Coax Temp Attenuation Tech Tip 4a6r47

Coaxial Cable Attenuation Changes versus Temperature A characteristic of coaxial cable is that its attenuation varies with changes in temperature. This is true for both drop and hardline cable. Coaxial cable attenuation is provided at a reference of 68°F (20°C) by the manufacturer. Attenuation increases as temperature increases and decreases as temperature decreases. This is because the electrons that make up the conductors have more difficulty propagating (moving) through a warmer conductor. As temperature varies from the reference, cable attenuation changes at a rate of about 1.0% for every 10°F change in temperature, or 0.1% for every 1°F change in temperature. Since this effect is predictable, we can calculate the new, expected attenuation at the new temperature using the formula: at = αREF[1 + 0.0011(tF – 68)]

where αt = attenuation at new temperature tF (°F) αREF = reference attenuation at 68°F tF = new temperature °F

Let’s work through an example.

Suppose you need to know the attenuation for a 200 foot length of drop cable during a nice 100 °F summer day.

Given αt = attenuation at new temperature tF (100°F)

αREF = reference attenuation at 68°F = 5.5 dB/100 feet, or 0.055/foot at 750MHz (from cable manufacturer’s published specifications) tF = 100°F

Plug the numbers into the formula:

αt = αREF[1 + 0.0011(tF – 68)] = 0.055 x [1 + 0.0011 x (100 – 68)] (always perform calculations within prentices first) = 0.055 x [1 + 0.0011 x 32] (always perform multiplication or division before addition or subtraction) = 0.055 x [1 + 0.0352] = 0.055 x 1.0352 = 0.056936 or 0.057dB/foot Your drop loss for the 200 feet long cable, at the reference attenuation, would be 200 x 0.055 or 11.0 dB. At 100°F, your drop loss would be 200 x 0.057 or 11.4 dB. Not a huge difference in our example, but it may affect your troubleshooting, depending upon the system design. The calculation works the same way for Celsius, although the formula is slightly different. Let’s work through an example using the following formula: αt = αREF[1 + 0.002(tc – 20)] where αt = attenuation at new temperature tc (°C) αREF = reference attenuation at 20°C tc = new temperature °C

100°F approximately equals 38°C, so we’ll use this for tc.

αt = αREF[1 + 0.002(tc – 20)] = 0.055 x [1 + 0.002 x (38 – 20)] = 0.055 x [1 + 0.002 x 18] = 0.055 x [1 + 0.036] = 0.055 x 1.036 = 0.05698 or 0.057dB/foot To assess your understanding of Coaxial Cable Attenuation Changes versus Temperature, please take this brief quiz.

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