Dielectric Loss in Coaxial Cables via Phase Delay Measurements

Determining dielectric loss as a function of frequency in high frequency coaxial cables is a requirement for efficient design of long cable systems. A method for determining dielectric loss based on attenuation and phase delay measurements along with low frequency capacitance data and nominal cable specifications is presented in Part I. The pi-point method of making precision phase delay measurements is reviewed and contrasted to earlier methods in Part II. A 10-nmi-length of SF ocean cable at frequencies up to 10 MHz is considered as an example. For this case the number of wavelengths N is approximately 1000 at 10 MHz. A measurement technique is described and a test setup, including circuitry, is presented for making pi-point phase measurements and for determining N with zero error. Measurement errors, converted to equivalent frequency errors, with detector noise, mismatch, attenuator phase shift, and frequency counter errors being the significant contributors are considered in Part III. When all worst case errors are added, the accuracy of the delay measurement is better than 0.3 ppm at 10 MHz for the sample case under study.