Transmission Line Calculator
Transform a complex load impedance through a lossless transmission line and estimate electrical length and matched cable loss.
This lossless-line transformation uses the exact tangent form for a lossless line, then reports the specified matched attenuation separately. For high-loss or precision work, use manufacturer S-parameters or a full lossy-line model.
Interpret electrical length and loss
Transmission-line behaviour depends on characteristic impedance, electrical length, velocity factor and the complex load. A cable that is physically short can still represent a significant electrical length at VHF and above.
Velocity factor describes how quickly the RF wave travels through the cable relative to free space. Use the manufacturer value for the specific cable rather than assuming every coax type has the same factor.
If the calculated input impedance seems surprising, check the cable length, frequency and load impedance first. A measurement at the radio end also includes connectors, adapters and any equipment between the instrument and the load.
Use the calculation to understand the feedline
Transmission-line calculations become important whenever the feedline is electrically significant compared with the wavelength. The result depends on frequency, physical length, characteristic impedance, velocity factor, loss and the load connected at the far end. A cable can transform the impedance seen at the transmitter, so the input impedance is not necessarily the same as the antenna's feedpoint impedance. Loss also changes with frequency and cable construction, while connectors and installation details add smaller discontinuities that a simple model does not include. Use the calculator to compare cable lengths, velocity factors and loads, or to understand why an analyser sees a different impedance at the shack than at the antenna. For final installation decisions, use manufacturer data for the actual cable and verify the finished system with measurements where practical. The calculator is especially useful for learning how a transmission line transforms impedance and why an apparently modest cable loss can become significant at higher frequencies or over long runs.