| This is a method to find out
the transmission line impedance of an unknown coaxial cable, twin lead
etc. You need a VNA analyzer which supports Smith chart plot. 1. First calibrate the reflection port (typically marked DUT) for so called S11 measurement with short, open and with a good 50 ohm load. 2. Connect the cable of interest in the DUT port. The cable length should be one wave length or more compared to the highest frequency scan of your VNA. 3. Terminate the end of the cable with a best guess resistor value of the transmission line impedance. 4: Run reflection measurement and activate Smith chart plot. 5. The Smith chart plot is circling the the transmission line impedance. 0.5 means 25 ohms, 1.0 means 50 ohms, 2.0 means 100, 5.0 means 250 ohms etc. 
See these Smith chart plots. The cables in this example are an unknown coax cable, a twisted 0.8mm enameld wire and a parallel 0.8mm enameld wire. |
Unknown foam coax cable, OD = 5mm 
Unknown coax cable terminated with 25 ohm resistor. The plot is circling abt 50 ohms. |
Unknown foam coax cable, OD = 5mm 
Unknown cable terminated with a good 50 ohm resistor. The plot is circling slightly higher than 1.0 which means abt 52 ohms. |
Unknown foam coax cable, OD = 5mm 
Unknown coax cable terminated with 100 ohm resistor. The plot is circling abt 1 = 50ohms. |
Twisted pair 0.8 mm enameled Cu wire 
Twisted pair 0.8 mm enameled Cu wire temiated with 50 ohm. The plot is circling 40 ohms (0.8) ohms at lower frequency up
to 30 MHz. In higher
frequency twisted pair is inductive so it is not so good for long transmission
line above HF. But twisted pair can be used also above HF with short lenghts
eg in RF transformers.
|
Twisted pair 0.8 mm enameled Cu wire 
Twisted pair 0.8 mm enameled Cu wire temiated with 39 ohm. The plot is circling 40 ohms (0.8) ohms at lower frequency up
to 30 MHz. In higher
frequency twisted pair is inductive so it is not so good for long transmission
line above HF. But twisted pair can be used also above HF with short lenghts
eg in RF transformers.
|
Parallel 0.8 mm enameled Cu wire 
Parallel 0.8 mm enameled Cu wire, average spacing 0.5mm-1mm.
The plot is circling abt 100 ohms (2.0). Parallel line is not showing
as much inductive series impedance compared to a twisted wire. Parallel
wire is better for transmission lines and it can be naturally used in
RF transformers. Smaller spacing between wires will provide lower than 100
ohm impedance.
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| Notes 1. The cable termination resistor is not so critical. You can start with a best guess resistor because the Smith plot is anyway circling the transmission line impedance. 2. Let your VNA to warm up enough (typically 1 hour) before calibration. 3. A coaxial cable can be coiled but a non coaxial eg twin lead should be straightened and quite free of conducting obstacles to get accurate results. 4. If you are using a PC based analyzer add a choke balun to the USB cable near the analyzer eg 5-10 turns into 31 or 43 material ferrite to avoid GND effects. You can chance VNA Smith plot reference resitance to get more accurate results. This helps especially if you are measuring a high impedance ladder line. Update 2022-03-02 OH7SV Back to OH7SV projects |