Lobe switching
Encyclopedia
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.
generally consisted of a number of small dipole antenna
s 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.
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.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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
Conical scanning
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...
systems by the end of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. The concept is also known as sequential lobing, although this terminology appears to be rare.
Description
Early radar antennasAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
generally consisted of a number of small dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
s 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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Gun laying
Gun laying is the process of aiming an artillery piece, such as a gun, howitzer or mortar on land or at sea against surface or air targets. It may be laying for direct fire, where the gun is aimed similarly to a rifle, or indirect fire, where firing data is calculated and applied to the sights...
. For instance, the US's SCR-268
SCR-268 radar
The SCR-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....
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
Searchlight
A searchlight is an apparatus that combines a bright light source with some form of curved reflector or other optics to project a powerful beam of light of approximately parallel rays in a particular direction, usually constructed so that it can be swiveled about.-Military use:The Royal Navy used...
, 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.
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
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
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radars. As such systems were introduced into service, lobe switching generally disappeared.
Jamming
It is possible to jamElectronic warfare
Electronic warfare refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. The purpose of electronic warfare is to deny the opponent the advantage of, and ensure friendly...
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.