired reduction in the polarization error was attained more or less uniformly over the whole of the 3–10 Mc/s range.Additional experiments were carried out at Martlesham on ground of such poor conductivity (107e.s.u., or about 105ohm-cm resistivity), that direct earth connection to the mat was ineffective. The results confirmed the previous deduction that the attachment of radial wires of suitable length and disposition makes the earth mat effective. It was also shown that both mat and radial extension wires may be laid on the ground if desired. Furthermore, it was shown that, with the mat in position, it was no longer necessary to bury the aerial feeders, but was sufficient to lay the feeders on the ground just under the mat, or along the surface of the mat, if the feeder screen was well bonded to the mat. An additional advantage of the use of such an arrangement is that it smooths out the effects of any irregularities in the ground at the site, and produces a corresponding improvement in the performance of the direction-finder.Comparison of observations made at the same site with two direction-finders, one equipped with an earth mat and one without, showed that when taking bearings on signals from stations at distances ranging from 200 km to nearly 800 km, the expected appreciable improvement in overall performance due to the mat was attained.