Leaky wave antenna
This type of wave radiates continuously along its length, and hence the propagation wavenumber kz is complex, consisting of both a phase and an attenuation constant.
Highly directive beams at an arbitrary specified angle can be achieved with this type of antenna, with a low sidelobe level.
The aperture distribution can also be easily tapered to control the sidelobe level or beam shape.
Leaky-wave antennas can be divided into two important categories, uniform and periodic, depending on the type of guiding structure.
From a more sophisticated point of view, the periodic modulation creates a guided wave that consists of an infinite number of space harmonics (Floquet modes).
A typical example of a uniform leaky-wave antenna is an air-filled rectangular waveguide with a longitudinal slot.
This simple structure illustrates the basic properties common to all uniform leaky-wave antennas.
The radiation causes the wavenumber kz of the propagating mode within the open waveguide structure to become complex.
As is typical for a uniform LWA, the beam cannot be scanned too close to broadside (θm=0), since this corresponds to the cutoff frequency of the waveguide.
In addition, the beam cannot be scanned too close to endfire (θm=90°,z direction) since this requires operation at frequencies significantly above cutoff, where higherorder modes can propagate, at least for an air-filled waveguide.
Scanning is limited to the forward quadrant only (0<θm<Π/2), for a wave traveling in the positive z direction.
Unlike the slow-wave structure, a very narrow beam can be created at any angle by choosing a sufficiently small value of α.
Since power is radiated continuously along the length, the aperture field of a leaky-wave antenna with strictly-uniform geometry has an exponential decay (usually slow), so that the sidelobe behavior is poor.
The practice is then to vary the value of α slowly along the length in a specified way while maintaining β constant (that is the angle of maximum radiation), so as to adjust the amplitude of the aperture distribution A(z) to yield the desired sidelobe performance.
In response to requirements at millimeter wavelengths, the new antennas were generally based on lower-loss open waveguides.
The spacing a between the metal plates is less than λ0/2 so that all junctions and discontinuities (also curves) that maintain symmetry become purely reactive, instead of possessing radiative content.
When the vertical metal plates in the NRD guide are sufficiently long, the dominant-mode field is completely bound, since it has decayed to negligible values as it reaches the upper and lower open ends.
3, a traveling-wave field of finite amplitude then exists at the upper open end, and if the dominant NRD guide mode is fast (it can be fast or slow depending on the frequency), power will be radiated away at an angle from this open end.
As a result, a small amount of net horizontal electric field is created, which produces a mode in the parallel-plate air region, which is a TEM mode, which propagates at an angle between the parallel plates until it reaches the open end and leaks away.
In addition, it is found that the value of β changes very little as the stub is moved, and α varies over a very large range.
The fact that the L-shaped structure strongly leaks may also be related to another leakage mechanism: the use of leaky higher modes.
In the structures based on rectangular waveguide, the asymmetry was achieved by placing the stub guide, or locating the longitudinal slot, off-center on the top surface.
Here the top surface is symmetrical, and the asymmetry is created by having unequal lengths on each side under the main-guide portion, as shown in Fig.
An analysis of the antenna behavior indicates that this geometry effectively permits independent control of the angle of maximum radiation θm and of the beamwidth Δθ.
The transverse equivalent network is slightly complicated by the presence of two additional changes in height of the waveguide, which can be modeled by means of shunt susceptances and ideal transformers.
Scanning in the cross plane, and therefore in azimuth, is produced by phase shifters arranged in the feed structure of the one-dimensional array of line sources.
The spacing between the line sources is chosen such that no grating lobes occur, and accurate analyses show that no blind spots appear anywhere.
The described arrays have been analyzed accurately by unit-cell approach that takes into account all mutual-coupling effects.
A key new feature of the array analysis is therefore the determination of the active admittance of the unit cell in the two-dimensional environment as a function of scan angle.
Typical data of this type exhibit fairly flat behavior for α/k0 until the curves drop quickly to zero as they reach the end of the conical-scan range, where the beam hits the ground.
