When transmitting data over a coax cable it is important to determine the amount of signal attenuation that occurs for data recovery purposes. Signal attenuation changes based on the cable construction, signal frequency, and cable length. Prior to ordering cable and measuring the attenuation a good first step is to approximate your application’s signal attenuation. Although cable datasheets provide a wealth of information, generally your application’s cable length and data frequency will not be listed.
This page helps to calculate a coax cable’s approximate attenuation for a selected signal frequency and cable length. The calculation uses a generic, cable attenuation approximation model that is built using data from the manufacturer’s cable datasheet. A JavaScript based cable attenuation calculator is provided that accepts vendor’s cable attenuation parameters as inputs and calculates the signal attenuation for a specified cable length.
Cable Attenuation Approximation Model
Cable attenuation for any coaxial cable can be approximated using the equation below where frequency (f) is in MHz. Once the coefficients a, b, and c are known, finding the cable attenuation for a selected frequency is easy by applying the formula below.
The coefficients a, b, and c are found by solving a set of linear equations using information found in the vendor’s datasheet. These equations are the different attenuation values (α1, α2, and α3 ) at different frequencies (f1, f2, and f3) for a common cable length. The cable datasheet usually provides these values but you can also measure these values in the lab.
Cramer’s Rule (method of determinates) is used to solve for coefficients a, b, and c using the linear equation frequency and attenuation values.
These coefficients create an attenuation based on frequency for the common cable length used in the linear equations. To calculate the cable attenuation for a different cable length first convert your answer to dB per unit length and then multiple by the application cable length.
Model Verification
To validate the above model we used Belden RG-174 catalog data. The vendor supplied data is for a 1000 feet of RG-174 coax cable at three different frequencies.
| n | Frequency (fn) | Attenuation (αn) for 1000 Feet |
| 1 | 1 MHz | 19dB |
| 2 | 10MHz | 33dB |
| 3 | 100MHz | 84dB |
Solving the linear equations yields the coefficient values of a = 12.871, b = 6.020, and c = 0.109 for 1000 feet of cable. Data from the Belden catalog shows that 100 feet of RG-174 has an attenuation of 5.8dB at 50MHz. Our model approximates the 100 feet, 50MHz attenuation at 6.09dB.
Cable Attenuation Calculator
In the form below, enter your cable parameters (Input Data: common cable length, and frequency vs. attenuation values) for three different frequencies/attenuations at a common cable length. Enter your application’s data frequency and cable length (Find For:) and then click Calculate to display the Solution: for coefficients, Δ determinate, and your attenuation value.
When using this model with calculated coefficients, don’t forget to convert the attenuation into unit length and then multiply by your cable length. For the validation model example the result using a, b, and c is 60.9dB / 1000′ × 100′ = 6.09dB.
Notes
Normally attenuation values are entered as negative values in dB. You can use either positive or negative numbers to calculate the cable attenuation. Also the length units are arbitrary. You can use feet, meters, km, etc. as long as the units are consistent. Finally frequency values are always entered in units of MHz.
