High-frequency direction finding

High frequency (HF) refers to a radio band that can effectively communicate over long distances; for example, between U-boats and their land-based headquarters.

In earlier RDF systems, the operator mechanically rotated a loop antenna or solenoid and listened for peaks or nulls in the signal to determine the bearing to the transmitter.

An early use was by the RAF Fighter Command as part of the Dowding system of interception control, while ground-based units were also widely used to collect information for the Admiralty to locate U-boats.

By taking several such measurements, or using some other form of navigational information to eliminate one of the ambiguous directions, the bearing to the broadcaster could be determined.

Although the rough location could be found by spinning the control rapidly, for more accurate measurements the operator had to "hunt" with increasingly small movements.

Robert Watson-Watt had demonstrated that measurements of these radio signals could be used to track thunderstorms and provide useful long-range warning for pilots and ships.

[6] The lightning strikes lasted such a short time that traditional RDF systems using loop antennas could not determine the bearing before they vanished.

These were found to have very poor performance due to the electrical characteristics of the Slough area, which made it difficult to determine if a signal was being received on a straight line or down from the sky.

[9] It was Watt's continuing desire to capture the location of individual lightning strikes that led to the final major developments in the basic huff-duff system.

As hoped, the radio signal produced a pattern on the screen that indicated the direction of the strike, and the slow-decay phosphor gave the operator ample time to measure it before the display faded.

[6][7] Watt and Herd wrote an extensive paper on the system in 1926, referring to it as "an instantaneous direct-reading radiogoniometer" and stating that it could be used to determine the direction of signals lasting as little as 0.001 seconds.

As the entire Dowding system of air control relied on ground direction, some solution to locating their own fighters was needed.

Every Sector Control, in charge of a selection of fighter squadrons, was equipped with a huff-duff receiver, along with two other sub-stations located at distant points, about 30 miles (48 km) away.

These stations would listen for broadcasts from the fighters, compare the angles to triangulate their location, and then relay that information to the control rooms.

Pip-squeak automatically sent out a steady tone for 14 seconds every minute, offering ample time for the huff-duff operators to track the signal.

At first, the UK's detection system consisted of a number of shore stations in the British Isles and North Atlantic, which would coordinate their interceptions to determine locations.

Four such groups were set up in Britain: at Ford End in Essex, Anstruther in Fife, Bower in the Scottish Highlands and Goonhavern in Cornwall.

[15] Land-based systems were used because there were severe technical problems operating on ships, mainly due to the effects of the superstructure on the wavefront of arriving radio signals.

These problems were overcome under the technical leadership of the Polish engineer Wacław Struszyński, working at the Admiralty Signal Establishment.

[16] As ships were equipped, a complex measurement series was carried out to determine these effects, and cards were supplied to the operators to show the required corrections at various frequencies.

At the same time, improved sets were introduced that included continuously motor-driven tuning, to scan the likely frequencies and sound an automatic alarm when any transmissions were detected.

This allowed hunter-killer ships and aircraft to be dispatched at high speed in the direction of the U-boat, which could be located by radar if still on the surface or ASDIC if submerged.

If one considers the signal received on one channel, say Y, the dot will move up and down, so rapidly that it would appear to be a straight vertical line, extending equal distances from the center of the display.

When the second channel is added, tuned to the same signal, the dot will move in both the X and Y directions at the same time, causing the line to become diagonal.

[18] This leaves the problem of determining whether the signal is north-east or south-west, as the ellipse is equally long on both sides of the display centre-point.

[20] For shipboard systems, the ship's superstructure presented a serious cause of interference, especially in phase, as the signals moved around the various metal obstructions.

[21] Naval units, notably the common HF4 set, included a rotating plastic plate with a line, the "cursor", used to help measure the angle.