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Impedance measurements made on a tape helix with an electron beam are described. Curves showing the measured impedances of the fundamental and first backward space‐harmonic components of the mode commonly used in traveling‐wave tubes are presented for values of ka (circumference to free‐space wave‐length ratio) ranging from 0.15 to 0.6. The impedance of the fundamental is found to be less than that calculated from the sheath model by a factor ranging from 0.8 to 0.3. Phase velocities of other modes and components were observed for ka from 0.1 to 1.1; these agree with an analysis by Sensiper which predicts that certain values of the phase constant are not allowed for a single wire helix. A relationship for the impedance of one space‐harmonic component in terms of the impedance of its fundamental is presented in approximate agreement with the experimental data. In addition to providing impedance data over a wide frequency range, the helix tester performed as a continuously voltage tunable backward‐wave oscillator from 1500 to 4300 mc at a beam current of 1 ma.