The polarization-error characteristics of H-type Adcock direction- finders operating in the v.h.f. band and in the upper part of the h.f. band are controlled to a large extent by resonance phenomena. A theoretical and experimental investigation of a rotating system for very high frequencies has demonstrated the feasibility of calculating, from the physical dimensions of the aerial system, the frequencies at which large errors are liable to occur, and also the magnitudes of the maximum errors. The lowest resonant frequency is that for which the product of aerial length and spacing is one-twentieth of a square wavelength. A second resonance occurs when the spacing is half the wavelength and, if the aerial length is approximately equal to the spacing, the third resonance occurs when the spacing is 0.8 times the wavelength.Three major causes of polarization errors are discussed and formulae are given for calculating these errors. The agreement between the calculated and measured errors at the resonant frequencies suggests that the three causes discussed account for nearly all the errors. Various methods of minimizing errors are examined theoretically and experimentally.Although the instrument used for the experimental investigation was designed for very high frequencies, the results have application to a wider band of frequencies and to instruments of both fixed and rotating types.