Introduction to Traveling-Wave antennas.pdf

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4 Introduction to TWAindependent of frequency. On the contrary, when the guiding structure is partlyfilled with dielectric, the transverse wavenumber k t is a function of frequency, sothat ∆θ changes as the beam is frequency scanned. On the other hand, with respectto frequency sensitivity, i.e., how quickly the beam angle scans as the frequency isvaried, the partly dielectric-loaded structure can scan over a larger range of anglesfor the same frequency change and is therefore preferred.Figure 2: Dispersion Curves (effective refractive index)In response to requirements at millimeter wavelengths, the new antennas weregenerally based on lower-loss open waveguides. One possible mechanism to obtainradiation is foreshortening a side. Let us consider for example the nonradiativedielectric guide (NRD).Figure 3: Non Radiative Dielectric guideThe spacing a between the metal plates is less than λo so that all junctions and2discontinuities (also curves) that maintain symmetry become purely reactive, insteadof possessing radiative content. When the vertical metal plates in the NRD guide areEuropean School of Antennas
5 Introduction to TWAsufficiently long, the dominant mode field is completely bound, since it has decayedto negligible values as it reaches the upper and lower open ends. If the upper portionof the plates is foreshortened, a traveling-wave field of finite amplitude then existsalong the length of 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 atan angle from this open end.Another possible mechanism is asymmetry. In the asymmetrical NRD guideantenna the structure is first bisected horizontally to provide radiation from one endonly; since the electric field is purely vertical in this midplane, the field structure innot altered by the bisection.Figure 4: Asymmetrical Non Radiative Dielectric guideAn air gap is then introduced into the dielectric region to produce asymmetry.As a result, a small amount of net horizontal electric field is created, which producesa mode in the parallel-plate air region, which is a TEM mode, which propagates atan angle between the parallel plates until it reaches the open end and leaks away. Itis necessary to maintain the parallel plates in the air region sufficiently long that thevertical electric field component of the original mode (represented in the stub guideby the below-cutoff TM 1 mode) has decayed to negligible values at the open end.Then the TEM mode, with its horizontal electric field, is the only field left, and thefield polarization is then essentially pure (the discontinuity at the open end does notintroduce any cross-polarized field components).Groove guide is a low-loss open waveguide for millimeter waves, somewhat similarto the NRD guide: the dielectric central region is replaced by an air region of greaterwidth. The field again decays exponentially vertically in the regions of narrowerwidth above and below.The leaky-wave antenna is created by first bisecting the offset groove guide horizontally.It also resembles a rectangular waveguide stub loaded. When the stub isoff-centered, the structure will radiate. When the offset is increased, the attenuationconstant α will increase and the beamwidth will increase too. When the stubis placed all the way to one end, the result is an L-shaped structure that radiatesvery strongly. In addition, it is found that the value of β changes very little as theEuropean School of Antennas
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