Lobe switching

Lobe switching is a method used on early radar sets to improve tracking accuracy. It used two slightly separated antenna elements to send the beam slightly to either side of the midline of the antenna, switching between the two to find which one gave the stronger return, thereby indicating which direction the antenna should be moved in order to point directly at the target. The concept was used only briefly, and was almost completely replaced by conical scanning systems by the end of World War II. The concept is also known as sequential lobing, although this terminology appears to be rare, and the associated output was sometimes known as a split display.

Early radar antennas generally consisted of a number of small dipole antennas in front of a passive reflector. The dipoles were placed in order to have them constructively interfere in front of the antenna, thereby "aiming" the signal in that direction. The beam's angular spread is a function of the number of elements, with more elements producing a more tightly focussed beam. A huge number of such elements would be ideal, but impractical due to them having to be placed at a specific distance to each other depending on the wavelength of the radio source being used. In early "longwave" systems, like those used by the British and US, this forced the elements to be placed several feet apart, limiting the number of dipoles to perhaps a dozen for any reasonably sized antenna.

The resulting beam angles for such a system were generally too wide to be used directly for gun laying. For instance, the US's SCR-268 had a beam width of 2 degrees, and once the target entered that beam, the operator could not easily say where in the beam it was. An angle accuracy of about 0.1 degree would be needed for direct gunlaying. In early use the system was simply paired with a searchlight, which would be directed to the target by the radar, and then the gunners would aim visually. In this role, even during the day, the range information the radar provided was still invaluable.

Lobe switching offered greatly improved accuracy for the addition of a small amount of complexity. Instead of a single set of dipole elements, two were placed at each point on the array. The radio signal was then alternately switched between the two sets of dipoles, normally through a motor-driven mechanical switch. The return signal from one set was sent through a small delay, shifting its "peak" on the operator's oscilloscope slightly to one side. Since the switching was faster than the eye could follow, the result appeared as two well formed peaks on the display.

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