Fundamental interaction impedance of a helix surrounded by a dielectric and a metal shield

The fundamental interaction impedance for a helix surrounded by a dielectric medium and a conducting shield is computed and the result is found to be in good agreement with measurements on several TWT structures. Results obtained in this paper indicate that the addition of a conducting shield around a helix reduces the interaction impedance; this impedance reduction becomes more severe as the shield is brought closer to the helix and as γa, the ratio of helix circumference to guide wavelength, is made smaller. A sheath helix surrounded by a dielectric medium and a conducting shield is used to obtain a factor representing the impedance reduction caused by the shield. This impedance reduction factor is then combined with Tien´s factors to compute the helix impedance.