An experimental spaced-loop direction-finder for very high frequencies

A description is given of an experimental spaced-loop direction-finder for use with vertically polarized waves in the frequency range 30¿100 Mc/s (10-3 m). The screened loops are 28 cm square and are mounted coaxially with a spacing of 1.57 m. The sensitivity of the system in conjunction with a good commercial receiver is such that a field strength of 20 ¿V/m is required for a silent swing of ± 5°. Means are provided for selecting the required minima of the polar diagram. Instrumental errors are liable to become excessive at the resonant frequencies of the aerial system or its screen, but may be minimized by careful construction and adjustment of the system. Reciprocal error can be reduced to less than ¿ deg at all frequencies in the above range. No satisfactory method exists for measuring any small residual polarization errors of a spaced-loop system, because the curvature of the wavefront from a local transmitter introduces polarization errors which would not be present with a distant transmitter. Errors measured with a local test transmitter at an angle of elevation of 25° with a dipole aerial inclined at 60° to the vertical have been compared with calculated errors due to wavefront curvature alone. It is estimated that there were no instrumental polarization errors in excess of 2 deg in the frequency range 30¿100 Mc/s under the above test conditions. A brief account is given of a more sensitive system with loops 92 cm square, designed for a small frequency range around 30 Mc/s (10 m). In the Appendix an analysis is made of the polarization errors due to the proximity of a local test transmitter.