There are several ways to measure coaxial cable, ranging from simple estimations to precise electronic measurements. The best method depends on the desired accuracy and the available tools.
Methods for Measuring Coaxial Cable
Here's a breakdown of common methods:
-
Estimation by Coil Measurement:
- This is a rough estimate, useful for determining if you have a general length.
- Process:
- Measure the diameter of one coil of the cable.
- Multiply the diameter by pi (approximately 3.14159). This gives the length of one coil.
- Count the number of coils.
- Multiply the length of one coil by the total number of coils.
- Formula: Length ≈ Diameter * π * Number of Coils
- Example: If a coil's diameter is 1 foot and there are 50 coils, the estimated length is approximately 1 * 3.14159 * 50 = 157 feet. This method is inherently imprecise due to variations in coil tightness and possible overlapping.
-
Using a Measuring Tape/Ruler:
- This is straightforward when the cable is accessible and uncoiled.
- Simply stretch the cable straight and measure its length using a measuring tape or ruler.
- Limitations: Impractical for long, installed cables or those that are difficult to access.
-
Time Domain Reflectometry (TDR):
- This is the most accurate method, especially for installed cables. It uses an electronic device to send a signal down the cable and measure the time it takes for the signal to reflect back. The instrument calculates cable length based on the signal's travel time and the cable's velocity of propagation.
- Tools: TDR-equipped cable testers, like some Fluke LAN/Cable testers, are specifically designed for this purpose.
- Calibration: TDR measurements often require calibration using a known length of the same type of coaxial cable to ensure accurate results. This accounts for variations in cable construction and dielectric properties.
-
Cable Length Meter:
- Some specialized cable length meters are designed specifically for measuring coaxial cables. These devices typically use TDR or similar techniques.
- Accuracy: Accuracy varies depending on the meter's specifications and calibration.
Summary Table of Measurement Methods
| Method | Accuracy | Ease of Use | Cost | Requirements | Best Use Case |
|---|---|---|---|---|---|
| Coil Measurement Estimation | Low | High | Low | None | Quick, rough estimate of coiled cable length. |
| Measuring Tape/Ruler | Medium | High | Low | Accessible, uncoiled cable | Direct measurement of accessible, uncoiled cable. |
| TDR (Cable Tester) | High | Medium | High | TDR-equipped cable tester, calibration cable | Accurate measurement, especially for installed cables. |
| Cable Length Meter | Medium/High | Medium | Medium/High | Cable length meter, possible calibration requirements | Accurate measurement; ease of use dependent on specific model |
Conclusion
Measuring coaxial cable can be accomplished through estimation, direct measurement with a tape measure, or sophisticated electronic methods like TDR. The best method depends on your needs and the available resources. For precise measurements, especially of installed cables, a TDR-based cable tester is recommended.
