Under this concept antennas are combined, which are built up as an antenna array and which get a special method in the feeding: The single antenna elements aren’t always together switched in phase! For different purposes various sums and differences can be formed from the received energy.

1. In the primary radar set 

• all antenna elements are fed in phase and the radiation patterns are summarized
• certain groups are only summarized in the reception time and their sums or differences feed their own receive channels

All signals are then compared as a video processor function and their difference is used to estimate the azimuth of the target more exactly. Therefore it can operate at a much lower rate of hits per scan.

1. In the secondary radar set 

• an impulse group is transmitted on the sum channel 
• a single impulse in the difference channel

The monopulse antenna is used for sidelobe suppression. These two examples illustrate that the monopulse antenna isn’t its own basic model, but a combination of a logarithmic periodical antenna and a phased array antenna.

Principle of Monopulse Antennas – Sum Channel and Difference Channel

Sectional Picture of Antenna Array of a Monopulse Antenna

9.7.1. Monopulse Concept

Monopulse radar find their origin in tracking systems. Since the late 1970s, the principle of monopulse has been adapted to suit PSR and SSR systems and is in common operational use worldwide today.

A target will be seen by radar from the moment it enters the main antenna beam or from the moment it is illuminated by the transmitted radar antenna beam. A search radar always makes an error in the determination of the direction of the target because it makes the assumption that the target is situated in the direction of the axis of the main beam of the antenna. This error is of the order of the beamwidth of the main antenna beam.

A crude way of determining angular position of a target is to move the antenna past the target direction and note the pointing direction that gives the maximum echo amplitude.

Unfortunately, the estimated azimuth position will be affected by thermal noise errors and target fluctuation errors (scintillation). The target fluctuation error is due to the cross section changing of the target during the time on target of the antenna, which gives a distortion of the envelope of the reflected pulse train.

Estimating of the angular position in older radar (not using a monopulse method)

Monopulse gives much better target azimuth measurements than the estimating of the angular position shown in figure 3. It can operate at a much lower interrogation rate to benefit others in the environment. Monopulse systems usually contain enhanced processing to give better-quality target code information. A single pulse is sufficient for monopulse bearing measurement (hence the use of the term monopulse).

The elements in the linear antenna array are divided into two halves. These two separate antennas arrays are placed symmetrically in the focal plane on each side of the axis of the radar antenna (this is often called boresight axis). In transmission (Tx) mode, both antennas arrays will be fed in phase and the radiation pattern is represented by the ice blue area, which is called the Σ or sum diagram. (shown in figure as blue graph and pattern)

In reception (Rx) mode an additional receiving way is possible. From the received signals of both separate antenna arrays, it is possible to calculate Σ (like the transmitted sum diagram) and the difference ΔAz, the so-called delta azimuth diagram. The antenna pattern is given by the red and green area on the same figure. Both signals are then compared as a reply processor function and their difference is used to estimate the azimuth of the target more exactly.

The angle between the axis of the antenna (boresight axis) and the direction of the target is also known as OBA value (off-boresight angle).

The elevation angle is also measured at 3D radar as a third coordinate. Well, the procedure is used twice now. Here the antenna is derived in addition to an upper half and a lower half. The second difference channel (ΔEl) is called “delta elevation.”

Principle of a Monopulse System

Sum – signal Σ ( I + II + III + IV )

Difference – signal ΔAz ( I + IV ) – ( II + III )

Difference – signal ΔEl ( I + II ) – ( III + IV )

• Auxiliary signal Ω also shall to complete the picture be mentioned, although this one isn’t tied to the monopulse antenna. This channel to the compensation of sidelobes always has practically an omnidirectional antenna. It has a wide antenna pattern and also serves to detect active jamming.

All these signals need their respective receiver channel. Modern 3D radar sets all have at least four parallel receiver channels.

If the primary radiators of the monopulse antenna consist of feed horns, then the distribution of the received signals can be performed with a monopulse duplexer.

Four Quadrants of a Monopulse Antenna

9.7.2. Amplitude or Phase Comparison

In the configuration of figure 1, the left antenna group produces the upper half of the difference beam and the right antenna group produces the bottom half of the difference diagram. The combination of the two groups produces the sum beam. With this antenna arrangement, the information about the angular position of the target is carried in the difference of the voltage levels from the two antenna groups: If the target in this figure were below the center axis of the diagrams in figure 1, the voltage from the right antenna array would be greater than the voltage from the left antenna array. If the target were above the center axis of the diagrams, the voltage from the left antenna array would be greater than the voltage from the right antenna array.

Since the angle information is determined from the voltage values or amplitudes, this type of antenna is termed an amplitude-comparison monopulse antenna, or shorter: amplitude monopulse antenna.

With a phased array antenna, the sum diagram and both difference halves would point in the same direction. In addition to the boresight axis of the antenna plane, a phase shift would cause the antenna diagram to steer. In this case, the angle information would not be carried in the relative amplitudes of the signals from the difference beams. Instead, the angle information is determined from the phase positions of the individual antennas or antenna arrays. Therefore, phased array antennas are also termed phase-comparison monopulse antennas, or shorter: phase monopulse antennas.

Old Russian VHF radar, like the P-3 and its successors up to the P-12, performed an elevation angle measurement with the phase comparison between two antenna planes. After that, this procedure was abandoned because it degraded the maximum range. In modern air traffic control radar like the ASR-NG, this phase comparison method received a renaissance. The ASR-NG uses three superimposed horn radiators for this procedure. Modern digital signal processing avoids range loss by dividing the received power into more channels.