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.