Filters are present in virtually all communication systems, ranging from radar systems, point-to-point radios, earth stations, base stations and repeaters. In view of this, filter design still draws a great deal of attention from RF engineers and academia, each focusing on various aspects of theoretical and practical filter designs.
At the lower end of the frequency communication spectrum (below 1 GHz), the weight and volume occupied by of RF hardware equipment can be significant. This poses significant challenges in terms of cost and deployment, to name but a few, to the network equipment suppliers. At this frequency spectrum, RF filters tend to occupy significant volume, primarily due to relatively long free-space wavelengths, but also because of the need for good electrical performance, which increases filters’ size.
In mobile cellular communications, coaxial cavity filters are usually the preferred choice, since, compared to waveguide dielectric filters, they are more cost effective, occupy lower volume and offer reasonably good performance. Nevertheless, in some applications, their size still dominates the volume of network equipment. Several solutions have been employed over the decades of research into this topic. For example, capacitive loading is one of the most common ways of reducing their size. As another example, a stepped resonator, described in can also be used to reduce the profile of the coaxial resonator, however, both approaches come at the expense of performance.
In this paper, we introduce a new way or reducing the profile of coaxial cavity resonators. The new approach relies on the filling of empty cavity spaces with a low-loss, low dielectric permittivity powders, suitably compacted so that it results in a stable mechanical performance, Fig. 1. In terms of electrical performance, the proposed resonators and filters sit between empty (air-filled) coaxial cavity resonators and ceramic resonators , allowing their use for a wide variety of applications. As an example of the proposed technology, a 5-pole filter operating at 779 MHz and with a bandwidth 17 MHz is designed, fabricated and tested. The filter cavities were filled with Evonik glass granules , exhibiting a measured dielectric permittivity of 2.29 nd tan(δ) = 1.83×10-4 . The fabricated filter and its performance are shown in Fig. 2.
The cavity is filled with dielectric powder (yellow). A Teflon-ring (green) is employed to allow the tuning screw to penetrate the cavity without affecting the powder (a) and resonator used for measurements of dielectric properties of powders (b)
 (2016). [Online]. Available: http://www.evonik.com