диафрагмированные волноводные фильтры / f2b19e29-623d-4c85-949f-1cb420876300

Abstract—This paper discusses the research related to 3D printed radiofrequency (RF) devices, using fused filament fabrication, carried out at Loughborough University. Different multidisciplinary aspects include the ability to 3D print unique shapes, the ability to tailor the infill to make functionally graded and heterogeneous substrates, characterizing the dielectric properties and how these materials can be used for antennas and other RF designs.

Index Terms — 3D printing, filaments, dielectric measurements, RF devices.

I.INTRODUCTION

There have been more than 100 hundred IEEE journal papers on the topic of 3D printing of RF devices in the last 5 or 6 years, representative examples include [1]–[11]. There has been considerable work in 3D printing waveguides and horn antennas and then metalizing them [12]–[16]. It is also possible to 3D print metal structures directly [17]–[19].

3D printing not only allows the external shape to be controlled, but by changing the way the internal structure is printed the local relative permittivity can be varied. Various papers have explored the concept via theory, simulations, and traditional manufacturing of using air or metallic inclusions to alter the dielectric properties [20]–[24]. As long as the inclusions are smaller than ~ λ/5, then an artificial dielectric is formed. Increasing the volume percentage of air, decreases the effective permittivity, while adding small subresonant metallic inclusions can increase the local effective permittivity. These graded structures can easily be 3D printed [25]–[27]. A valuable demonstrator is the graded index (GRIN) lens [28]–[32] where one filament with different local volume fractions of air inclusions can create different effective permittivities. Ideally, a smooth variation is desired, but simulations have indicated that six heterogeneous rings are sufficient to achieve a high level of performance. The inclusion of metal allows the imagination of new components such as 3D capacitors or filters [33], [34].

Conventional 3D printing filaments include polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). These

were originally developed as filaments for their physical rather than their electromagnetic properties. There is therefore a need to develop new materials that can be 3D printed to increase the relative permittivity and decrease the losses [35]–[40]. These emerging materials are often more challenging to print than conventional filaments and require specialist knowledge and / or suitable 3D printers. It is essential to be able to measure their dielectric properties [41], [42]. Note, generally the achievable dielectric properties are less than the bulk values as there are inevitably some air gaps in the final printed designs.

II.FABRICATION AND CHARACTERISATION CAPABILITY

A.3D Printing Capability

Loughborough University has a wide range of 3D

printing equipment for fused filament fabrication:

Hyrel3D Hydra 16A

Hyrel3D System30M

Hyrel3D High Resolution Engine

Raise3D Pro2 and Pro2 Plus

MakerBot Replicator 2X

Ultimaker 3

These enable different external and internal shapes to be realized, see Fig. 1. Internal shapes enable anisotropy.

Fig. 1. Examples of the external and internal geometries that are enabled via 3D printing.

B. Dielectric properties measurement facilities.

The Wireless Communications Research Group at Loughborough University, has extensive facilities to measure dielectric properties [41], [42]. This includes waveguides, split post dielectric resonators (SPDRs), resonant cavity methods, and an in-house device using a resonant complementary frequency selective surface (CFSS) [41], [43]. Every technique can typically measure properties at a specific frequency or range of frequencies, measure a particular size of material under test (MUT), and also measure the fields in a particular orientation. Note, if the inclusions are not in the form of spheres or cubes, then the effective relative permittivity will be anisotropic. These techniques are summarized in Table 1.

TABLE I. DIELECTRIC CHARACTERISATION FACILITIES AT

LOUGHBOROUGH UNIVERSITY

III.MEASURED RESULTS OF DIELECTRIC PROPERTIES

A range of commercially available filaments have been printed. Their dielectric properties have been measured. These were measured using the SPDR or cavity resonator. Note, that the relative permittivity tends to decrease with increasing frequency, however, this effect is negligible unless the frequency increases by many times. A summary of these results can be found in Table II. Different materials have different relative permittivity values. It is worth noting that the values are typically less than their bulk values, and there is skill and practice in the 3D printing settings to maximize the values. By changing the local infill settings different effective permittivities can be achieved from close to 1 up to the maximum value. There must be some support material there and for higher relative permittivity filaments, the minimum value than be achieved will be higher. For example, the minimum achievable value might be ~1.05 when starting with a PLA or ABS or closer to 2 with the

higher permittivity filaments. It is also possible to print materials with relative permittivities ~ 10, 12 or 14 but these are more challenging to print.

IV. CONCLUSION

This paper has discussed some of the work related to 3D printing and the advantages of being able to control the external and internal geometry as well as the material properties. Various filaments have been 3D printed and measured. The presentation will present results using these materials for antennas and metamaterials designs.

ACKNOWLEDGMENT

This work was funded by EPSRC grants ‘SYnthesizing 3D METAmaterials for RF, Microwave and THz Applications (SYMETA)’, EPSRC reference: EP/N010493/1; and ‘Anisotropic Microwave/Terahertz Metamaterials for Satellite Applications (ANISAT)’, EPSRC reference: EP/S030301/1.

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