Why the Magnetic Loss Tangent Is Not a Relevant Constraint for Permeable Conformal Antennas

Recent experiments on high-efficiency permeable conformal antennas have drawn attention to a long-standing misconception on the theory of the ferrite rod antenna family. Deriving in closed form the radiation efficiency of these antennas and using full-wave simulations, we show that their radiation efficiency is not necessarily reduced by the loss tangent of the permeable material. It is shown that the key material parameter controlling the radiation efficiency of these antennas is not the loss tangent but the maximum of the Heaviside magnetic conductivity, a quantity proportional to Snoek's product. This yields the clear design rules for obtaining permeable antennas with maximized efficiency-bandwidth product (EBWP). Because the radiating currents of these antennas are the magnetic polarization currents in the material, they can be placed close to a metal ground plane or inside a low impedance medium without performance degradation while having an EBWP much higher than the metal-and-dielectric antennas having the same conformal volume, in spite of the presence of the magnetic loss. As an example, an antenna operating from HF to VHF can attain an EBWP 10 dB higher than the corresponding metal antenna. Similarly, using the Best-Yaghjian formalism, it is shown that these antennas are easier to match and can attain wider bandwidths than the equivalent conformal metal antennas.