Conventional antennas for the frequency band of 3 - 30 MHz (HF) and 30 - 300 MHz (VHF) are physically large, and therefore not suitable for portable applications. A pertinent challenge is to reduce the physical dimensions without affecting its electrical performances. From the laws of physics, it is potentially possible to use material loading to scale down the antenna’s physical dimension by a factor of n (refractive index of material), with its electrical dimension unchanged. A class of materials that serves this purpose is the magneto-dielectric materials with high refractive index (n=(m΄ε΄)1/2), and almost matching m΄and ε΄. The latter condition is particularly important for matching the antenna impedance to free space environment. Moreover, the materials must have sufficiently low magnetic and dielectric loss tangent (≤0.01) to minimize losses in the antennas. However, no such materials have been found in nature. This project aims to develop magneto-dielectric materials for miniaturization of HF and VHF antennas ferrite ceramics.
Materials and Strategies
It has been shown that ferrites are good candidates for magneto-dielectric materials, because they have both magnetic and dielectric properties. The key problem of ferrite ceramics is their poor sintering behavior, which requires high sintering temperature. High temperature sintering leads to high conductivity and thus high dielectric loss tangent. Therefore, the first issue is to reduce the dielectric loss tangent of ferrite ceramics by reducing their sintering temperatures. There are various methods to reduce the sintering temperature of ferrite ceramics, among which using sintering aids is the best one. Our research results indicate that Bi2O3 is the most promising sintering aids for the reduction of dielectric loss tangent of ferrite ceramics. The magnetic properties of ferrites can be optimized by adjusting compositions.
By these strategies, we have fabricated various ferrite ceramics, with almost equal real permeability and permittivity, and sufficiently low magnetic and dielectric loss tangents, over 3 – 30 MHz and 30 – 90 MHz. This class of materials is expected to useful for antenna miniaturizations.