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Two-Way beamwidth

The transmitter’s
antenna pattern
8 simultaneously generated
narrow antenna patterns
during receiving time
Two-Way beamwidth
of 6th receiver channel

Figure 1: Exemplary digitally shaped antenna pattern with Two-Way beamwidth

The transmitter’s
antenna pattern
8 simultaneously generated
narrow antenna patterns
during receiving time
Two-Way beamwidth
of 6th receiver channel

Figure 1: Exemplary digitally shaped antenna pattern with Two-Way beamwidth

Two-Way beamwidth

A Two-Way beamwidth generally occurs when radar uses a different antenna pattern during transmitting time than during the receiving time. This is the case, for example, with multi-function radars using digital beamforming. Such radar must illuminate the entire room during the transmission time, for example, with a fan-beam pattern, which should then be searched during the receiving time by several simultaneously formed narrow pencil beams. But the radar receiver can only recognize this object, which is also illuminated by the transmit power. The superimposition of the transmitted antenna pattern with the effective antenna pattern during the receiving time is then a two-way antenna pattern with a given beamwidth.

Figure 2: Two-way antenna diagram through different aspect angles

Figure 2: Two-way antenna diagram through different aspect angles

Application to bistatic SAR and SLAR

A second possibility for the arising of a two-way antenna pattern is the use of two identical patterns but from different aspect angles.[1] A side-looking airborne radar generates, for example, a symmetrical (circular) antenna pattern. For example, if a symmetrical antenna pattern from a satellite falls obliquely on the earth's surface, this results in an illuminated area of an ellipse whose area can be calculated from the lengths of the major axis and the minor axis.

(1)

  • ΘB = half power beam width of the antenna
  • R·ΘB = applicable approximation at small angles, will be shortened later anyway
  • R = slant range
  • γ = depression angle

The depression angle is the angle between the horizontal line and the radar's line of sight.

A second satellite from a different position carries the receiver and an antenna with the same antenna pattern and also targets this area. It also creates an ellipse but it’s twisted relative to the first one. Only the area illuminated by both ellipses can be observed. In extreme cases, these ellipses are crossed one above the other. In case of many aspect angles, this is a circle with diameter of B. The ratio of the area of the circle to the area of the ellipse is then equal to sin(γ). With a depression angle of 30°, this results in a factor of exactly 0.5. This factor reduces the reflecting area and thus also the reflected power. The usable beamwidth of these superimposed antenna patterns thus decreases approximately to the -1.5 dB power level.

In the case of a synthetic aperture radar, this case also arises, for example, in spotlight mode, since all received echo signals from different satellite positions must be used for signal processing. Only this part of the surface can be used to calculate the radar image from the synthetic aperture that the satellite can illuminate uniformly from different aspect angles. This fact also improves the cross-range resolution resulting from the smaller beamwidth of the two-way antenna diagram (-1.5 dB beamwidth instead of the −3 dB beamwidth).

It is interesting to note that this relationship can also be applied to a monostatic radar, which radiates vertically downwards: Here, the depression angle is then equal to 90° and the sine from this angle is equal to one. Thus its resolution remains at the usual -3 dB.

Further applications

Figure 3: An FMCW radar module with different antenna sizes for transmit and receive.
(courtesy of RFbeam Microwave GmbH)

Figure 3: An FMCW radar module with different antenna sizes for transmit and receive.
(courtesy of
RFbeam Microwave GmbH)

Another use case exists, for example, for a radar with a conical scan on receive only (COSRO), such as the historical Type 275 target tracking radar. However, more modern radar sets also use different transmit and receive antennas, such as the K-MC4 from RFbeam Microwave GmbH, which operates in the 24-GHz frequency band (Figure 3). With this radar module, it is possible to implement the monopulse method in one plane. However, only one receiving channel can also be used. Therefore, the antenna patterns of the transmitting and receiving antennas overlap. The sidelobes of the transmitting and receiving antennas are on different side angles, therefore, the intensity of the sidelobes are reduced.

Source:

  1. D.Jenn, ''Microwave Devices & Radar'', Lecture Notes Volume 2, Version 7.2, Naval Postgraduate School, p. 52 (online)