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A new ferrite isolator is described, which uses a helical transmission line. The main advantage of the new isolator, as compared to other isolators, is that no externally applied magnetic field is required. This has been achieved by using closed rings of soft ferrites with a square-loop magnetization curve. Although designed primarily for use in traveling-wave tubes, the new isolator should also find other applications. By suitable modifications, it can be used as an isolator, switch, or modulator with helical as well as hallow waveguide and coaxial lines. The best ratio of backward-loss to forward-loss in decibels obtained with the new isolator is 25:1 at 6500 mc, with a ratio better than 15:1 over a 25 per cent frequency band. Ratios of 100:1 were measured when a small external magnetic field was applied. Switching times of less than a microsecond are feasible.