Measurement of the capacity of data cables, signal and control lines

Loss-free transmission of high-frequency, analog or digital signals is an essential quality feature for LAN, coaxial, telephone or RF cables. The characteristic impedance of the cable must be optimized for the system components and the specified frequency range to enable the lowest possible attenuation and signal reflection.

The transmission quality of a signal or data cable is largely determined by 4 parameters in addition to the characteristic impedance:

  • Line resistance
  • Inductance
  • Capacitance
  • Conductance

Capacitance measurement therefore already plays an important role during cable production, as it interacts with inductance and resistance and, taking into account the geometry and insulation of the cable, significantly determines the characteristic impedance and thus the transmission quality of the cable.


For inline capacitance measurement SIKORA offers the CAPACITANCE 2000. This is installed in the cooling trough and measures the capacitance of the cable during production. At the same time, the system determines periodic capacitance changes (FFT) as well as the resulting return loss (SRL), provides an extremely fast measured value update and detects bare spots.

System design

This technology is realized by combining a short and a long measuring electrode (multi-zone technology), which are integrated in a measuring tube.

The short measuring electrode determines periodic capacitance fluctuations with high spatial resolution using Fast Fourier Transformation (FFT). The structural return loss (SRL) is determined from the FFT data and provides information about the expected attenuation of the RF signal during data transmission.

The long measuring electrode measures the average value of the capacitance. Avoiding periodic changes in the cable is a prerequisite for optimum attenuation behavior, whereby the critical periodic interval becomes shorter the higher the data transmission rate targeted for the cable increases. These periodic changes are determined with the multi-zone technology of the CAPACITANCE 2000, since all parameters that influence the quality (e.g. conductor diameter, foaming, outer diameter) are reflected in the capacitance.

CD-control, FFT & SRL

The interaction of diameter and capacity is analyzed with a CD control (Capacity – Diameter) and controlled to target specifications. This is achieved by automatic adjustment of the cooling trough and control of the line speed.

The detection of periodic capacitance variations and the prediction of return loss (FFT and SRL) are available as special features directly at the measuring tube via a diagnostic interface.

Display and analysis

For a pure display of the measured values, SIKORA provides the REMOTE 2000. For graphical processing, display of statistics, trends as well as logging, the devices of the ECOCONTROL series are suitable.

Advantages of capacity measurement with SIKORA

The measuring instruments of the CAPACITANCE 2000 series offer a number of other advantages in addition to pure capacitance measurement:

  • Multi-zone electrode for capacitance measurement with simultaneous FFT analysis and prediction of return loss SRL.
  • High resolutions
  • All calculations without interference directly in the measuring head
  • Bare patch detection
  • CD control, e.g. in conjunction with a LASER Series 2000 or LASER PRO device