Design and performance of a double-tuned bird-cage coil

Abstract The “high-pass” and “low-pass” designations commonly given to bird-cage coils derive in each case from the relationship of the individual sections of the bird cage to simple filters. By extension of the concept of constructing a bird-cage coil from elementary filter sections, it is possible to create coils which support “bird-cage mode” resonances at more than one frequency. Designs based on both simple bandpass and band-stop filters, for example, provide means to realize bird-cage coils which may be tuned simultaneously to two frequencies. Construction and performance of a band-stop bird cage operating at both 200 and 81 MHz are described.