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Register a new userFundamental Limits on Substructure Dielectric Resonator Antennas
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We show theoretically that the characteristic modes of dielectric resonator antennas (DRAs) must be capacitive in the low frequency limit, and show that as a consequence of this constraint and the Poincar{e} Separation Theorem, the modes of any DRA consisting of partial elements of an encompassing super-structure cannot resonate at a frequency that is lower than that of the encompassing structure. Thus, design techniques relying on complex sub-structures to miniaturize the antenna, including topology optimization and meandered windings, cannot apply to DRAs. Due to the capacitive nature of the DRA modes, it is also shown that the Q factor of any DRA sub-structure will be bounded from below by that of the super-structure at frequencies below the first self-resonance of the super-structure. We demonstrate these bounding relations with numerical examples.
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Studies were made into the arise and an evolution of the beam breakup (BBU) instability in a rectangular dielectric resonator under excitation by a sequence of relativistic electron bunches. The dielectric resonator is a metal rectangular waveguide $R_{26}$ $(45mmtimes 90mm)$ with Teflon dielectric slabs $8.2mm$ thick (dielectric constant $varepsilon=2.051$) located along the wide side of the resonator. The wavelength of the $LM_{21}$ operating mode having a symmetric profile of the longitudinal electric field component is $53.2mm$. The electron energy of bunches is $4.5MeV$ , the charge of each bunch is $6.4nC$, the bunch repetition period is equal to twice the wavelength of the $LM_{21}$ mode. By the use of numerical PIC simulations, the charge losses of electron bunches on the dielectric plates were investigated as the bunches were displaced relative to the cavity axis. It is found that the charge losses on the dielectric slabs due to the BBU instability do not exceed $5%$. When the bunch repetition period is changed to a multiple of another eigenfrequency (e.g., the $LM_{11}$ mode), the charge losses of drive bunches do not change appreciably.
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