Transmission line analysis & design software

Input Impedance of Balanced-Pair Transmission Lines Terminated with a Load

Author: R.J.Edwards G4FGQ © 6th April 2006

This program is a companion to Input_Z which deals with coaxial lines.

The input impedance of a transmission line is often needed. Much experience is required just to guess what it might be. This program accurately calculates it.

The load impedance, R+jX, can be anywhere between a short circuit and a virtual open circuit. Input impedance is calculated in the same series R+jX form.

The program can be used to assist with design of transmission line impedance-matching transformers when one or both of the terminations are complex.

The reflection coefficient is calculated. Also SWR for both ends of the line.

Attenuation and power efficiency are calculated.

Note how line loss increases when high standing waves are present.

Power dissipated in the line as a percent of input power is 100% - Efficiency %.

Typical Velocity Factors

Line Type (Impedances are nominal values) Velocity
Open wire air-spaced lines of all impedances 0.995
450-ohm ladder/window lines 0.91
Domestic quality 300-ohm TV ribbon downlead 0.85
ubstantial 300-ohm ladder line 0.8
130-ohm, 16 to 20 AWG, figure-of-eight cables 0.7
75-ohm, close-spaced wires embedded in solid polyethylene 0.68

When attenuation exceeds 40 dB the input impedance is equal to Zo of the line. So when the load impedance is put equal to Zo = Ro+jXo, the reflection coefficient becomes zero and the SWR becomes 1-to-1.

If attenuation exceeds 250 decibels then the program may abort due to numerical overflow. Decrease line length, increase conductor diameter, or reduce frequency.

Run this Program from the Web or Download and Run it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link Line_Zin then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.

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