Cable TDR

The Cable measurement mode analyzes transmission lines to determine length, characteristic impedance, and loss. It uses S11 measurements to characterize cable properties.

Enable Cable Mode

Connect the cable to CH0 (leave far end open or shorted)
Set sweep from 50 kHz to 300 MHz or higher
Go to DISPLAY > MEASURE
Tap CABLE
Measurements appear on screen

Display Output

S11 CABLE
Z0 = 51.2 ohm
Length = 12.45 m (VF=66%)
Loss = 1.23 dB at 145.000 MHz
Att (dB/100m): 9.88 dB at 145.000 MHz

Parameters

Parameter	Description
Z0	Characteristic impedance
Length	Cable length using velocity factor
VF	Velocity factor if real length entered
Loss	Total cable loss at marker frequency
Att	Attenuation per 100 meters

Shell Commands

# Enable cable analysis mode
measure cable

# Disable measurement mode
measure none

Measurement Method

The cable analyzer works by:

Finding the frequency where reactance (imaginary S11) crosses zero
This frequency corresponds to quarter-wave resonance
From quarter-wave frequency, calculating electrical length
Using velocity factor to convert to physical length
Measuring loss from S11 magnitude

Setting Velocity Factor

The velocity factor affects length calculation. Set it via DISPLAY > TRANSFORM > VELOCITY F.:

If you know the cable length, enter it as “real length”
The firmware calculates velocity factor
Or enter known velocity factor to calculate length

Common velocity factors:

Cable Type	Velocity Factor
RG-58 foam	79%
RG-58 solid	66%
RG-174	66%
RG-213	66%
LMR-400	85%
RG-8X	82%

Known Length Mode

If you know the actual cable length:

Enter the known length in settings
The analyzer calculates velocity factor from the measurement
This is useful for characterizing unknown cable types

Impedance Measurement

Z0 is measured from the reactance at 1/8 wavelength:

Leave far end of cable open
Run measurement
Z0 is calculated from reactive impedance at quarter-wave/2

The accuracy depends on cable length and frequency range coverage.

Loss Measurement

Cable loss is measured using parabolic regression on S11 data:

Loss increases with square root of frequency (skin effect)
Multiple data points across the sweep are used
Move the marker to see loss at different frequencies

Practical Examples

Find Cable Length

Connect unknown cable to CH0
Leave far end open
Set velocity factor for cable type (or use 66% default)
Enable CABLE mode
Read length from display

Determine Velocity Factor

Measure cable length with tape measure
Enter length in settings
Connect cable to CH0
Enable CABLE mode
Read calculated velocity factor

Measure Cable Loss

Connect cable to CH0
Enable CABLE mode
Position marker at frequency of interest
Read loss from display
dB/100m shows loss normalized to 100 meters

Compare Cable Quality

Measure reference (good) cable
Note Z0 and loss at specific frequency
Measure test cable
Compare Z0 (should match within 5%)
Compare loss (should be similar or better)

Fault Location

For finding cable faults, use Time Domain mode instead:

Go to DISPLAY > TRANSFORM
Enable transform with LOW PASS IMPULSE
Set correct velocity factor
Look for reflection peaks indicating faults

See the Time Domain Transform guide for detailed TDR fault location.

Accuracy Considerations

Optimal Frequency Range

Cable Length	Recommended Sweep
< 1 m	50 kHz - 900 MHz
1-5 m	50 kHz - 500 MHz
5-20 m	50 kHz - 300 MHz
20-50 m	50 kHz - 150 MHz
> 50 m	50 kHz - 50 MHz

The sweep should cover at least one quarter-wave resonance for accurate measurement.