Properties of annular ring microstrip antennas printed on magnetically biased ferrite substrates are reported. Experimental results for the fundamental TM11 mode and a higher-order TM21 mode are described. Circular polarisation is generated using a single electromagnetically coupled feed for both modes. The resonance frequency can be tuned over a 15% range using a magnetic bias normal to the patch and ferrite.
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Tsang et al. (1994) studied this question.
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