Abstract :
In Part 1 of the paper the present state of medium-frequency direction-finding in H.M. ships is reviewed. There have been no fundamental changes in the system employed in recent years. The standard method in use is that of a screened crossed-loop aerial with low-capacitance screened feeder cables connected to a radiogoniometer and a receiver, using audio observation on the null signal to determine bearing. A number of advances have been made in the radio equipment used, both in the aerial system and its associated feeder cables, in the design of radiogoniometers and bearing corrector circuits, and in the compact assembly of the various units into one d.f. receiver housing. Part 2 consists of a general discussion of the effect of reradiation from the ship´s hull and deck structures in medium-frequency direction-finding. When the wavelength exceeds about four times the length of the ship, the calibration curve for a Bellini-Tosi direction-finder becomes independent of frequency, and the ¿blurring¿ becomes negligible. These low-frequency calibration curves are of a very simple type and are somewhat analogous to the calibration curves of a magnetic compass. They have an equation of type tan (¿¿¿0) = C tan (¿¿¿0+¿) where ¿0 and C are constants, ¿ is the relative d.f. bearing and ¿ is the correction (i.e.minus the error). As with the magnetic compass, the errors can easily be compensated. This is done by turning the loops through a certain angle ¿0 and applying ¿inductance correction¿, that is to say inserting extra inductance in one or other of the loop circuits. It is shown how the constants in the above equation may be calculated from hydrodynamical potential theory in many cases, and detailed calculations are made for a deck of rectangular cross-section and for a site near the edge of the bridge. The phase-quadrature effect, giving rise to ¿blurring¿, is also considered.
