Lobe switching

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.


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.

Since each lobe was slightly off center, if the target was not centered down the middle of the antenna (as a whole), one of the two return signals would have greater strength than the other. Thus the operator could keep the antenna pointed at the target simply by moving it to make both returns equal height on the display. Since the lobes were arranged to overlap only slightly, there was only a very small angle where the two returns would be equal – even slight movements of the target out of the centerline would quickly make one signal much stronger. The resulting measurement was therefore much more accurate.

Conical scanning was similar in concept to lobe switching, but as the name implies it was operated in a rotary fashion instead of two directions. This allowed the operator to get a 2-D view of which direction had the strongest return, and was much easier to operate as a result. However, conical scanning could only easily be used on an antenna with a single feed horn, which is only practical with microwave radars. As such systems were introduced into service, lobe switching generally disappeared.


It is possible to jam a lobe switching radar with relative ease if one knows the basic operating frequencies of the radar. In the case of a lobe switching set that switches lobes 30 times a second, a jammer can be constructed to send out a weak signal on the same frequency that also varies 30 times a second, but only sends out a signal when the radar's lobe is pointed away from the aircraft – which is easily found by looking for a low point in the received signal. At the receiver end the two signals are mixed, and the additional signal from the aircraft's jammer "smooths out" the strong/weak signal that would otherwise be seen. This denies the radar angle information, and can make anything but gross angle tracking difficult.

One way to avoid this problem is used in the LORO radar, short for "Lobe On Receive Only", which uses one set of antenna elements to send a non-lobed signal, and two additional sets for lobing on reception. Operation is basically identical to a normal lobing radar, but it denies any information about the lobing to the target aircraft, for the cost of some additional antenna elements (or more commonly, a second antenna). Unsynchronized "blocks" of signal can be used to jam LORO radars, although it is not as effective as against a "normal" lobing system and generally makes the operator's job difficult, as opposed to impossible.

Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • Frontal lobe disorder — The frontal lobe of the brain plays a key role in higher mental functions such as motivation, planning, social behaviour, and speech production. A frontal lobe syndrome can result from a range of causes inlcuding head trauma, tumours,… …   Wikipedia

  • Conical scanning — is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used on some of the… …   Wikipedia

  • History of radar — The history of radar starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell s seminal work on electromagnetism.… …   Wikipedia

  • River delta — Nile River delta, as seen from Earth orbit. The Nile is an example of a wave dominated delta that has the classic Greek delta (Δ) shape after which river deltas were named. A delta is a landform that is formed at the mouth of a river where that… …   Wikipedia

  • Pantalla de radar — Saltar a navegación, búsqueda Los sistemas modernos de radar usan algún tipo de pantalla de barrido de trama para producir una imagen tipo carta geográfica. En el pasado, especialmente en sus primeras etapas, tales displays eran difíciles de… …   Wikipedia Español

  • SCR-268 radar — The SCR 268 (for Signal Corps Radio no. 268 ) was the US Army s first radar system. It was developed to provide accurate aiming information and used in gun laying systems and directing searchlights against aircraft.The system was already… …   Wikipedia

  • scan — i. To search the sky for any hostile aircraft. The area of the scan for a typical two aircraft formation is indicated in the illustration. In flight, the term means “Search the sector indicated and report any contacts.” ii. To make a sweep of… …   Aviation dictionary

  • scan — 1. The path periodically followed by a radiation beam. 2. In electronic intelligence, the motion of an electronic beam through space looking for a target. Scanning is produced by the motion of the antenna or by lobe switching. See also electronic …   Military dictionary

  • Radar — For other uses, see Radar (disambiguation). A long range radar antenna, known as ALTAIR, used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on Kwajalein Atoll …   Wikipedia

  • Würzburg radar — Infobox Radar name = Giant Würzburg radar country = Germany introdate = 1941 number = around 1500 range = up to 70 km (44 mi) diameter = 7.5 m (24 ft 7 in) azimuth = 0 360º elevation = 0 90º precision = ­±15 m (49 ft 2½ in)The Würzburg… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”