UNCLASSIFIED
TM-CAL-013
COAX CABLE CAPACITANCE STANDARD
Known-Length Coaxial Cable Sections as Precision Capacitance References
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers use of known-length coaxial cable sections as precision capacitance standards. Coaxial cable has a well-characterized capacitance per unit length; a measured length provides a calculable capacitance suitable for calibrating LCR meters, capacitance bridges, and DMM capacitance functions.

CHAPTER 2 — THEORY OF OPERATION

2-1. COAX CAPACITANCE

The capacitance per unit length of coaxial cable is determined by the dielectric constant of the insulation and the conductor geometry:

Capacitance per unit lengthC/l = 2πε0εr / ln(D/d)

where D = inner diameter of outer conductor, d = outer diameter of center conductor, εr = relative permittivity of dielectric.

Coax Capacitance Reference Values
Cable TypepF/ftpF/mVFZ0
RG-58A/U28.593.50.6650 Ω
RG-174/U26.085.30.6650 Ω
RG-8X (mini-8)26.085.30.7850 Ω
RG-213/U29.095.10.6650 Ω
RG-6 (75 Ω)16.052.50.8275 Ω
LMR-40023.978.40.8550 Ω

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. STANDARD PREPARATION

Cut a section of RG-58 to an exact measured length. Accuracy of the capacitance standard depends directly on length measurement accuracy.

  1. Measure RG-58 section to ±1 mm using steel rule.
  2. Install BNC connectors on both ends. Verify center pin fully seated and dielectric flush with BNC shoulder.
  3. Mark cable: length in cm, calculated capacitance, date.
  4. Short one end (connect center to shield at one BNC).
  5. The other BNC becomes the measurement port.

CHAPTER 4 — ASSEMBLY PROCEDURES

NOTEFor best accuracy, cut cable square and ensure BNC compression/crimp is fully bottomed. Improperly installed connectors add 0.5–2 pF each.

CHAPTER 5 — CALIBRATION PROCEDURE

  1. Calculate expected capacitance: C = (pF/ft) × length(ft).
  2. Example: 1 m of RG-58 = 3.281 ft × 28.5 pF/ft = 93.5 pF.
  3. Measure with DMM capacitance function or LCR meter: connect shorting BNC to one end, measure at open BNC.
  4. Compare measured vs. calculated. Error reflects meter accuracy.
  5. Error = (measured − calculated) / calculated × 100%.
  6. If meter reads 91.2 pF and calculated is 93.5 pF: error = −2.5%.
  7. Apply correction factor to future capacitance measurements.

CHAPTER 6 — TUNING AND ADJUSTMENT

The standard length is fixed. For a higher-accuracy standard, verify the actual capacitance/length ratio by measuring a long section (≥3 m) and back-calculating from a well-calibrated instrument. Record the actual pF/m for that cable spool.

CHAPTER 7 — VERIFICATION

  1. Prepare two lengths: 0.5 m and 1.0 m of same cable reel.
  2. 1.0 m should measure exactly 2× the 0.5 m reading (within ±1%).
  3. This ratio check is independent of meter accuracy and verifies consistency.
  4. Log: cable type, length, calculated C, measured C, date.

APPENDIX A — CALCULATIONS AND FORMULAS

Total capacitanceCtotal = (C/m) × lmeters = (C/ft) × lfeet
RG-58, 1.524 m (5 ft)C = 28.5 × 5 = 142.5 pF
Velocity factor and capacitance relationshipVF = 1/√εr, so C/m = 1/(Z0 × VF × c)

APPENDIX B — EXAMPLE RESULTS

Coax Capacitance Standards
CableLength (m)C calc (pF)C meas (pF)ErrorNotes
RG-580.50046.846.2−1.3%DMM capacitance
RG-581.00093.592.9−0.6%LCR meter at 1 kHz
RG-1741.00085.384.8−0.6%LCR meter at 1 kHz
← Back to Calibration Standards