Cable Loss Measurement

This tutorial covers measuring coaxial cable characteristics using the NanoVNA-H, including loss per unit length, electrical length, and velocity factor.

What You Will Learn

• Measuring cable loss using S11 and S21 methods
• Determining cable electrical length
• Calculating velocity factor
• Using Time Domain Reflectometry (TDR) mode
• Finding cable faults

Cable Parameters

Parameter	Description	Typical Values
Loss (dB/m)	Signal attenuation per meter	0.01 - 1 dB/m (frequency dependent)
Velocity Factor (VF)	Speed of signal vs. light	0.66 - 0.85
Impedance	Characteristic impedance	50 or 75 ohms
Electrical length	Length in wavelengths	Varies with frequency

Note

Cable loss increases with frequency. Always specify the measurement frequency when reporting loss values.

Method 1: S21 Transmission Loss

The most direct method - measure signal loss from one end to the other.

1. Set frequency range

   Cover the frequencies you care about:

   • HF work: 1 MHz - 30 MHz
   • VHF/UHF: 50 MHz - 500 MHz

2. Calibrate with THRU

   Connect your Port 1 cable directly to Port 2 cable (without the cable under test).

   Perform THRU calibration: CAL > CALIBRATE > THRU > DONE

3. Connect the cable under test

   • Port 1 to one end of the cable
   • Port 2 to the other end

4. Read the loss

   The S21 trace shows transmission loss in dB.

   At each frequency, the loss (in dB) equals the cable’s total loss at that frequency.

5. Calculate loss per unit length

   Loss/meter = Total loss (dB) / Cable length (m)

   Example: 3.5 dB loss for a 25m cable = 0.14 dB/m

Method 2: S11 Return Loss with Short

Use reflection measurement when you only have access to one end of the cable.

1. Short the far end

   Place a short circuit at the far end of the cable. You can:

   • Use a commercial short standard
   • Touch inner and outer conductors together
   • Solder a wire across the connector

2. Calibrate Port 1

   Perform OPEN, SHORT, LOAD calibration at Port 1.

3. Connect the cable

   Connect the near end of the cable to Port 1.

4. Read S11

   The signal travels down the cable, reflects from the short, and returns.

   One-way loss = |S11 in dB| / 2

   Example: S11 = -2.4 dB means one-way loss = 1.2 dB

Tip

The short method works because a perfect short reflects all power. Any loss in S11 is due to round-trip cable loss.

Measuring Electrical Length

The electrical length determines how the cable transforms impedance.

1. Short the far end

2. Set a narrow frequency span

   For a quarter-wave section at 100 MHz:

   • CENTER: 100M
   • SPAN: 50M

3. Enable S11 phase display

   Go to DISPLAY > FORMAT S11 (REFL) > PHASE

4. Find quarter-wave frequency

   At odd multiples of quarter-wavelength, the phase crosses through 0 degrees (going from +90 to -90 or vice versa).

5. Calculate electrical length

   At frequency F where phase = 0:

   • Quarter-wave length at F: lambda/4 = c / (4 * F * VF)

Calculating Velocity Factor

Velocity factor tells you how fast signals travel in the cable compared to free space.

1. Measure the physical length

   Measure the actual cable length in meters.

2. Find the quarter-wave frequency

   Using the method above, find where the shorted cable shows 0-degree phase.

3. Calculate velocity factor

   VF = (4 * Physical length * Frequency) / c

   Where c = 299,792,458 m/s (speed of light)

   Example:

   • Physical length: 2.5 m
   • Quarter-wave frequency: 24 MHz
   • VF = (4 * 2.5 * 24,000,000) / 299,792,458 = 0.80

Common Velocity Factors

Cable Type	Velocity Factor
RG-58, RG-8 (solid PE)	0.66
RG-213	0.66
RG-8X, RG-58 foam	0.78
LMR-400	0.85
Air-dielectric hardline	0.92 - 0.95
Twin-lead (300 ohm)	0.82

Using the Cable Measurement Mode

The NanoVNA firmware includes a cable measurement function.

1. Short the far end of the cable

2. Set the velocity factor

   Go to MARKER > MEASURE > CABLE (S11)

   Or use DISPLAY > TRANSFORM > VELOCITY F.

   Enter your cable’s velocity factor (e.g., 66 for 0.66).

3. Read the calculated length

   The display shows estimated cable length based on the velocity factor and measured electrical properties.

4. Adjust velocity factor for accuracy

   If you know the physical length, adjust VF until the measured length matches.

Time Domain Reflectometry (TDR)

TDR mode converts frequency-domain data to distance, showing reflections along the cable.

1. Enable TDR mode

   Go to DISPLAY > TRANSFORM

   Select TRANSFORM ON

2. Choose transform type

   • LOW PASS IMPULSE: Shows individual reflections
   • LOW PASS STEP: Shows impedance changes

3. Set velocity factor

   DISPLAY > TRANSFORM > VELOCITY F.

   Enter your cable’s VF as a percentage (66 for 0.66).

4. Set window function

   DISPLAY > TRANSFORM > WINDOW

   • MINIMUM: Sharp resolution, more ringing
   • NORMAL: Balanced
   • MAXIMUM: Smooth, reduced resolution

5. Interpret the display

   The horizontal axis now shows distance. Peaks indicate reflections:

   • Peak at the far end = normal termination
   • Peaks in between = faults, connectors, or impedance discontinuities

Caution

TDR resolution depends on frequency span. Wider span = better distance resolution. For accurate TDR, sweep at least 100 MHz.

Finding Cable Faults

Open Circuit Fault

TDR signature: Large positive peak

In frequency domain: S11 magnitude stays near 0 dB (all power reflected), phase changes with frequency

Finding the fault:

1. Enable TDR mode
2. Set correct velocity factor
3. Peak location shows distance to break

Short Circuit Fault

TDR signature: Large negative peak

In frequency domain: S11 magnitude stays near 0 dB, 180-degree phase shift pattern

Finding the fault:

1. Enable TDR mode
2. Peak location shows distance to short
3. Often caused by water ingress or crushed cable

Partial Damage

TDR signature: Smaller peak, possibly with ringing

In frequency domain: S11 varies with frequency, may show periodic pattern

Finding the fault:

1. Enable TDR mode
2. Look for intermediate peaks (not at the end)
3. May indicate water, kinks, or damaged connector

Comparing Cable Loss

To compare cables or check for degradation:

1. Create a reference measurement

   Measure a known-good cable section.

2. Save the reference

   Use DISPLAY > TRACE > STORE TRACE to save the reference.

3. Measure the test cable

4. Compare visually

   The stored trace appears as a reference line. Differences indicate:

   • Higher loss: Possible water ingress, damage, or poor connectors
   • Irregular response: Possible intermittent connection

Loss vs. Frequency Table

Measure and record loss at multiple frequencies for complete characterization:

Frequency	Total Loss (dB)	Loss/100m (dB)
3.5 MHz
7 MHz
14 MHz
28 MHz
50 MHz
144 MHz
432 MHz

Practical Tips

Minimizing measurement error

• Use quality connectors and adapters
• Keep adapter count to minimum
• Calibrate including any necessary adapters
• For long cables, use the S11/short method to avoid extra adapters at the far end

When to be concerned

• Loss significantly higher than manufacturer specification
• Loss that varies with frequency differently than expected
• TDR showing unexpected reflections
• Intermittent results when cable is moved

Next Steps

• Your First S11 Measurement - Basic reflection measurements
• Full Calibration - Accurate calibration procedure
• Tuning HF Antennas - Use cable knowledge for antenna work