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The impedance parameter of a circular helix, from which the gain of a helix-type traveling wave amplifier is computed, is investigated for a "Tape-Helix" model. Results obtained in this paper indicate that the impedance has a smaller value than for the "Sheath-Helix" model, and is considerably reduced at larger values of ka, the ratio of the helix circumference to the free space wavelength. A tape helix surrounded by a dielectric medium is analyzed. It is shown that the results obtained from the theory can be used to evaluate the helix impedance for usual types of traveling wave tubes. They have been found to be in agreement with measurements on many tube designs.