1. 1 Ferrite loaded volumetric spiral antenna I. Tzanidis, C.-C. Chen and J. L. Volakis The ElectroScience Laboratory Dept. of Electrical and Computer Engineering The Ohio State University Columbus, OH 43212 E-mail: {tzanidis.1, chen.118, volakis.1}@osu.edu

2. 2 Goal no coils large coils small coils 150MHz with dia=250mil 6’’ -15dB theoretical limit @118MHz Fabricated 6’’ weaved spiral antenna Can we go lower than 150 MHz using materials? Combined magnetic/dielectric loading to improve miniaturization of weaved spiral antenna 6’’ Ferrite core Dielectric matrix

3. 3 Proof: what do materials offer miniaturization gain drops at high frequencies (tapering absence) theoretical limits 118 MHz 158 MHz 6’’ planar spiral embedded in ε’ r =μ’ r material Materials offer wave slow-down (=miniaturization) & if ε’ r =μ’ r (impedance matched material) antenna impedance is the same before and after loading Application: ferrite loaded coiled loop antenna 6’’ (152.4 mm) 2mm pitch~23mm 6’’ (152.4 mm) 2mm μ’ r =2

4. 4 limit Miniaturization looking at -15 dBi frequency point error bars : resonance ripples cause ambiguity in picking -15 dB frequency point f/f o : loaded/free-space frequency @ -15 dBi Realized Gain k b (k o ): wavenumber in material(free space) at frequency f (f o ) t : material thickness Diminishing returns for thickness beyond (k b - k o )t = 0.6 Miniaturization limit at ~ 55% when (k b - k o )t= 1 extending the material width from 6 to 8 inches Optimal thickness t ε’ r =μ’ r, lossless How much material do we need? How much can the benefit be?

5. 5 Practical materials High values of ε’ r, μ’ r (10 to 14) ε’ r ≠ μ’ r High losses in freqs >100 MHz Mixing with PDMS to reduce losses and adjust ε’ r = μ’ r Magnetic losses reduced, initial permeability dropped almost by a factor of 10 Composites offer tailoring capabilities but cannot change dispersive behavior of ferrites μ’ r μ’’ r Typical dispersion in ferrites

6. 6 Measured samples Magnetic Dielectric + + = PDMS (silicon) mixed thoroughly T2 (curing agent) ferrite is addedMixed & de-gassed poured into molds Measuring equipment Fabrication process of composites

7. 7 Every 0.005 of losses reduces gain by 1 dB or 5% loss of miniaturization Losses affect miniaturization The figure of merit of miniaturization is the Realized Gain → Miniaturization is affected by losses

8. 8 New material measurements (1) Co 2 Z composites offer lower magnetic losses for a larger bandwidth

9. 9 New material measurements (2)

10. 10 E field @ 220MHzH field @ 220MHz 6’’ (152.4 mm) Effective loading: Dielectric material outside the coiling, magnetic within coil Field distributions of first mode on xy plane Ferrite coreFerrimagnetic matrix Dielectric matrix Best practices for material loading What is the optimum location of the loading? Magnetic core Dielectric coating Application: dielectric coated coiled loop antenna 6’’ Bisection view

11. 11 Loading of the weaved spiral antenna Magnetic loading only Magnetic-dielectric loading No loading The wave is slowed down as we load more Although the wave is slowed down, the gain curve does not shift lower This shows that the freestanding design has already reached it’s optimal miniaturization limit in terms of gain Need to change design to go lower frequencies material loading does not improve this particular antenna 6’’

12. 12 Ground plane issue The presence of a ground plane degrades the antenna performance data: F. Erkmen

13. 13 Ground plane material treatment Material treatment of the ground plane recovers free-space performance at low frequencies data: F. Erkmen Ferrite PEC

14. 14 Future work Height=1 inch Magneto-dielectric material loading We will use ferrite composites to lower the height and operational frequency of the daizy-like antenna